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Chapter 3 PATTTERNS OF EVOLUTION Phylogenetic and systematic studies enable us to describe past changes in genes genomes biochemical and physiological features development and morphology and life histories and behavior as well as associated changes in l 39 39 quot quot 39 habitat 39 39 and ecological interactions between different species EVOLUTIONARY HISTORY AND CLASSIFCATION Evolution has two major features cladogenesis the branching of a lineage into two or more descendant lines and anagenesis the evolutionary change in each of the characteristics in each of the descendants Polyphyletic group taxonomic groups that are the result of parallel evolution due to different ancestral groups evolving into the same level of organization and function e g different groups of therapsids evolved independent of one another into the mammalian level of organization Gynmosperm conifers cycads and ginkgoes Monophyletic group all organisms within a group or taxon are derived from a single ancestral population Paraphyletic group a taxon phylogenetic tree or gene tree whose members are all derived from a single ancestor monophyletic but which does not include all the descendants of that ancestor eg the family Pongidae includes the orangutans gorillas and chimpanzees but humans the closest relatives of the chimpanzees have been placed in the family Hominidae therefore the Pongidae is a paraphyletic taxon More and more systematists have adopted the idea that taxa should be monophyletic and thus re ect a common ancestry The cladistic method or phylogenetic analysis focuses on the branching of one group from another in the course of evolution It attempts to identify monophyletic groups The groups or clades are identi ed by the possession of unique features called shared derived character states synapomophorphies Widespread features are called ancestral or preexisting character states plesiomorphies The analysis results in a cladogram There are practical dif culties for example an extinct group of species early in evolutionary time which may be termed a stem group such as the mammallike reptiles called Therapsida gives rise to a later group with distinctive derived characters referred to as a crown group such as the Mammalia Any name for the stem group that excludes the crown group would be designated paraphyletic taxon and therefore unacceptable in cladistics The decision to split a monophyletic group or to lump all its members into one taxon can be very arbitrary and there is no consensus in many cases INFERRING THE HISTORY OF CHARACTER EVOLUTION Systematic studies that parsimoniously map changes in character states onto a phylogeny that has been derived from other have yielded information on common patterns and principles of character evolution See example ofNils Adey s experiment on page 47 SOME PATTERNS OF EVOLUTIONARY CHANGE INFERRED FROM SYSTEMATICS COMPARATIVE ANATOMY An important principle of evolution is that features of organisms almost always evolve from pre eXisting features of their ancestors They do not arise de novo Structures with a similar underlying plan can be eXplained by relationship through common ancestry Homologous vs analogous Homologous structures have been derived from a common ancestor Homologous features have the same basic structural pattern even though the structure may be used differently Homologous structures are considered to indicate evolutionary affinity among organisms possessing them A character may be homologous among species but a give character state may not be Homoplasy is common Homoplasy the independent evolution of a character or character state in different taxa includes convergent evolution and evolutionary reversal Homoplastic or analogous features have similar function but lack the basic pattern of homologous structures eg wings of birds and insects the eyes of vertebrates and cephalopod mollusks Homoplastic features shown what is called convergent evolution It is due to the selection for similar habitats in different evolutionary lineages Parallelism a similar feature occurs in different species but their immediate common ancestor was different and did not have the feature The similar feature occurs in different species but it is not present in their immediate common ancestor They are functional adaptations Similar developmental mechanisms are one possible explanation for this phenomenon o The 439 quot quot between 1 quot quot and evolution may be arbitrary because there is no rule to specify how far back in the past one can establish a common ancestor for parallel evolution and even convergent lineages have common albeit distant ancestors o Eg the development of maXillipeds on the rst thoracic segment in some crustaceans rather than on the head segment See page 51 and 52 Evolutionary reversal involves the return from an advanced or derived character state to a more primitive or ancestral state 0 eg all frogs lack teeth in the lower jaw but frogs are descended from ancestors that have teeth One genus of frogs Amphignathodon reevolved teeth in the lower jaw its immediate ancestor lacked teeth in the lower jaw Mimicry is an example of convergent evolution Mimicry is the close resemblance of one organism the mimic to another organism the model in order to deceive a third the operator 0 The model and the mimic are not closely related 0 Both live in the same area It is usually presumed that mimicry is an evolutionary development in animals but there is evidence that animals in search for food may have stimulated mimicry in plants 0 e g Butter y Helicom39us may have stimulated mimicry in its food plant the passion ower Passi ora species to resemble species inedible to Helicom39us larvae l Batesian mimicry is when an edible mimic resembles a poisonous model 2 Mullerian mimicry occurs when several poisonous or distasteful organisms resemble each other It is advantageous to both Rates of character evolution differ Different characters evolve at different rates Conservative characters remain unchanged over long periods of time among the man descendants of an ancestor eg humans conserve the pentadactyl limb ve digits that rst evolved in amphibians body size evolves rapidly The rate of DNA sequence evolution varies among genes among segments within genes and among the three positions within amino acidcoding triplets of bases codons Evolution of different characters at different rates within a lineage is called mosaic evolution A species evolved not as a whole but in piecemeal There are exceptions to the independent rate of character evolution characters that function together may evolve in concert Species have a mixture of primitive and advanced features as a result of mosaic evolution Evolution is often gradual Darwin proposed that evolution takes place in small changes over a long period of time this is called gradualism Others have proposed that evolution takes place in large leaps saltation Gradation among living species is common Many higher taxa that diverged in the distant past are not bridged by intermediate forms either among living species or in the fossil record 0 Examples of these are the animal phyla and many order of insects and mammals Change in form is often correlated with change in function As function changes characters change thus producing a great variety of form in homologous characters 0 Plants that have evolved the climbing habit roots leaves and stems have been modi ed into clinging organs Similarity between species changes throughout ontogeny Baer Karl Ernst Von 1792 1876 Estonian naturalist and embryologist One of the founders of the modern science of development and one of his era s most in uential scientists He proposed what is now called Von Baer39s Law 0 Von Baer s Law states that structures that are present early in development are widely distributed among animals while structures that are present late in development are less widely distributed Features common to a more inclusive taxon higher taxa subphylum Vertebrata often appear in ontogeny development before the speci c characters of lowerlevel taxa orders and families Example tetrapod vertebrate embryos have gill slits paddlelike limb buds notochord and segmentation appear before features typical of their class and order Ernst Haeckel 18341919 proposed in 1866 the Biogenetic Law 0 Biogenetic Law Ontogeny recapitulates phylogeny Species are often more similar as embryos than as adults The development of the individual repeats the ancestral sequence that occurred during evolution the evolutionary history of the species The biogenetic law seldom holds true and is not an infallible guide to the evolutionary history of the species Development underlies some common patterns of morphological evolution In the past classi cation and phylogenetic studies relied chie y on analyses of morphological characters including changes during embryological development One of the most active research area concerns the genetic and developmental basis of such evolutionary changes 1 INDIVIDUALIZATION The bodies of many organisms consist of distinct unit that have distinct genetic speci cations developmental patterns locations and interactions with other units These units are called modules Some modules lack individual identities and may be considered aspects of a single character they are repeated along the body aXis Such structures are called serially homologous is they are located along the aXis of the body eg vertebrae teeth leaves Homonymous structures are not arrayed along the body aXis Individualization occurs when these different units acquire their own identities which in turn is an important basis for mosaic evolution 0 Eg teeth of reptiles are fairly uniform but acquired their own identity as incisors premolars and molars during the evolution of mammals 2 HETEROCHRONY Heterochrony is an evolutionary change in the timing or rate of development events Somatic features may be altered relative to the time of maturation of gonads Organisms that reproduce While retaining juvenile characteristics eg axolotl Paedomorphosis is the term used for reproducing juvenilized organisms evolution of a juvenilized morphology of a reproducing adult Forms of paedomorphosis o Neoteny caused by a reduction in the growth rate Without changing the duration of growth o Progenesis a cessation of growth altogether at an earlier age before reaching the adult form Peramorphosis is the delaying of maturity while the development of the adult is eXtended 3 ALLOMETRY Allom etry or allometric growth refers to the differential rate of growth of different parts or dimensions of an organism during its ontogeny o In humans the head grows at a slower rate than the rest of the body and the legs at a faster rate digits in the wings of the bats Allometry is local heterochrony that alters the shape of one or several characters Allometric growth is described by the formula y bXa Where x and y are two measurements such as the height and width of a tooth a is the allometric coefficient If a l the growth is isometric meaning that the two structures grow at the same rate If y increases faster than X as for human leg length relative to body size or weight a gt 1 positive allometry if the increase is relatively slowly as for human head size a lt 1 negative allometry See graphs fig 315 and 316 on pages 58 and 59 o Neoteny caused by a reduction in the growth rate without changing the duration of growth 0 Progenesis a cessation of growth altogether at an earlier age before reaching the adult form 4 HETEROTOPY Heterotopy is an evolutionary change in the position within an organism at which a phenotypic character is eXpressed o eg leaves are the photosynthetic organs of plants but in cactus the stems carry on photosynthesis 5 INCREASES AND DECREASES IN COMPLEXITY Earlier organisms must have had very few genes and have had a simple form Many phylogenetic and paleontological studies show that there have been great increases in compleXity during the history of life Reduction and loss of structures simpli cation of morphology is a common trend in clades Many of these changes can be ascribed to increased functional efficiency PHYLOGENETIC ANALYSIS DOCUMENTS EVOLUTIONARY TRENDS The term evolutionary trend can refer to a succession of changes of a character in the same direction either within a single lineage or in many lineages independently 0 Among owering plants the following trends are observed in many different groups low to high chromosome number and DNA content from many to few ower parts stamens petals etc from separate to fused ower parts from radial to bilateral symmetry of the ower from animal to wind pollination and from woody to herbaceous structure MANY CLADES DESPLAY ADAPTIVE RADIATION The divergent evolution of numerous related lineages in a relatively short time is called adaptive radiation A common ancestor Modi ed for different ways of life occupy different niches in the environment No directional trend evolved in many directions In a geographic region Galapagos nches cichlids of the Great Lakes of Eastern Africa Hawaiian silverswords Hawaiian drosophilid ies Anolis lizards in the Caribbean Islands etc Chapter 13 EVOLUTION OF PHENOTYPIC TRAITS Variation in most phenotypic characters is continuous or quantitative and it is based on effects of several or many variable gene loci as well as those of the environment 0 Height nger length eye color skin color etc Quantitative genetics studies continuous characters and the mechanisms that produce them 0 It includes traits that are determined by several genes polygenic traits 0 These traits may have a strong environmental component Quantitative genetics deals with the genetics of continuously varying characters Rather than considering changes in the frequencies of specific alleles of genotypes quantitative genetics seeks to quotquanti xquot changes in the frequency distribution of traits that cannot easily be placed in discrete phenotypic classes The reason for the continuous variation is usually that the traits are polygenic controlled by many genes and there are environmental e bcts that alter the phenotypic state of each individual httn39 11inde hmvvn edn Four 9 RIOAR 10 0mm genetic HTMI EVOLUTION OBSERVED Rapid adaptation at rates far greater than the average evolutionary rates documented in the fossil record is most often seen when a species is introduced into a new regions or when humans alter features of its environment Many of these instances of observed rapid evolutionary rate involved quantitative character See examples ofthe coalling moth German blackcap bird heavy metal tolerance in plants and insect adaptations to new plant species Page 298 and 299 COMPONENTS OF PHENOTYPIC VARIATION Variance average of the squared deviations of observations from the arithmetic mean of a sample V Standard deviation the square root of the variance s lV Frequency Frequency x Higher variance Lower variance ar an Variable Variable u Iln39 u n u httpwww rhssr salfnrd 2r Ariam e htm m l an 0 WAan M n tw H Im u New wnn Vsnattnn vanznce tn aphenutyplc wan VP may Include genene vanznce vs and vsnanee due tn mmmnmental teeters V2 V2 and nunraddmve genene vansnee due tn dummance and eptstssts are the basis fur respunse tn seleetmn WALhm pupulanun Only the adattnve vsnanee Creates a cunelanun betwem patents and uffspnng Only VA is sereetea by salecuun results 39nm the sum quhe gmntypic vs and Envinmmmml V2 van snees VF V5VE tnhentanee is the adamve MA MA AzAz 71 n 1 genutype sthe sumufthephmutyplcvalues ufeanh nftnelnn and therefure zreLhebasls fur respunsetu seleetmn WALth pupulahuns generanun The proportion of the genetic variance to the total variance is heritability Heritability does not indicate the degree to which a trait is genetic it measures the proportion of the phenotypic variance that is the result of genetic factors Two speci c types of heritability can be estimated 0 The broadsense heritability is the ratio of total genetic variance to total phenotypic variance VGVp o The narrowsense heritability is the ratio of additive genetic variance to the total phenotypic variance h2 V AVp Narrow sense heritability quantifies only the portion ofthe phenotypic variation that is additive allelic by nature When interested in improving livestock via artificial selection for example knowing t e narrow sense heritability ofthe trait ofinterest will allow predicting how much the mean ofthe trait will increase in the next generation as afunction ofhow much the mean ofthe selectedparents differsfrom the mean ofthe populationfrom which the selectedparents were chosen The observed response to selection leads to an estimate ofthe narrow sense heritability called realizedheritubility http enwikipedia orgwikiHeritabilitv HOW POLYGENIC ARE POLYGENIC CHARACTERS Quantitative multifactorial or polygenic inheritance refers to phenotypic characteristics that are the result of two or more genes and their interaction with the environment Polygenic traits do not follow Mendelian inheritance and the phenotypes vary along a gradient depicted by a bell shape curve Quantitative trait loci QTLs are stretches of DNA that are closely linked to the genes that underlie the trait in question It may contain several genes that affect the trait being studied QTLs are loci that in uence a quantitative trait A quantitative trait locus QTL is a region of DNA that is associated with a particular phenotypic trait these QTLs are often found on different chromosomes Knowing the number of QTLs that eXplains variation in the phenotypic trait tells us about the genetic architecture of a trait o It may tell us that plant height is controlled by many genes of small effect or by a few genes of large effect Some QTLs have large effects while other have small effects Many QTLs have both additive effects on the phenotype and strong epistatic interaction Strong epistatic effect means that the joint effect of some pairs of loci differed from the sum of their individual effects QTL mapping is the statistical study of the alleles that occur in a locus and the phenotypes physical forms or traits that they produce 0 QTL mapping is the collective name for a suite of related techniques that employ marker loci to scan chromosomes and identify regions containing genes that contribute to a quantitative trait Freeman and Herron 2007 A marker is segment of DNA that is signi cantly more likely to cooccur with the trait than eXpected by chance that is a marker that has a statistical association with the trait Candidate genes are genes thought to be involved in the evolution of a phenotypic trait based on its mutant phenotype of protein they encode LINKAGE DISEQUILIBRIUM Linkage disequilibrium is a term used in population genetics When alleles at two loci are randomly associated with each other they are said to in a state of linkage equilibrium when they are nonrandomly associated they are in linkage disequilibrium Linkage disequilibrium The association oftwo alleles at two or more loci more or less frequently than predicted by their individual frequencies The loci in linkage disequilibrium are usually located in different chromosomes Linkage disequilibrium is generally caused by selection in the form of epistatic tness interactions between genes genetic linkage and the rate of recombination random drift or nonrandom mating and population structure Epistasis occurs when the effect of a gene is affected by the activity of other genes located in different loci o A gene produces a protein that prevents transcription of another gene The coef cient of linkage disequilibrium is the measure of the deviation from the twolocus equilibrium Positive linkage disequilibrium increases the frequency of phenotypic variance The level of linkage disequilibrium declines due to recombination in the double heterozygotes with the passage of generations Linkage disequilibrium is caused by 1 Natural selection If selection favors individuals with particular combinations of alleles then it produces linkage disequilibrium If two or more gene combinations are much tter than recombinant genotypes linkage disequilibrium will be favored 2 Nonrandom mating 3 When a new mutation arises the single copy is necessarily associated with speci c alleles at other loci on the chromosome and therefore is in linkage disequilibrium with those alleles The copies of this mutation in subsequent generations will retain this association until it is broken down by recombination 4 The linkage equilibrium has not yet been reached It takes a number of generations for recombination to do its randomizing work and particularly for tightly linked genes linkage disequilibrium can persist for some time 5 Recombination is very slow or nonexistent Recombination tends to randomize genotypes at one locus with respect to genotypes at another It decreases the frequency of overrepresented haplotypes and increases the frequencies of underrepresented haplotypes 6 Random drift processes can cause persistent linkage disequilibrium If random sampling produces by chance an excess of a haplotype in a generation linkage disequilibrium will have arisen Any haplotype could be favored by chance so the disequilibrium is equally likely to have D gt 0 or D lt 0 EVOLUTION OF QUANTITATIVE CHARACTERS GENETIC VARIANCE IN NATURAL POPULATION S Heritability is determined by the additive genetic variance V A which depends on allele frequencies and by the environmental variance VE which depends in part on how variable the environmental factors are that affect the development or expression of the character V1 is the phenotypic variance h2 VAVp Where VP VG VE and VG is the sum of V A and nonadditive genetic components The causes of genetic variation in natural populations are uncertain but input by mutation may balance losses due to selection and genetic drift The paucity of genetic variation would be a genetic constraint that could affect the direction of evolution or prevent adaptation altogether Example Some grasses growing near mines have developed tolerance to high copper and zinc concentration in the soil other grass species have not and disappeared from the contaminated areas Experiments showed that seeds from plants growing in normal soil produced a small number of tolerant seedlings in the species that evolved tolerance in contaminated soils no tolerant seedlings were found in those species that had not formed tolerant populations in contaminated soil The genetic variation that allowed for tolerance was apparently missing in the intolerant species RESPONSE TO SELECTION The response of a quantitative character to selection depends on the heritability of the character and the selection differential The selection differential is the difference between the mean of a population and the mean of the individuals selected to be parents of the next generation The response to selection R is the change in the mean phenotype after selection Example The trait to be selected is the length of the tail in a population of rats the trait has a normal frequency distribution z bellshaped curve A normal distribution is expected if a large number of loci all with relatively small effects on the character freely recombine The experimenter imposes a selection for tail length by breeding only those rats with a tail longer than a certain value The mean tail length of the selected parents differs from the mean of the entire population this is the selection differential S The average tail length among the offspring of the selected parents differs from that on the parental generation as a whole by an amount R the response to selection The magnitude of R is proportionate to the heritability of the trait As selection proceeds it increases the frequencies of those alleles that produce phenotypes closer to the optimum value As those frequencies increase combination of alleles at different loci multilocus genotypes that were extremely rare become more common In this way phenotypes arise that were absent before The mean of a polygenic character shifts beyond the original range of variation as directional selection proceeds even if no further mutations occur As selection proceeds low alleles are removed and additive genetic variation is reduced As the VA decreases the heritability decreases h2 VAVp Prolonged directional selection should ultimately x all favored alleles eliminating genetic variation Alleles with the most favorable effects are likely to be xed rst Will selection come to a halt Probably not because mutation is constantly introducing a trickle of new alleles each with different additive effects Mutational variance the infusion of new additive genetic variance by mutation is on the order of 10393 x VE per quot A fully 39 39 could attain a heritability of 05 in one thousand generations If mutations have harmful pleiotropic effects and have a net selective disadvantage the usable advantageous mutational variance would be much less Under this view a continual mutationselection balance might explain the gradual changes in phenotype seen over long evolutionary times 1 1 RESPONSES TO ARTIFICIAL SELECTION Arti cial selection is the intentional selection of trait or combination of traits by humans The criterion for survival and reproduction is the trait chosen rather than tness as determined by the entire genotype This is in contrast with natural selection which focuses on the organism ability to survive and reproduce in its natural habitat the overall tness An experiment by Yoo in which he selected for an increased number of bristles in Drosophila melanogaster showed that arti cial selection has accomplished an enormous evolutionary change a rate far higher than is usually observed in the fossil record Several populations eventually stopped responding they reached a selection plateau in which response to selection stopped Selection plateau is caused by natural selection and not by loss of genetic variation 0 Extreme genotypes have low tness 0 Low tness is due to hitchhiking of deleterious genes and pleiotropy After the experiment was stopped the mean bristle declined showing that genetic variation was still present and that genotypes with fewer bristles had higher tness The response to selection is based on both genetic variation in the original population and new mutations that occur SELECTION IN NATURAL POPULATIONS MEASURING NATURAL SELECTION ON QUANTITATIVE CHARACTERS An attempt has been made to measure the strength of natural selection on quantitative traits One index uses the mean and variance of a trait measured within a single generation before and after selection has occurred 0 E g the measurements are made on juveniles and then on those individuals that successfully survive to adulthood and reproduce EXAMPLES OF SELECTION ON QUANTITATIVE CHARACTERS pages 309 31 1 See the examples of The Darwin s nch Gossips forties on Daphne Island The con icting selection pressures create stabilizing selection that on average favors an intermediate bill size Birth weight in humans Antagonistic agents of selection The larvae of the goldenrod gall y its parasitoids and predators Taken together these enemies imposed rather strong stabilizing selection o Vestigial features in cave organisms STRENGTH OF NATURAL SELECTION The strength of natural selection appears to be quite modest although strong selection has often been recorded Stabilizing selection and diversifying selection appear to be about equally common There is a tendency for the strength of selection due to variation in mating success and female fecundity to be greater on average than that of selection due to differences in survival A NEUTRAL MODEL OF THE EVOLUTION OF QUANTITATIVE CHARACTERS If alleles that contribute to variation in a polygenic trait are selectively neutral variation and evolution of the trait are affected only by mutation which increases variation and genetic drift which erodes it At equilibrium mutation is balanced by genetic drift the genetic variance and heritability should theoretically reach a stable value which should be quite high if the effective population size is large As mutations are xed by genetic drift the mean will uctuate randomly Mutation and genetic drift can cause divergence between two isolated populations that originated from a uniform ancestral population Studies on the rate of evolution show that many features uctuate rather rapidly but show little net change so that the rate of evolution over the long term is much less than it is over shorter intervals of time WHAT MAINTAINS GENETIC VARIATION IN QUANTITATIVE CHARACTERS There are high levels of variation in quantitative traits in both large and small populations Alleles that contribute to quantitative traits are probably seldom selectively neutral Genes contributing to quantitative traits have pleiotropic effects of survival and other tness components Gene ow between populations with optimal phenotypes can maintain variation in both populations but studies of captive populations in which gene ow does not take place show that these populations do not differ substantially from wild populations MUTATIONSELECTION BALANCE HYPOTHE SIS Levels of polygenic variation re ect a balance between the erosion of variation by stabilizing selection and the input of new variation by mutation There is some doubt that variation by mutation can counter balance the strong stabilizing selection that acts on many traits CORRELATED EVOLUTION OF QUANTITATIVE TRAITS Evolutionary change in one character is often correlated with change in other features If changes by selection occur in one trait other traits will also change The overall phenotype must be maintained in order to avoid deleterious changes CORRELATED SELECTION Characters that are functionally related are usually selected together GENETIC CORRELATION The evolution of a trait is governed both by selection on that trait and by selection on other traits with which it is genetically correlated Phenotypic correlation may have a genetic and an environmental component 0 E g difference in nutrition may affect two phenotypic traits of individuals with the same genotype 0 This is referred as environmental correlation Correlated variation may be caused by genetic differences that affect two characters causing genetic correlation Causes of genetic correlation 0 Linkage disequilibrium among the genes that independently affect each character 0 Pleiotropy the in uence of the same gene in two characters I pleiotropy Perfect genetic correlation occurs when one gene affects all characters in the same direction all increase or decrease Imperfect genetic correlation occurs when one gene affects some characters in one direction and others in another direction or if the gene affects only character and does not affect the others Linkage disequilibrium will decline with time due to recombination unless selection for the adaptive gene combinations maintains it Natural selection may affect modi er alleles that alter the pleiotropic effects of other loci See the example of Australian blow ies on page 313 Genetic correlation among character and environmental correlation give rise to phenotypic correlation HOW GENETIC CORRELATION AFFECTS EVOLUTION Genetic correlation may be positive or negative Genetic correlations among characters can cause them to evolve in concert Multiple characters may evolve as an integrated ensemble more rapidly if they are genetically correlated as when they are subject to the same developmental controls 0 E g characters that are functionally related like the organs of the body and the body size 0 Body size would evolve much more slowly in response to selection if every organ had to undergo independent genetic change than if there were coordinated increases or decreases in the sizes of the various organs 0 During development the various organs grow in concert and alleles that change body size have correlated effects on most body parts Genetic correlation among characters may retard the rate of adaptive evolution depending on circumstances A con ict may eXist between the genetic correlation of characters and directional selection on those characters When such a con ict eXists the two characters may evolve to their optimal states only slowly and may evolve temporarily in a maladaptive direction 0 E g selection for a deeper bill in the Galapagos nch Geospizafortis caused average ill length to increase even though selection favored a shorter bill o Eg An example from beef cattle breeding Selection for fast growth is a common goal of selection By doing so it is hard to avoid a change in the nal adult weight At the same time the selection will also result in larger calves being born which can cause dif culties during calving if the ratio between the weight of the calf and the cow is changed unfavorably httpwww knrsns k l dk hare vetgen I 39 3 4htm CAN GENETICS PREDICT LONG TERM EVOLUTION Some characters may have very little variation and may constraint or determine the initial direction of evolution Studies have shown that genetic correlation between characters may remain consistent for a long time and can in uence the direction of selection Some investigators have found that the genetic correlations among certain characters are fairly similar among geographic populations of a species or among closely related species Other studies have found lower similarities showing that the strength of genetic correlations can evolve rather rapidly NORMS OF REACTION The norm of reaction of a genotype is the set of phenotypes it expresses in different environments For every genotype phenotypic trait and environmental variable a different norm of reaction can exist in other words an enormous complexity can exist in the interrelationships between genetic and environmental factors in determining traits httpenwikipediaorgwikiNorms of reaction PHENOTYPIC PLASTICITY Phenotypic plasticity Organisms with a given genotype may change its phenotype in order to adapt to environmental changes Adaptive phenotypic plasticity has evolved by natural selection for those genotypes with norms of reaction that most nearly yield the optimal phenotype for the various environments the organism commonly encounters o Eg the leaf form of the aquatic plantRanunculus aquatilis depends on whether it is submerged aerial or situated at the airwater interface during development The following examples are from httpenwikipediaorgwikiPheno pic plasticity o E g In social insects colonies of which depend on the division of their members into distinct castes such as workers and guards These two castes differ dramatically in appearance and behavior However these differences are not genetic they arise during development and depend on the manner of treatment of the eggs by the queen and the workers who manipulate such factors as embryonic diet and incubation temperature The genome of each individual contains all the instructions needed to develop into any one of several morphs but only the genes that form part of one developmental program are activated 0 E g In epidemiology a popular theory is that rising incidences of coronary heart disease and Type II diabetes in human populations undergoing industrialization is due to a mismatch between a metabolic phenotype determined in development and the nutritional environment an individual is subsequently exposed to This is known as the Thrifty phenotype hypothesis CANALIZATION Canalization is the buffering of developmental pathways that tend to produce a standard phenotype despite 39 39 and 39 39 39 genetic variability developmental homeostasis 0 Uniform expression of traits or patterns of development despite slight changes in environment or genotype 0 Maintenance of a single adaptive phenotype A trait is canalized when the same phenotype is produced in a wide range of genotypic and environmental backgrounds Canalization occurs when selection allows a larger number of different genotypes to produce the same phenotype 0 Selection for genotypes that can produce the same phenotype despite their variability A character state that initially developed in response to the environment can become genetically determined his called genetic assimilation Canalized characters include threshold characters in which underlying polygenic variation is not phenotypically expressed unless a drastic mutation or environmental perturbation breaks down canalization The following quotation refers to the experiment performed by Waddington on the effect of heat shock and the development of crossvainless trait in Drosophila on page 319 As explained by WADDINGTON the development of crossveins and other apparently very stable morphological traits can be in uenced by environmental disturbances above a certain threshold of intensity but individuals from wildtype populations have a threshold so high that only an unusually strong stimulus such as a heat shock can effectively induce a modi ed expression According to WADDINGTON39s explanation the phenotypic uniformity generally observed in these traits can easily coexist with the abundant genetic variability by arti cial election 39 39 quot 39 xperiments Although different genotypes available in a population are sensitive to different threshold values of external stimuli phenotypic variation does not arise if all of them have too high a threshold to be affected by the disturbances prevailing in the usual environment However when an exceptionally severe disturbance occurs the subpopulation of individuals in which a phenotypic change is induced is necessarily enriched for the most sensitive genotypes which provide the material for arti cial selection Canalization Genetic 39 quot quot and 39 39 A f Genetic Model Ilan Eshel and Carlo Matessi Genetics Vol 149 21192133 August 1998 Genotypes of ies differ in their degree of susceptibility to the in uence of the environment heat shock 7 they differ in their degree of canalization so that some are less easily de ected into an aberrant developmental pattern Selection for this pattern favors alleles that canalize development into the newly favored pathway As such alleles accumulate less environmental stimulus is required to produce the new phenotype Waddington explained that genetic assimilation provided a potentially bene cial mechanism for populations under stress The phenotype produced under stress becomes the phenotype for every condition Canalization implies that a genotype39s phenotype remains relatively invariant when individuals of a particular genotype are exposedto different environments environmental canalization or when individuals of the same single or multilocus genotype differ in their genetic background genetic quot 39 The 39 genetics of quot 39 52 Rev Biol 2005 Sep39 803287316 EVOLUTION OF VARIABILITY Variation refers to the differences actually present in a sample or species Variability refers to the potential to vary The variability of individual characters is affected by the evolution of canalization A character that is insensitive to alteration by environmental factors in environmentally canalized Genetic canalization describes insensitivity of a character to mutations In genetic canalization the phenotype remains unchanged when the genes underlying its development vary genetic canalization can be viewed as a particular kind of epistasis and environmental canalization and phenotypic plasticity are two aspects of the same phenomenon Canalization results in the accumulation of phenotypically cryptic genetic variation which can be released a er a quotdecanalizingquot event Thus canalized genotypes maintain a cryptic potential for expressing particular phenotypes which are only uncovered under particular decanalizing environmental or genetic conditions Selection may then act on this newly released genetic variation The evolutionary genetics of canalization 52 Rev Biol 803287 316 2005 Sep Threshold traits are controlled by polygenic variation rather than by single loci The polygenic variation is not expressed phenotypically unless development is disturbed by substantially beyond the threshold by a large enough genetic or environmental change The evolution of canalization may explain the constancy of some characters over vast periods of evolutionary time o E g Early Devonian amphibians had a variable number of eight or nine toes then vetoed limbs evolved and became canalized so vertebrate tetrapods since then have had more than ve digits The hypothesis of morphological or phenotypic integration holds that functionally related characteristics should be genetically correlated with one another Wagner and Altenberg 1996 have shown theoretically that prolonged directional selection favors modi er alleles that enhance a l 39 39 1 39 39 39 between L 39 quot related traits o Eg if it were functionally important for the upper and lower mandibles of a bird s bill to be the same length then selection for a longer bill would include selection for alleles that coordinate the development of the two mandibles creating a pleiotropic correlation between them u r u For additional information see httn39 eehweh ari7nna badvaeVpapers73pdf Chapter 9 VARIATION Mutations and recombinations give rise to variation among organisms This genetic variation is the foundation of evolution Phenotype refers to the eXtemal appearance of the organism eg Seed shape round or wrinkled it also includes internal anatomy physiology and behavior Genotype refers to the genetic makeup of the organism 0 Individuals with the same appearance phenotype may differ in their genetic makeup genotype Locus a site on a chromosome or the gene that occupies that site Allele refers to genes that govern variations of the same feature eg yellow seed and green seed are determined by two alleles of the same gene Haplotype is one of the sequences of a gene or DNA segment that can be distinguished from homologous sequences by molecular methods such as DNS sequencing Gene copy refers to the number of representatives of a gene It does not distinguish between alleles o In a diploid population each individual carries two copies of a gene eg 100 individuals 200 gene copies Allele copy distinguishes between alleles o A heterozygous individual has two copies of the gene eg allele A1 and allele A2 DISTINGUISHING COPIES OF PHENOTYPIC VARIATION Individuals may differ in their phenotypes because of genetic differences environmental differences or both Maternal effects effects of a mother on her offspring that are due not genes inherited from her but rather to nongenetic in uences such as the amount or composition of yolk in her eggs the amount and kind of maternal care or physiological condition while carrying the eggs or embryos Congenital differences among individuals may be caused by genes by nongenetic maternal effects or by environmental factors that act on the embryo before birth or hatching To determine whether variation among individuals is genetic environmental or both several methods can be used Testing for Mendelian inheritance by using the backcross method Correlation between the average phenotype of offspring and that of their parents or greater resemblance among siblings than among unrelated individuals Suggests that genetic variation contributes to phenotypic variation o Rearing offspring from different parents in a common environment for a few generations help to distinguish differences due to genotype from maternal or environmental effects FUNDAMENTAL PRINCIPLES OF GENETIC VARIATION IN POPULATIONS At any given locus a population may contain two or more alleles that have arisen over time by mutations An allele may be more common that others and is often called the wild type Some times two or more of the alleles are very common The relative commonness or rarity of an allele fits proportion of all gene copies in the populationi is called the allele frequency In sexual reproduction alleles may combine forming a homozygous or heterozygous genotype Genotype frequency is the proportion of a population that has a certain genotype Any alteration in genotype frequencies in one generation will alter the frequencies of the alleles carried by the population s gametes when reproduction occurs The genotype frequencies of the neXt generation will be altered as well Such alteration from generation to generation is the central process of evolutionary change The factors that can cause the frequencies to change are the causes of evolution FREQUENCIES OF ALLELES AND GENOTYPES THE HARDY WINBERG PRINCIPLE The principles states that a population at genetic equilibrium allele and genotype frequencies do not change from generation to generation The principle shows that the process of inheritance by itself does not cause changes in allele frequency It also eXplains why dominant alleles are not necessarily more common that recessive ones Punnett Square ofthe Hardy Weinberg Equilibrium A a A AA p2 Aapq a Aapq aaq2 Example p2 qu q2 1 where p is the frequency of the dominant allele B and q the frequency of the recessive allele b p2 is the frequency of AA 2pq is the frequency of Aa q2 is the frequency of aa 1 is the total population The allele frequencies do not change from one generation to the neXt The genotype frequencies will remain unchanged Departure from genetic equilibrium HardyWeinberg principle indicates the amount of evolutionary change Change from to quot is 39 called miu THE SIGNIFICANCE OF THE HARDY WEINBERG PRINCIPLE FACTORS IN EVOLUTION The following conditions have to be met for a population to remain in genetic equilibrium 1 Random mating Each individual of the population has equal chance of mating o Panmictic random interbreeding population 2 No net mutations The frequencies of genes must not change due to mutations 3 Large population size in order to avoid frequency changes due to random uctuations 4 No migration There can be no exchange of genes with other populations that might have different allele frequencies 0 Mating among individuals from different populations is called gene flow or migration 5 No natural selection in order to avoid that some genotypes be favored over others The required conditions for the HardyWeinberg principle to work probably do not occur in the real wor Inasmuch as nonrandom mating chance gene ow mutation and selection can alter the frequencies of alleles and genotypes these are the major factors that cause evolutionary change within populations FREQUENCIES OF ALLELES GENOTYPES AND PHENOTYPES At HardyWeinberg equilibrium the frequency of heterozygotes is greatest when the alleles have equal frequencies When an allele is very rare almost all its carriers are heterozygotes o This is called concealed genetic variation INBREEDING Inbreeding is a form of nonrandom mating Gene copies are identical by descent if the have descended by replication from a common ancestor Inbreeding coef cient of A measure of how close two people are genetically to each another The coef cient of inbreeding symbolized by the letter F is the probability that a person with two identical genes received both genes from one ancestor ttDwww determ 39 39 39 19962007 MedicineNet Inc If the genes are identical by descent the individual is said to be autozygous In self fertilization or sel ng all loci are affected equally Inbreeding increases the proportion of homozygotes and decreases the proportion of heterozygotes For a good eXplanation of39 39 see httpwww netnets 39 39 39 39 39 html GENETIC VARIATION IN NATURAL POPULATIONS Genetic polymorphism is the presence in a population of two or more variants alleles or haplotypes o Multiples alleles human blood groups The term includes genetically determined phenotypes and variations at the molecular level GENETIC VARIATION IN VIABILITY Studies on fruit ies and the mortality of children from marriages between relatives have shown that organisms carry several lethal recessive genes 0 The average person carries 35 lethal alleles acting between late fetal and adult stages Morton et al 1956 These genes are lethal only in the homozygous condition Natural populations carry an enormous amount of concealed genetic variation INBREEDING DEPRESSION Populations with a high rate of inbreeding often show a decline in components of tness like fecundity and survival This decline is called inbreeding depression Inbreeding may increase the risk of extinction of small populations in the wild Inbreeding is a problem in captive populations and is highly controlled in zoo populations GENETIC VARIATION IN PROTEINS To know how much genetic variation is present in a population it is necessary to nd out what fraction of the loci are polymorphic how many alleles are present in each locus and what their frequencies are Enzyme electrophoresis is used to nd the polymorphic loci 0 About one third of them in Drosophila pseudoobscura o In one study 28 our 71 loci were polymorphic in a human population 0 The estimated number of polymorphic loci in humans is 3000 Variant forms of an enzyme are called allozymes They are coded by different alleles in the same locus Allozymes move at different rates depending on the amino acids that have been substituted Almost every individual in a sexually reproducing species is genetically unique VARIATION AT THE DNA LEVEL DNA sequencing distinguishes the extent of synonymous and nonsynonymous nucleotide substitutions in the amino acid coding region of the DNA It is also possible to compare changes in the coding and noncoding regions of the DNA Chromosomal and mitochondrial DNA and RNA sequences have been used Considerable sequence variation especially synonymous variation has been found in most of the gene and organisms that have been studied MULTIPLE LOCI AND THE EFFECTS OF LINKAGE Genes that are found on the same chromosome are said to be linked Linkage refers only to a physical association and not to a functional association Linked genes tend to be inherited together Linkage disequilibrium is a term used in the study of population genetics for the nonrandom association of alleles at two or more loci not necessarily on the same chromosome It is not the same as linkage which describes the association of two or more loci on a chromosome with limited recombination between them Linkage disequilibrium describes a situation in which some combinations of alleles or genetic markers occur more or less frequently in a population than would be expected from a random formation of haplotypes from alleles based on their frequencies Nonrandom associations between genes at different loci are measured by the degree of linkage disequilibrium D httpenwikipediaorgwikiLinkageidisequilibrium If there is no association of the alleles the loci are in linkage equilibrium Two loci are in linkage equilibrium if genotype frequencies at one locus are independent of genotype frequencies at the second locus otherwise the two loci are in linkage disequilibrium httpevotutororgEvoGenEG4A htrnl What causes linkage disequilibrium Recombination breaks down nonrandom genetic associations and yet in some cases non random associations exist Four causes of linkage disequilibrium 1 Natural selection If selection favors individuals with particular combinations of alleles then it produces linkage disequilibrium It is this process that has most interested evolutionary biologists and is probably responsible for the Papilo memnon genes 2 The linkage equilibrium has not yet been reached It takes a number of generations for recombination to do its randomizing work and particularly for tightly linked genes linkage disequilibrium can persist for some time 3 Random dri in a nite population Random processes can cause persistent linkage disequilibrium If random sampling produces by chance an excess of a haplotype in a generation linkage disequilibrium will have arisen Any haplotype could be 39favored39 by chance so the disequilibrium is equally likely to have D gt 0 or D lt 0 As a population approaches linkage equilibrium all random uctuations in haplotype frequencies will tend to be away from the linkage equilibrium values if a population is well away from the point of linkage equilibrium random sampling is equally likely to move it towards as away from the equilibrium Most natural populations are probably near linkage equilibrium and then the balance between the random creation of linkage disequilibrium and its destruction by recombination in small enough populations is such that linkage disequilibrium will persist 4 Nonrandom mating If individuals with gene A1 tend to mate with El types rather than B2 types AlBl haplotypes will have excess frequency over that for random mating hm m1 1 n 1 1 population mph002 VARIATION IN QUANTITATIVE TRAITS Continuous metric quantitative polygenic variation of a character is the result of several genes located in different loci working to produce a give phenotype eg nose shape and skin color in human These phenotypes vary along a continuous gradient These variations are said to be polygenic because they depend on the interaction of several genes among themselves and with the environment The proportion of the phenotypic variance that is due to genetic variation genetic variance is the heritability of the trait ESTIMATING COMPONENTS OF VARIATION The description and analysis of quantitative variation are based on statistical measures because the loci that contribute to quantitative variation generally cannot be singled out for study The variance in a phenotypic character is the sum of genetic variance and environmental variance Each genotype in a population has an average phenotypic value Individuals with that genotype vary in their phenotypes because of environmental in uences The amount of variation among the averages of the different genotypes is the genetic variance The average amount of variation among individuals with the same genotype is the environmental variance By performing speci c experiments quantitative geneticists can estimate the proportion of the total variance that is attributable to the total genetic variance and the environment genetic variance If geneticists are trying to improve a speci c quantitative trait such as crop yield or weight gain of an animal estimates of the proportion of these variances to the total variance provide direction to their research If a large portion of the variance is genetic then gains can be made from selecting individuals with the metric value you wish to obtain On the other hand if the genetic variance is low which implies that the environmental variance is high more success would be obtained if the environmental conditions under which the individual will be grown are optimized Copyright 1997 Phillip McClean httpwww nd 11 39 Heritability is the variation of phenotypes due to genetic variation genetic factors in the population In general physical characters are highly heritable e g nger length height RESPONSES TO ARTIFICIAL SELECTION A character can be altered by selection only if it is genetically variable Under arti cial selection the reproductive success of individuals is determined largely by a single characteristic chosen by the investigator rather than by their overall capacity for survival and reproduction Most of our domestic animals and plants are the result of arti cial selection Species contain genetic variation that could serve as the foundation for the evolution of almost all of their characteristics VARIATION AMONG POPULATIONS In a panmictic population all individuals have the potential to mate with all other individuals there are no mating restrictions Few species consist of a single panmictic population e g Devil s Hole pup sh the eel Anguilla rostrata Most species consist of separate population with most mating taking place between members of the same population Populations of a single species in different geographic areas often differ genetically This is called geographic variation A subspecies or geographic race in zoological taxonomy means a recognizable distinct population or group of populations that occupies a different geographic area from other populations of the same species In botanical taxonomy subspecies names are sometimes given to sympatric interbreeding forms Populations with overlapping range in which individuals frequently encounter each other are called sympatric e g eastern subspecies of the Northern icker Populations with adjacent nonoverlapping ranges that come into contact are called parapatric e g the hybrid zone of the Northern icker in which hybrid forms of the eastern and western subspecies interbreed Populations with separated distributions are allopatric e g the eastern and western subspecies of the Northern icker A gradual change in a character or in allele frequencies over geographic distance is called a cline 0 There is direct relationship between latitude and body size in mammals and birds this cline has been called Bergmann s rule 0 Larger body size reduces the surface area relative to body mass reducing heat loss an adaptive geographic variation An ecotype is a population of a species adapted to a particular environment and having distinct characteristics different from other populations in other environments 0 The distinct characters are the result of natural selection taking place in a speci c environment Populations of a species usually exhibit at least some degree of genetic differentiation among geographic localities This geographic structuring has several causes such as social structure mating system dispersal capability cohesion of parents with their offspring and habitat fragmentation These processes lead to certain patterns of gene ow genetic recombination natural selection and random dri which in turn have an impact on the structure Avise 1994 http herkulesoulu isbn95 l4255364htmlX706html ADAPTIVE GEOGRAPHIC VARIATION Some differences among populations are correlated with the environmental differences and appear to be adaptive o Bergmann s rule latitudinal variation in body size 0 Allen s rule shorter appendages in colder climates o Gloger s rule pale fur color in arid environments Genetic differences compensate for constraints imposed by the environment e g larva of Rana clamitans see page 216 Latitudinal counter gradient variation in the common frog Rana temporaria development ratesevidence for local adaptation Laugen AT LauIila A Rasanen K MeIila J JEvol Biol 2003 Sep1659961005 Adaptive genetic differentiation along a climatic gradient as a response to natural selection is not necessarily expressed at phenotypic level if environmental effects on population mean phenotypes oppose the genotypic effects This form of cryptic evolutioncalled counter gradient var1ation See the entire article in httpwww H quots ner comdoipdflO1046il4209l01 70m 00560 Morphological differences resulting from competition may result in what is called character displacement Sympatric populations of two species differ more than allopatric populations 0 See the example of the ground Galapagos nches on page 216 GENE FLOW The exchange ofgenes between two populations is called gene flow or gene migration Migrants that do not reproduce do not contribute to gene ow Gene ow tends to homogenize the allele frequencies unless is counterbalanced by genetic drift or natural selection 0 Gene ow may introduce or reintroduce genes that had disappeared or become rare due to genetic drift 0 Gene ow may reduce the chance of speciation The rate of gene ow measures the change allele frequencies per generation due to migration into the population If a population becomes extinct and the area is recolonized by individuals from several populations the allele frequencies are a mixture of those among the source populations Different populations will be genetically similar if they had been recently founded by migrants from a single population Gene ow is greatest among mobile organisms eg marine invertebrates with planktonic larva Animals that move little are generally divided into small genetically distinct populations ALLELE FREQUENCY DIFFERENCES AMONG POPULATION S Populations that are found at the extreme distances of their range are seldom exchange genes They are isolated by distance Isolation by distance produces populations with greater divergence in allele frequency than adjacent populations that exchange genes Nei s Index of Genetic Distance is a measure of the genetic difference between two populations It measures how likely it is that gene copies taken from two populations will be different alleles given data on allele frequencies It measures how dissimilar two species or two populations of the same species are The genetic distance ofChimpanzees and Human beings is only 16 they are about 984 identical suggests that Human beings and Chimpanzees last had a common ancestor about 5 million years ago and that Chimpanzees are more closely related to Humans than they are to Gorillas about 9 million years ago and Orangutan about 12 million years ago httpenwikipediaorgwikiGenetic distance This index is used to construct phenograms Phenograms are diagrams that show the similarity and differences between populations DNA sequencing may be used to show the similarities between population by producing clusters of populations GEOGRAPHIC VARIATION AMONG HUMANS Human races Homo sapiens is a single biological species The number of human races is arbitrary Between 3 and 60 races have been described Genetic differences among human populations consist of allele frequency differences only One early study of allozymes variation showed that about 85 of the genetic variation in the human species is among individuals within populations and only about 8 is among the major races 0 Although strictly speaking allozymes represent different alleles of the same gene and isazymes represent d erent genes whose products catalyse the same reaction the two words are usually used 39 1 L7 quothttpen wikinediz quot 39 Another study came to the conclusion that differences among members of the same population account for 93 to 95 of human genetic diversity and differences among populations account for only about 5 An interesting article htm 39 quot sampleresultsdnatribes lobalsurve re ional W 39t39 ndf Based on the slight allele frequency difference among populations ve major geographic clusters could be distinguished subSaharan Africa EuropeCentral Asia East Asia Oceania and native America No native Australians were included in the study In spite of these similarities some loci show strong patterns of geographic variation the sicklecell hemoglobin allele is most frequent in parts of Africa and cystic brosis mutations are most prevalent in northern Europe VARIATION IN COGNITIVE ABILITIES General cognitive ability IQ appears to be substantially heritable and the heritability increases with age from childhood to adolescence to adulthood and old age Despite that substantial heritability of IQ there is abundant evidence that education and an enriched environment can substantially increase IQ scores Chapter 15 S The diversity of organisms is the consequence of cladogenesis the branching or multiplication of lineages each of which the evolved by anagenesis along it own path Each branching in the phylogenetic tree marks a speciation event the origin of two species from one WHAT ARE SPECIES The word species in Latin means kind There are many de nitions of species PHYLOGENETIC SPECIES CONCEPTS PSC quotAn irreducible basal cluster of organisms diagnosably different from other such cluster and within which there is a parental pattern of ancestry and descent Cracraft 1989 Phylogenetic species concepts emphasize the phylogenetic history of organisms common ancestry A species is the smallest lineage that can be united by synapomorphic characters 0 Synapomorphies are characters shared by two or more taxa that are derived from a common ancestor The members of the species should share characteristics that other groups lack these characteristics are diagnostic This de nition makes no reference to reproductive boundaries BIOLOGICAL SPECIES CONCEPT BSC It was de ned by Ernest Mayr in 1942 Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups Populations of similar organisms that interbreed in the wild and produce viable and fertile offspring Based on Buffon s ideas Reproductive isolation no genetic exchange is a key element of this de nition Based on sexual reproduction Reproduction without human interference Morphological similarities and differences do no not suf ce to de ne species Variation within populations characteristics vary among the members of a single population of interbreeding individuals Geographic variation populations of a species differ there eXists a spectrum from slight to great difference eg human populations Sibling species these are reproductively isolated populations that are difficult or impossible to distinguish by morphological features but which are often recognized by differences in ecology behavior chromosomes and other characters DOMAIN AND APPLICATION OF THE BIOLOGICAL SPECIES CONCEPT All concepts have limitations Dif culties in Testing allopatric populations Evaluating differences in fertility of offspring Fossils cannot be tested Asexual reproducing organisms are genetically isolated because they cannot reproduce sexually Selfpollinated plants 6 Plasmidmediated horizontal gene exchange between different species JawNH U DOMAIN The domain of the BSC is restricted to sexual outcrossing organisms and to short intervals of time E g Fossils cannot be tested asexual reproducing organisms are genetically isolated because they cannot reproduce sexually selfpollinated plants A second meaning of species is a taxonomic category just like genus or family BORDERLINE CASES Interbreeding versus reproductive isolation is not an eitheror all or none situation Narrow hybrid zones exist where genetically distinct populations meet and interbreed to a limited extent but in which there exist partial barriers to gene exchange The hybridizing entities are often recognized as species but may be called semispecies A collection of semispecies is a superspecies The biological species concept is sometimes difficult to apply in botany Geographic variation in status occurs when genetically different populations appear to be conspeci c in certain geographic regions but to be different species elsewhere PRACTICAL DIFFICULTIES E g Testing allopatric populations evaluating differences in fertility of offspring The greatest practical limitation of the BSC lies in determining whether or not geographically segregated allopatric populations belong to the same species Populations with intrinsic barriers to gene exchange can undergo independent evolutionary change even if they should become sympatric Range extension or colonization could well bring presently isolated populations in contact so the evolutionary future of the populations depends on whether or not they have evolved reproductive isolation Allopatric populations have been classi ed as species if their differences in phenotype or in DNA sequence are as great as those usually displayed by sympatric species in the same group WHEN SPECIES CONCEPTS CONFLICT Allopatric populations that can be distinguished by xed characters are species according to the PSC but if the diagnostic differences are slight advocates of the BSC may recognize the populations as geographic variants of a single species In some cases a local population of widespread species evolves reproductive isolation from other populations which remain reproductively compatible with one another Under the BSC two species would be recognized under the PSC the various distinguishable populations of the paraphyletic group might be named as distinct species BARRIERS TO GENE FLOW Gene ow between biological species is largely or entirely prevented by biological differences that have often been called isolating mechanisms Other terms are isolating barriers or barriers to gene ow Under BSC speciation consists of the evolution of biological barriers to gene ow Premating barriers A Ecological isolation potential mates do not meet 1 Temporal isolation reproductive period occurs at different time of the year 2 Habitat isolation live in the same locality but in different habitats eg primarily aquatic while the other mostly terrestrial B Potential mates meet but do not mate 3 Behavioral isolation differences in courtship or life style 4 Pollinator isolation different pollinators respond to different colors scent or form of owers Postmating prezygotic barriers Mating between species occurs but the fertilization of ova does not occur 5 Mechanical isolation copulation occurs but no transfer of male gametes takes place because of failure of mechanical t of reproductive structures 6 Copulatory isolation failure of fertilization because of behavior during copulation or because genitalia fail to stimulate properly 7 Gametic isolation gamete recognition is based on the presence of speci c molecules on the coats around the egg which adhere only to complementary molecules in the sperm Postzygotic barriers prevent the hybrid zygote to develop into a viable fertile adult Extrinsic hybrid tness depends on context 1 Ecological inviability hybrids do not have ecological niche in which they are competitively equal to parent species 2 Behavioral sterility hybrids are less successful than parent species in obtaining mates Intrinsic hybrid tness is low because of problems that are relatively independent of environmental context 1 Reduced hybrid viability hybrid zygote dies in the early stages of development or fails to reach sexual maturity 2 Reduced hybrid fertility hybrid does not produce functional gametes 3 Hybrid breakdown offspring of hybrids fail to produce functional gametes or do not reach sexual maturity Possible causes of hybrid sterility 1 Reduced fertility of hybrids can be caused by structural differences between the chromosomes that cause segregation of some aneuploid gametes during meiosis unbalanced number of chromosomes 2 Differences between genes from the two parents interact disharmoniously Haldane s Rule Hybrid sterility and inviability is often reduced to the heterogametic sex male in mammals and most insects female in birds and butter ies When in the o spring 0ftw0 d erent animal races one sex is absent rare or sterile that sex is the hetem gous heterogametic sex Haldane J B S 1922 Sex ratio and unisexual sterility in hybrid animals J Genet 12 101109 Hybrid breakdown occurs in the F2 generation and backcross offspring between species and among different geographic populations of the same species The common interpretation of this phenomenon is that the F1 generation produced various combinations of alleles that were disharmonious Alleles at different loci Within the same population have presumably been selected to from harmonious combinations They are coadapted and the population is said to have a coadapted gene pool HOW SPECIES ARE DIAGNOSED Morphological and other phenotypic characters serve as markers for reproductive isolation If a sample of sympatric organisms falls into two discrete clusters that differ in two or more character it is likely to represent two species DIFFERENCES AMONG SPECIES 0 Reproductive isolation 0 Adaptive differences to different ecological factors 0 Neutral differences that have arisen by genetic drift and mutation Character differences may have evolved during the process of speciation and partly after the reproductive barriers evolve The strength of both prezygotic and post zygotic isolation increases gradually with the time since the separation of the populations Equot The time required for full reproductive isolation to evolve is very variable but both average it is achieved when the genetic distance D is about 030 053 based on a molecular clock for Drosophila 0 Genetic distance is a measure of the dissimilarity of genetic material between different species or individuals of the same species 0 Genetic distance between humans and chimps is 16 and it suggests that the last common ancestor eXisted about 5 million years ago W Among recently diverge populations or species premating isolation is overall a stronger barrier to gene exchange that postzygotic isolation hybrid sterility or inviability 5 In the early stages of speciation hybrid sterility or inviability is almost always seen in males only female sterility or inviability appears only when taxa are older Postzygotic isolation evolves more rapidly in males than in females Because differences continue to accumulate long after two species achieve complete reproductive isolation Some of the genes and traits that now cause reproductive isolation may not have been the ones that were instrumental in forming the species in the rst place THE GENETIC BASIS OF REPRODUCTIVE BARRIERS GENES AFFECTING REPRODUCTIVE ISOLATION The most eXtensive information on genetic reproductive barriers has been obtained from studies conducted on certain Drosophila species Wu and his coworkers 1998 have suggested that as many as 40 gene differences on the X chromosome and 120 in the genome as a whole might cause hybrid male sterility among closely related species studied A similar study conducted by Presgraves 2003 suggested that about 200 genes can contribute to hybrid inviability Orr and Irving 2001 found that male hybrids between two populations of Drosophila pseudoobscura one from the US and the other form near Bogota Colombia appears to be based on differences in about ve gene regions of which four are required for sterility It seems that early hybrid sterility requires few gene differences These genes do not affect nonhybrid individuals Sterility and inviability must stem from interactions between genes in the two different species Epistatic interactions contribute to postzygotic isolation Evidence of complex epistatic interactions supports the idea that species consist of distinct coadapted gene pools or systems of genes that interact harmoniously within species but interact disharmoniously if mixed together Experiments have shown that the X chromosome has greater effect on causing sterility than autosomes Sterility must be caused by the epistatic interaction of the X chromosome of one species with the autosomal genes of another species It has been suggested that favorable Xlinked genes evolve faster than autosomal genes because they are subject to greater natural selection due to the males carrying only one Autosomal genes affecting male sterility have diverged faster than those affecting female sterility possibly because of sexual selection Premating isolation is frequently based on polygenic traits although in some cases a few traits are involved Male and female components of communication that result in sexual isolation are usual genetically independent FUNCTIONS OF GENES THAT CAUSE REPRODUCTIVE ISOLATION There is little understanding of the function of the genes that cause inviability and sterility Presgraves and collaborators 2003 have shown that selection drove the nonsynonymous substitutions in one of the 30 or so proteins making the nuclear pore complex that controls the passage of RNA and proteins between the nucleus and the cytoplasm This selection renders the hybrids between Drosophila melanogaster and D simulans that inherit the Nup96 gene inviable CHROMOSOME DIFFERENCES AND POSTZYGOTIC ISOLATION Chromosome differences among species include alterations of chromosome structure and differences in the number 0 chromosome sets polyploidy Reciprocal translocations can align the arms of a metacentric chromosome of a hybrid individual with two arms of two different chromosomes that came from the other parent causing aneuploid segregation Chromosome fusion causes differences in the number of chromosome pairs in the burrowing mole rat Spalax ehrenbegeri in two populations in Egypt Turkey Syria Israel and Lebanon The species consists of four parapatric groups found in Israel with the following number of chromosomes 2n 52 2n 54 2n 58 and 2n 60 This is called a superspecies Hybrids are found in a very narrow zone 28 to 3 km wide Aneuploidy in hybrids may reduce fertility Horses have 64 chromosomes and donkeys 62 Their hybrid the mule is viable and has 63 chromosomes Furthermore the 39 are not 39 39 The 39 of the parents fail to pair up during meiosis thus causing the sterility of the mule Evidence that chromosome heterozygotes have reduced fertility because of meiotic irregularities is convincing for some plants and mammals but not for other organisms CYTOPLASMIC INCOMPATIBILITY A possible cause of or contributor to speciation in insects is cytoplasmic incompatibility caused by the endosymbiotic bacteria of the genus Wolbachia Offspring of infected or uninfected males and an infected female develop normally Offspring of an infected male and an uninfected female are unviable because the paternal chromosomes are destroyed very early in development Wolbachia modi es the chromosomes of the infected male in such a way that they have to be xed by the Wolbachia in the eggs cytoplasm THE SIGNIFICANCE OF GENETIC STUDIES OF REPRODUCTIVE ISOLATION The Dobzhansky Muller incompatibility is the result of epistatic incompatible gene interactions between diverging populationsspecies and is recognized as the basis of postzygotic reproductive isolation Functional mismatch between genes gives rise to hybrid sterility or inviability Reproductive isolation requires that populations diverge by at least two allele substitutions The number of gene differences that suf ce for postzygotic isolation may be rather small but more differences are incorporated over time Reproductive isolation eventually becomes irreversible and the evolutionary lineages undergo independent genetic change thereafter Abstract New species arise as reproductive isolation evolves between diverging populations Here we review recent work in the genetics of postzygotic reproductive isolation the sterility and inviability of species hybrids Over the last few years research has taken two new directions First we have begun to learn a good deal about thepopulation geneticforces driving the evolution of postzygotic isolation It has for instance become increasingly clear that conflictdriven processes like sexual selection and meiotic drive may contribute to the evolution ofhybrid sterility Second we have begun to learn something about the identity and molecular characteristics ofthe actual genes causing hybridproblems Although molecular genetic data are limited early findings suggest that uspeciation genes correspond to loci having normal mctions within species and that these loci sometimes diverge as a consequence ofevolution in gene regulation BioEssays 22 10851094 2000 2000 John Wiley amp Sons Inc Speciation by postzygotic isolation forces genes and molecules H Allen Orr Daven C Presgraves MOLECULAR DIVERGENCE AMONG SPECIES Differences between species in allozymes and DNA sequences are presumably selectively neutral or nearly so Not speci c level of allozymes or DNA divergence can tell that two populations have become separate species Some reproductively isolated populations display little or no divergence in molecular markers presumably because reproductive isolation has evolved very recently This isolation may be the result of genetic changes in one or a few characters Two populations that become isolated from each other at rst share many of the same gene lineages inherited from their polymorphic common ancestor With respect to some loci individuals in each population are genealogically less closely related to one another than they are to some individuals in the other population Genetic drift or directional selection for a favorable mutation in each population eventually results in the loss of all the ancestral lineages of DNA sequence variants except one 7 Coalescent Theory Gene lineages are lost by genetic drift at a rate inversely proportional to the effective population s12e At one point one population will become monophyletic for a single gene lineage while the other population if it is larger retains both this and other gene lineages At this time the more genetically diverse population will be paraphyletic with respect to this gene and some gene copies sample from population 2 will be more closely related to some gene copies in population 1 Eventually both populations will become monophyletic for gene lineages and the relation ship among genes will re ect the relationship among populations This process of sorting of gene lineages into species is called lineage sorting Shared polymorphism can be maintained for a long time if natural selection maintains the variation in both species Incomplete lineage sorting between closely related species if possible HYBRIDIZATION Hybridization occurs when offspring are produced by interbreeding between genetically distinct populations In some cases hybridization may be the source of new adaptations or even of new species PREVIARY AND SECONDARY HYBRID ZONES A hybrid zone is a region where genetically distinct populations meet and mate resulting in at leas t some offspring of mixed ancestry A character or locus that changes across a hybrid zone eXhibits a cline that may be quite steep This is called a cline Primary hybrid zones originate in situ as natural selection alters allele frequencies in a series ofmore or less continuously distributed populations 0 The position of the zone is likely to correspond to a sharp change in one or more environmental factors 0 Natural selection on different loci or characters would result in clines with different geographic positions and that selectively neutral variation would not display a clinal pattern Secondary hybrid zones are formed when two formerly allopatric populations that have become genetically differentiated eXpand so that they meet and interbreed o Hybrids between populations that meet at secondary hybrid zones often have low intrinsic tness due to heterozygote disadvantage or breakdown of coadapted gene complexes 7 tension zones 0 The clines in characters that differentiate the populations need not match changes in the environment and are eXpected to be coincident o Clines in selectively neutral markers should be coincident with the others GENETIC DYNAMICS IN A HYBRID ZONE Dispersal selection and linkage all affect the distribution of alleles and phenotypic characters in hybrid zones The geographic position of a tension zone is not determined by ecological factors Dispersal of semispecies into the range of the other followed by random mating constitutes gene ow that tends to make the cline in allele frequency broader and shallower The alleles of one population cannot increase in frequency within the other population because of heterozygote disadvantage Heterozygotes hybrids act as a barrier to gene ow if they have low tness Introgression in genetics particularly plant genetics is the movement of a gene from one species into the gene pool of another by backcrossing an interspeci c hybrid with one of its parents 0 An example of introgression is that of a transgene from a transgenic plant to a wild relative as the result of a successful hybridization Unlinked alleles of one population will diffuse into the other population creating the introgression of those genes These loci will have shallow cline A lowered tness of hybrids at one locus reduces the ow of neutral or advantageous alleles between populations but the reduction is greater for alleles at closely linked loci that a t loosely liked or unlinked loci The cline of a gene in the hybrid zone will be steep if the hybrids have reduced tness or are linked to a locus that is selected against The steepness of a cline depends on the rate of dispersal the strength of selection and linkage to selected loci THE FATE OF HYBRID ZONES Hybrid zones may have several fates l A hybrid zone may persist inde nitely with selection maintaining steep clines at some loci even while the clines in neutral alleles dissipate due to introgression 2 Natural selection may favor alleles that enhance prezygotic isolation resulting ultimately in full reproductive isolation 3 Alleles that improve the tness of hybrids may increase in frequency In the eXtreme case the postzygotic barrier to gene exchange may break down and the semispecies may merge into on SpCCICS 4 Some hybrids may become reproductively isolated from the parent forms and become a third species Cenozoic Time Scale Associated Hominids Hominoids and Proto hominoids Time In Millions of Years Epoch Period Homo sapiens sapiens Present001 Holocene Quaternary Homo sapiens sapiens Homo sapiens neanderthalis Archaic Homo sapiens Homo erect OO116 mya Pleistocen e Quaternary Homo erectus Homo habilis Australopithecus boisei Australopithecus robustus Australopithecus aethiopicus Australogithecus africanus Australogithecus afarensis Australopithecus anamensis Australopithecus bahrelgazali u ramidus 1653 mya Pliocene Tertiary Ouranopithecus Kenyapithecus Sivapithecus Dryopithecus Proconsul 53 237 mya Miocene Tertiary Aegyptopithecus 237366 mya Oligocene Tertiary Adagidae Omomyidae 366578 mya Eocene Tertiary glesiadagiforms 578 60 mya Paleocen e Tertiary Chapter 12 THE GENETICAL THEORY OF NATURAL SELECTION Important points to remember about natural selection 1 Natural selection is not the same as evolution Evolution requires 0 the origin of variation by mutation or recombination 0 followed by the change in gene frequencies 0 natural selection and genetic drift do not account for the origin of variation 2 Natural selection is different from evolution by natural selection 3 Natural selection can have no evolutionary effect unless phenotypes differ in genotype 4 Natural selection is variation in average reproductive success a feature cannot evolve by natural selection unless it makes a positive contribution to the reproduction or survival of individuals that bear it 5 In some instances genotypes differ in survival and fecundity but the proportion of genotypes and alleles stay the same from one generation to another FITNESS The relationship between phenotype and tness and genotype and phenotype causes a relationship between tness and genotype which determines whether or not evolutionary change occurs MODES 0F SELECTION The relationship between phenotype and tness can be described as one of three modes of selection For a continuously varying trait selection is l Directional if one eXtreme phenotype is ttest 2 Stabilizing or normalizing if an intermediate phenotype is ttest 3 Diversifying or disruptive if two or more phenotypes are tter than the intermediate Which genotype has the greatest tness depends on the relationship between phenotype and genotype The relationship between phenotype and tness can depend on the environment because different environmental conditions can favor different phenotypes DEFINING FITNESS A genotype is likely to have different phenotypic eXpressions as a result of environmental in uences on development so the tness of genotype is the mean of the tness of its several phenotypes Reproductive success includes not only the number of offspring produced but also survival since survival is a prerequisite for reproduction The tness of a genotype is the average lifetime contribution of individuals of that genotype to the population after one or more generations measured at the same stage in the life history Genotype absolute tness 0 proportion surviving X average fecundity R per capita replacement rate or biotic potential The relative tness of a genotype is its value of R relative to that of some reference genotype By convention the reference genotype is often the one with the highest value The mean tness is the average tness of individuals in a population relative to the ttest genotype 0 Frequency of genotype A X R A frequency of genotype B X RB mean tness Coef cient of selection is the amount by which the tness of one genotype is reduced relative to the reference genotype 0 If the relative tness ofa genotype is 075 then the coefficient of selection is 025 The coefficient of selection measures the selective advantage of the tter genotype or the intensity of selection against the less t genotype The rate of genetic change under selection depends on the relative not the absolute tnesses of genotypes COMPONENTS OF FITNESS Survival and female fecundity are two components of tness Rate of population increase When generations overlap the usual way of measuring the absolute tness of a genotype is eXpressed as the per capita rate of population increase per unit time The rate at which the population is increasing or decreasing in a given year expressed as apercentage ofthe base population size It takes into i it at i a t e I I I 39 growth namely births deaths and migration httpnewhstorgzaindicindicphp 18 This rate depends on the proportion of individuals surviving to each age class and on the fecundity of each age class The age at which female have offspring is important and not only the number of females should be considered A genotype may have superior survival and another genotype may have superior fecundity but the overall tness determines the outcome of natural selection COMPONENTS OF SELECTION IN SEXUALLY REPRODUCING ORGANISMS ZYGOTIC SELECTION A Viability The probability of survival of the genotype through each of the ages at which reproduction can occur B Mating success The number of mates obtained by an individual affects the individual s progeny as is often the case for males but less often for females whose eggs may all be fertilized by a single male C Fecundity The average number of viable offspring per female GAMETIC SELECTION A Segregation advantage And allele has an advantage if it segregates into more than half the gametes of a heterozygote B Gamete Viability Dependence ofa gamete s viability on the allele it canies 0 Fertilization success An allele may affect the gamete s ability to fertilize an ovum MODELS OF SELECTION DIRECTIONAL SELECTION The replacement of relatively disadvantageous alleles by more advantageous ones is the fundamental basis of adaptive evolution Replacement occurs when the homozygote for an advantageous allele has tness equal to or greater than that of a heterozygote or any other genotype in the population 0 Advantageous alleles may be very common if under previous environmental conditions it was selectively neutral or was maintained by on of several forms of balancing selection 0 An advantageous allele may be initially very rare if it is newly arisen by mutation or if it was disadvantageous before an environmental change made it advantageous An advantageous allele than increases from a very low frequency is said to invade the population Unless an allele can increase in frequency when it is very rare it is unlikely to become xed in the population 0 The mystery of the brilliant pattern of the coral snake A character state with even a minuscule advantage will be xed by natural selection as long as no other evolutionary factors intervene Purifying selection reduces the frequency of a deleterious allele The number of generations required for an advantageous allele to replace one that is deleterious depends on the initial allele frequencies the selection coef cient and the degree of dominance 0 And advantageous allele will take very long time to increase in frequency if it is very rare because very few homozygous are formed 0 Once established and advantageous recessive reaches xation very rapidly as the deleterious dominant is eliminated An advantageous allele can increase in frequency more rapidly if it is dominant After a dominant advantageous allele attains high frequency the deleterious recessive allele is eliminated very slowly because a rare recessive allele occurs mostly in heterozygous form and is thus shielded from selection DELETERIOUS ALLELES IN NATURAL POPULATION S Directional selection xes advantageous alleles but deleterious alleles often persist because they are repeatedly reintroduced either by mutation or by gene ow from other populations in which they are favored by a different environment Selection and mutation The frequency of the deleterious allele moves toward a stable equilibrium that is a balance between the rate at which it is eliminated by selection and the rate at which it is introduced by mutation or gene ow The equilibrium frequency of a deleterious recessive allele is directly proportional to the mutation rate and inversely proportional to the strength of selection If the deleterious allele is dominant or partially dominant its equilibrium frequency will be lower than if it was recessive because of the selection against both the homozygous and heterozygous carriers Selection and gene flow Very often different environmental conditions favor different alleles in different populations of a species Frequency of deleterious allele will arrive at an equilibrium between the gene ow of the deleterious allele and the elimination by selection A smooth cline in allele frequencies may be established if there is gene ow among populations along a gradient over which the tness of different genotypes changes In the absence of gene ow there will be an abrupt change in gene frequencies among populations of the gradient The width of the cline over which gene changes is directly proportional to the distance that genes disperse and inversely proportional to the strength of selection against the allele If selection is strong relative to gene ow a steep cline in allele frequency will result and the populations are strongly differentiated POLYMORPHISM MAINTAINED BY BLANCING SELECTION A wealth of variation is maintained by several means 1 Recurrent mutation producing deleterious alleles which are subject to only weak selection 2 Gene ow of locally deleterious alleles from other populations in which they are favored by selection 3 Selective neutrality e g genetic drift 4 Maintenance of polymorphism by natural selection Balancing selection is selection that maintains polymorphism Polymorphism refers to a variety of alleles in a population that usually represent different phenotypes Balancing selection maintains two or more stable frequencies of phenotypic forms HETEROZYGOTE ADVANTAGE Heterozygote advantage occurs when the heterozygote has greater tness than either homozygous Overdominance and singlelocus heterosis are other names Heterozygous advantage depends on the balance between the tness values of the two homozygotes Heterozygous individuals carriers of the sickle cell anemia have the advantage of showing little anemia and having the resistance to malaria while both homozygous are either anemic or vulnerable to malaria ANTAGONISTIC AND VARYING SELECTION The opposing forces acting on the sicklecell polymorphism are and example of antagonistic selection Polymorphism is maintained because the heterozygote has greater tness than both homozygotes Temporal fluctuation occurs when a changing environment favors one genotype over another in different generations Spatial fluctuation occurs when different genotypes are best adapted to different microhabitats or resources Spatial variation in the environment is most likely to maintain polymorphism if different homozygotes with a single population are best adapted to different microhabitats o This is sometimes called multiple niche polymorphism Soft selection occurs when the number of survivors in a population is determined by competition for a limiting factor such space or food and the relatively superior genotype has a higher probability of surv1va Hard selection occurs when the survival of an individual depends on its absolute tness not on the density of its competitors FREQUENCY DEPENDEN T SELECTION The tness of a genotype depends on the genotype frequencies in the population The population then undergoes frequencydependent selection Inverse frequencydependent selection Inverse frequencydependent selection occurs when the rarer a phenotype is in the population the greater is fitness The selfincompatibility alleles of many plant species are an example The S locus in the cabbage family 1 The quotS locusquot consism in reality of three loci 2 There are multiple alleles of these genes up to 50 3 The proteins coded by these loci are located one in the membrane of the stigma cells another in the cell wall of the stigma cells and the third is secreted by mature pollen grains 4 If the proteins secreted by the pollen are the same as one or both of the proteins in the cell membrane and wall of the stigma the pollen grain does not form a pollen tube Similarity of alleles means that they are probable from the same plant 5 If the proteins secreted by the pollen tube are different from both of those in the cell wall and the cell membrane of the stigma the pollen tube forms The pollen comes from a different plant a 6 5 5 xs sz 9 c 6 552 x552 Q 535 x Wynulvlo Peamm Euuunnn lnn but th z Emlamln Cmnmnvas Also see the example ofthe cichlidPerlLs sodus microlepis on page 284 The more intense competition among individuals of the same genotype would then impose inverse frequencydependent selection THE EVOLUTION OF THE SEX RATIO Individuals that very in the sex ration of their progeny can differ in the number of their grandchildren and later descendanm and thus differ in fitness if this is measured over two or more generations The average per capita reproductive success of the minority sex is greater than the majority sex Selection favors genotypes whose individual sex ratios are biased toward that sex that is in the minority in the population as a whole MULTIPLE OUTCOMES OF EVOLUTIONARY CHANGE The initial genetic conditions usually determine the path of genetic change a population will follow In other words the evolution of a population often depends on its previous evolutionary history POSITIVE FREQUENCY DEPENDENT SELECTION In positive frequency dependent selection the tness ofa genotype is greater the more frequent it is in a population Whichever allele is initially more frequent will be xed See the example ofthe Heliconius butterflies on page 286 Heliconius butter ies have very distinct races in Central and South America When one race butter ies are released in the area of another race they are selected against by strong bird predation o The phenotypes common to the area are recognized by predators but the introduced phenotypes are not 0 The recapture of both phenotypes showed a strong selection against the introduced race against their genotypes HETEROZYGOTE DISADVANTAGE Underdominance is another name for heterozygote disadvantage If monomorphism is the stable solution heterozygotes will be selected against If a mutant gene appears or an allele is introduced in a monomorphic population it will be carried mostly by heterozygotes If these heterozygotes are less t than the homozygotes it will be eliminated ADAPTIVE LANDSCAPE Population size affects natural selection 0 Advantageous mutations may be lost by chance rather than become xed deleterious mutation may become xed for the same reason 0 Population bottleneck provide the opportunity to increase the frequency of deleterious alleles When the heterozygote is less t than either homozygote genetic drift is necessary to initiate a shift form one homozygous equilibrium to another Adaptive landscapes are graphs that visualize the relationship of genotypes and reproductive success The graphs plots tness y aXis against genotype or mutants X aXis Each point of the curve represents the average tness of individuals An evolving population typically climbs uphill in the tness landscape by a series of small genetic changes until a local optimum is reached Fig 1 There it remains unless a rare mutation opens a path to a new higher tness peak Note however that at high mutation rates this picture is somewhat simplistic A population may not be able to climb a very sharp peak if the mutation rate is too high or it may drift away from a peak it had already found consequently reducing the tness of the system The process of drifting away from a peak is often referred to as Muller39s ratchet Muller s Ratchet refers to the process by which asexual organisms accumulate deleterious mutations See for more information httpen wikinedia quot 39 quot 27s ratchet Figure 1 Sketch of a tness landscape The arrows indicate the preferred ow of a population on the landscape and the points A B and C are local optima The red ball indicates a population that moves from a very low tness value to the top ofa peak Illustration by CO Wilke 2001 Source of Adaptive Landscape httpenwikipediaorgwikiFitness landscape INTERACTION 0F SELECTION AND GENETIC DRIFT In nite l 39 39 allelef l 39 are 39 39 39 affected by both selection and chance The effective size of a population Ne and the strength of selection both affect changes in allele frequencies Random genetic drift is negligible if selection on a locus is strong relative to the population size If selection is so weak that the allele frequencies change mostly by genetic drift the alleles are nearly neutral 0 Selection determines the outcome of evolution if the population is large 0 In a suf ciently small population genetic drift is more powerful than selection Effects of population size on the ef cacy of selection have several important consequences 1 If the population is large the equilibrium allele frequency predicted from its genotypes tnesses would likely wander by genetic drift in the vicinity of the equilibrium frequency 2 A slightly advantageous mutation is less likely to be xed by selection if the population is small than if it is large because it is more likely to be lost simply by chance Conversely deleterious mutations can become xed by genetic drift especially if selection is weak and the population is small 3 Population bottlenecks provide temporary conditions under which genetic drift may counteract selection so that a deleterious allele may increase in frequency When the heterozygote is less t than either homozygote genetic drift is necessary to initiate the shift from one homozygous equilibrium state to the other MOLECULAR SIGNATURES OF NATURAL SELECTION If a new mutation occurs in a gene than increases tness of the carrier versus other members of the population natural selection will favor this individual and its descendants that inherit the gene With time the newly mutated variant of the gene will increase in frequency relative to other variants alleles of the gene As the frequency of this allele increases neutral or slightly deleterious mutations linked to this gene will also increase this is called genetic hitchhiking A strong selective sweep results in a region of the genome where the positively selected haplotype the mutated allele and its neighbours are essentially the only ones that exist in the population resulting in a large reduction of the total genetic variation in that chromosome region Whether a selective sweep has occurred or not can be investigated by measuring linkage disequilibrium ie whether a given haplotype is overrepresented in the population Under neutral evolution genetic recombination will result in the reshuf ing of the different alleles within a haplotype and no single haplotype will dominate the population However during a selective sweep selection for a positively selected gene variant will also result in selection of neighbouring alleles and less opportunity for recombination Therefore the presence of strong linkage disequilibrium might indicate that there has been a 39recent39 selective sweep and can be used to identify sites recently under selection A study of genetic variation among 269 humans found evidence for selective sweeps on chromosomes 1 2 4 8 12 and 22 m 1 quotA haplotype map of the human genome 2005 by the International HapMap Consortium inNature Volume 437 pages 1299 1320 httpenwikipediaorgwiki Selective sweep The effects of balancing selection eg heterozygote advantage or frequencydependent selection are opposite to those of positive directional selection 0 Balancing selection favors polymorphism multiple alleles two or more stable frequencies 0 Directional selection favors a single allele When deleterious mutations are eliminated from a population selectively neutral mutations linked to it are eliminated as well This called background selection Background mutations reduce neutral polymorphism THE STRENGTH OF NATURAL SELECTION Selection through both survival and reproduction can be very strong Studies have shown that natural selection can be very strong in some circumstances and therefore a strong force of evolution