ORGANISMAL BIOLOGY BIO 1331
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Microevolution within population change in allele frequency between generations Gene ow Genetic drift Natural selection Mutations Macroevolution Evolutionary change leading to the origin ofnew taxonomic groups How does the process of natural selection acting within a population microevolution to macroevolution Microevolution can take little time to notice but Macroevolution can take up to millions of years Key concepts 0 Speciation occurs when population of the same species become genetically isolated by lack of gene ow 0 They diverge from each other due to natural selection genetic drift or mutation o Populations can be recognized as distinct species if they are reproductively isolated from each other 0 If they have distinct morphological characteristics or if they form independent branches on a phylogenetic tree Isolation Effect on Speciation o Ifwe were to separate the class and put them in separate environments 0 One part of the class would adapt to live in their conditions 0 The other would adapt to live in their own condition as well 0 This prologue isolation can breeding to adapt to the new trait we will produce a new human species 0 Genetic isolation time and environmental pressure will cause speciation Macroevolution figure 261 0 An ancestral population ofbirds 0 Population splits into different island and characteristics diverge o Produces different species over time on each island 0 Selection pressure produces new species Biological species concept 0 Reproductively isolated group of interbreeding individuals Most common used definition Species gen pool Must form viable offspring Problems with definition 0 Only applicable to the present 0 Doesn t apply to asexual organisms o Hybrids produced are no fertile 0 Hybrid slide for the Liger o Liger is crossed with a male lion and female tiger 0 With seX ip opped is a tiglon 0 Parents are the same genus but different species 0 Separation by nature buy can reproduce by man Biological Species concept 0 Biologists categorize the mechanisms that stop gene ow between populations as being either prezygotic or postzygotic o Prezygotic isolation I Occurs when individuals of different species are prevented from mating o Postzygotic isolation I Occurs when individuals from different population are separated after birth from separation I Populations are isolated because the breed at different times Mechanisms of Reproduction Isolation figure 261 o Prezygotic Isolation 0 Temporal I Breed different times 0 Habitat I Breed in different habitats o Behavioral I Different courtship 0 Genetic barrier I Mating fails egg and sperm no compatible 0 Mechanical I Fails because male and female reproduce systems are incompatible o Postzygotic Isolation 0 Hybrid viability I Hybrid offspring do not develop normally as embryos 0 Hybrid sterilization I Hybrid offspring are born sterile The biological species concepts 0 The biological species concept considers population to be evolutionary independent 0 If they are reproductively isolated from each other 0 They do no interbreed 0 They fail to produce viable fertile offspring 0 Therefore no gene ow occurs between population 0 The bio spec con has disadvantages 0 Cannot be applies to fossil records Phenetic morphospecies species concept 0 Cluster in morph space 0 Extension ofhow species is defines and practiced 0 Works for sexual asexual and extinct species 0 Problems 0 Morphological differences do not always mean biological difference I 1 Sexual dimorphism I Black widow spider cannot make correlation between male and female with the size we know the male is just used for sperm I 2 Life cycle stages I Barnacles a lot ofinvertebrates that go though very different life stages I 3 Polymorphism I Can have tons ofmorphological variation 0 Plants and other animals vary in characteristics greatly in diversity I 4 Sibling species I Species that look extremely similar but cannot mate with each other I Types of crabs frogs etc Sometimes phenotypes cannot be ass connected and the gene typical relationship Phylogenetic Species Concept 0 In phylogeny we re construct the phylogenic or evolutionary tree Monophyletic all come the a single branch Trees can be built on characters morphology and genetic information Example 0 Salamanders in a particular environment are all similar I They might all be in an individual species or they can just be different I Man cannot legal drive these animals to extinction by our own development This argument is what they use to identify is something is a species and it if small enough it may be an endangered species Ch agter 26 Sgeciation If gene ow ends allele frequencies in isolated populations are free to diverge meaning that the populations begin to evolve independently of each other Ie when a new mutation creates an allele that changes the phenotype of individuals in one population there is no longer any way for that allele to appear in the other population If mutation selection and genetic drift cause isolated populations to diverge sufficiently distinct types or species form that is the process of speciation takes place Speciation is a splitting event tat creates two or more distinct species from a single ancestral group When speciation is complete a new branch has been added to the tree of life Speciation results from genetic isolation and genetic divergence Genetic isolation results from lack of gene ow and divergence occurs because selection genetic drift and mutation precede independently in the isolated population 261 How are Species De ned and Identified Genetic distinctions occur because of mutation selection and drift act on each species independently ofwhat is happening in other populations Species an evolutionarily independent population or group ofpopulations The Biological Species Concept According to the biological species concept the critical criterion for identifying species is reproductive isolation This is a logical yardstick because no gene ow occurs between populations that are reproductively isolated from each other Prezygotic isolation prevents individuals of different species from mating Postzygotic isolation the offspring of mattings between members of different species do not survive or reproduce Disadvantages ofBiological Species Concept Cannot be evaluated in fossils or asexual species The Morphospecies Concept Researches identify evolutionarily independent lineages by differences in size shape or other morphological features Distinguishingfeatures are most likely to arise ifpopulations are independent and isolated from gene ow Disadvantages It cannot identify cryptic species which differ in traits other than morphology such as meadowlarks in table 261 Is subjective The Phylogenetic Species concept Monophyletic group or clade or lineage consists of an ancestral population all ofits descendants and only those descendants Synapomorphy is a trait found in certain groups of organisms that exist in no others Homologous trait one that was inherited from another ancestor that is unique to certain populations or lineages Ie Fur and lactation are synapomorphies that identify mammals as a monophyletic group Figure 262 Under the phylogenetic species concept species are defined as the smallest monophyletic groups on the tree oflife Advantages of Phylogenetic concept Can be applied to any population fossils asexual or sexual Logical because different species have different synapomorphies on if they are isolated form gene ow and have evolved independently Disadvantages Available only for a tine subset ofpopulations Species De nitions in Action The Case ofthe Seaside Sparrow Subspecies populations that live in discrete geographic areas and have distinguishing features but are not considered distinct enough to be called separate morphospecies 262 Isolation and Divergence in Allopatry Genetic isolation happens when populations become physically separated Physical isolation in turn occurs in one of two ways Dispersal population can disperse to a new habitat colonize it and found a new population Vicariance physical splitting of habitat mountains rivers Allopatric speciation speciation that begins with physical isolation ie either dispersal or vicariance Populations that live in different areas are said to be in allopatry Dispersal and Colonization Isolate Populations Characteristics of a colonizing population are likely to be different from the characteristics of the source population due to chance that is genetic drift Vicariance Isolates Populations Ifa new physical barrier mountain river etc splits the geographic range of a species vicariance has taken place Panama canal split Pacific Ocean from Caribbean Sea splitting the snapping shrimp population 263 Isolation and Divergence in Sympatry Sympatry when populations or species live in the same geographic area or at least close enough to one another to make interbreeding possible Can Natural Selection Cause Speciation Even When Gene Flow is Possible Sympatric speciation speciation that occurs even though gene ow is possible Even though sympatric populations are not physically isolated they may be isolated by preferences for different habitats Ie Maggot ies Chapter 24 Evolution by Natural Selection 0 241 the Evolution of Evolutionary Thought 0 Scientific revolution overturns an existing idea about how nature works and replaces it with another radically different idea 0 Darwin and Wallace overturned that species were specially not naturally created 0 Plato and Typological Thinking o Plato claimed that every organism was created by God and was an example ofa perfect essence and was unchanging o Typological thinking is based on the idea that species are unchanging types and variations within species are unimportant or misleading o Aristotle and the Great Chain ofBeing 0 Proposed that species were organized into a sequence based on increased size and complexity with humans at the top 0 Also claimed that species did not change over time 0 Central claims 0 Species are fixed types 0 Some species are higher more complex or better than others 0 Figure 241 o Landmarck and the Idea of Evolution as Change through Time 0 Theory states that species are not static but change through time Species change through time via the inheritance of acquired characteristics 0 Le Giraffes develop long necks as they stretch for leaves 0 Darwin and Wallace and Evolution by Natural Selection 0 The process responsible for change through time evolution occurs because traits vary among the individuals in a population and because individuals with certain traits leave more offspring than others do 0 Population consists ofindividuals of the same species that are living in the same area at the same time 0 Population thinking variation among individuals in a population was the key to understanding the nature of species 0 242 The Pattern of Evolution Have Species Changed through Time 0 Decent with modification species that lived in the past are the ancestors of the species existing today and that species and their descendant species change through time Nature of species 0 Species change through time 0 Species are related by common ancestry 0 Evidence for Change through Time Fossil any trace of an organism that lived in the past Fossil record consists of all the fossils that have been found and described in the scientific literature I The Vastness of Geologic Time Sedimentary rocks form from sand or mud or other materials deposited at locations such as beached or river mouths 0 Known to form in layers with younger layers deposited on top of older layers The geologic record indicated that earth was much older than what was proclaimed by proponents of the theory of special creation Earth is about 46 billion years old earliest sign oflife appear in rocks that formed 3438 billion years ago I Extinction changes the Species Present Over Time 99 of all species that have ever lived are now extinct I Transitional Features Link Older and Younger Species Transitional feature is a trait in a fossil species that is intermediate between those of older and younger species 0 Le Found that fossils that document a gradual change over timefrom aquatic animals that had find to terrestrial animals that had limbs Data like these are consistent with predictions from the theory of evolution if the traits observed in more recent species evolved form traits in more ancient species then intermediate forms are expected to occur in the appropriate time sequence I Vestigial Traits are Evidence of Change Through Time Vestigial trait is a reduced or incompletely developed structure that has no function or reduced function but is clearly similar to functioning organs or structures in closely relates species 0 Le Hip bone in whalessnakes Evidence that the characteristics of species have changed over time I Current examples of Change Through Time Bacteria has evolved resistance to drugs Insects have evolved resistance to pesticides Weedy plants have evolved resistance to herbicides Species are dynamic NOT static unchanging and fixed types as claimed by Plato Aristotle and the theory of special creation 0 Evidence ofDescent from a Common Ancestor I Similar Species are Found in the Same Geographic Area 0 Darwin began to realize that species are related by common ancestry just as individuals within a family are 0 Le Galapagos Islands mockingbirds is an example of Decent with Modification 0 Mockingbirds are a part ofa phylogeny family tree ofpopulations or species I Homology is Evidence of Descent From a Common Ancestor o Homology is a similarity that eXists in species because they both inherited the trait from a common ancestor o ie human hair and dog fur o Homolog can be recognized in three levels 0 Genetic Homology I Occurs in DNA sequence Eyeless gene in fruit ies and the Aniridia gene in humans are so similar that their protein products are 90 identical in amino acid 0 Developmental Homology I Recognized in embryos Early chick human and cat embryos have tails and gill pouches 0 Structural homology I Similarity in adult morphology or form 0 Le similarities in Human Horse Bird Bat Seal and turtle bones The 3 levels ofhomology interact Genetic homologies cause the developmental homologies observed in embryos which then lead to the structural homologies recognized in adults 0 The theory of evolution by natural selection predicts that homologies will occur If species were created independently of one another as the theory of special creation claims these types of similarities would not occur 0 Evolution s quotInternal Consistency the Importance of IndependentDatasets o The most powerful evidence for any scientific theory including evolution by natural selection is what scientist call quotinternal consistency This is the observation that data from independent sources agree in supporting predictions made by a theory Table 241pg422 0 Prediction 1 Species are not static but change through time I Most species have gone eXtinct I Fossil extinct species frequently resemble living species found in the same area Transitional features document change in traits through time Vestigial traitsare common The characteristics ofpopulations can be observed changing daily 0 Prediction 2 species are related not independent I Closely related species often live in the same geographic area I llomologous traits are common and are recognized at 3 levels 0 Genetic 0 Developmental 0 Structural I The formation ofnew species from preexisting species can be observed today 0 243 The Process of Evolution How Does Natural Selection Work 0 Darwin39s 4 Postulates process of evolution by natural selection into 4steps in logical sequence 1 The individual organisms that make up a population varies in thetraits they possess such as their size and shape 2 Some of the trait differences are heritable meaning that they are passed on to offspring genetically Ie Tall parents may tend to have tall offspring 3 In each generation many more offspring are produced than can possibly survive Thus only some individuals in the population survive long enough to produce offspring and among the individuals that produce offspring some will produce more than others 4 The subset of individuals that survive best and produce the most offspring is not a random sample of the population Instead individuals with certain heritable traits are more likely to survive and reproduce Natural selectionoccurs when individuals with certain characteristics produce more offspring than do individuals without those characteristics The individuals are selected naturally meaning by the environment 0 1 Individuals vary 0 2 Some variations are heritable o 3 More offspring are produced than can survive o 4 Individuals with traits that confer an advantage are more likely to survive and reproduce I Summary Evolution by natural selection occurs whenever heritable variation leads to differential success in survival and reproduction Modern form of 4 postulates Evolution by Natural Selection occurs when 0 1 Heritable variation leads to o 2 Differential reproductive success 0 The Biological Definitions of Fitness and Adaptations Biological Fitness the ability of an individual to produce survian offspring relative to that ability in other individuals in that population 0 Fitness is a measureable quantity When researchers study a population they estimate fitness of individuals by counting of offspring each individual produces Adaptation is a heritable trait that increases the fitness of an individual in a particular environment relative to individuals lacking their trait Adaptations increase fitness the abfility to produce offspring 244 Evolution in Action Recent Research on Natural Selection 0 Case Study 1 How Did Mycobacterium tuberculosis Become Resistantto Antibiotics A Patient History Patient diagnoses with TB took medication and 10 months later showed no signs of TB 2 months later went to hospital for same symptoms and died 10 days later TB was growing in his lungs again but this time the bacterial cells were completely resistant to medication Drug resistant bacteria killed the patient A Mutation in Bacterial Gene Confers Resistance Testing Darwin s Postulates 1 Did variation exist in the population Yes Due to mutation both resistant and nonresistant strains ofTB were present prior to administration of the drug 2 Was this Variation heritable Yes The variation in the phenotypes of the 2 strains from drug susceptibility to drug resistance was due to variation in their genotypes Because the mutant gene is passed on to daughter cells when Mycobacterium replicates the allele and the phenotype it produces drug resistance are passed to offspring 3 Was there variation in reproductive success Yes Only a tiny fraction of M TB cells in the patient survived the first round of antibiotics long enough to reproduce 4 Did selection occur Yes When rifampin was present certain cells those with the drug resistant allele had higher reproductive success than cells with the normal allele 0 Natural selection acts on individuals because individuals experience differently reproductive success But only populations evolve Allele frequencies change in populations not in individuals 0 Case Study 2 Why are Beak Size Shape and Body Size Changing inGalapagos Finches I Selection During Drought Conditions During a drought few plants were able to produce seeds and 84 ofmedium ground finch population disappeared 2 observations support hypothesis that they died of starvation 0 Researchers found a total of 38 dead birds and all were emaciated to be extremely thin None of the missing individuals were spotted on nearby islands and none reappeared once the drought had ended and food supplies returned to normal Survivors tended to have deeper beaks than the non survivors o The type of seeds available to the finches had changed Deeper beaks were able to crack the fruits in order to survive Showed that natural selection led to an increase in average beak depth When breeding resumed later offspring had deeper beaks Confirmed that evolution occurred 0 I Which Genes are Under Selection o 245 Common Misconceptions About Natural Selection and Adaptation 0 Selection Acts on Individuals but Evolutionary Change Occurs inPopulations Most important point to clarify about natural selection is that during the process individuals do not change only the population does 0 0 During the drought the beaks ofindividual finches did not deepen Rather the average beak depth in the population increased over time becaue deepbeaked individuals produce more offspring than shallowbeaked individuals did Natural selection acted on individuals but evolutionary change occurred in the characteristics of the population Individual bacterial cells did not change when rifampin was introduced to their environment Each M tuberculosis cell had the same polymerase alleles all its life But because the mutant allele increased in frequency over time the characteristics ofthe bacterial population changed I A Contrast With quotLamarckianquot Inheritance Sharp contrast between evolution by natural selection and evolution by inheritance of acquired characteristics 0 O In Lamarck key claim is that the important evolutionary changes occur in individuals Darwin realized that individuals do not change when they are selected Instead they simply produce more offspring that other individuals do When this happens alleles found in the selected individuals become more frequent in the population I Acclimation is NOT Adaption Acclimation describes changes in an individual s phenotype that occur in response to changes in environmental conditions 0 Key is realizing that phenotypic changes due to acclimation are not passed on to offspring because no alleles have changed in composition As a result acclimation does NOT cause evolution 0 Evolution is Not Goal Directed The mutation that conferred resistance did not occur because of the presence of the drug It just happened I Evolution is not Progressive organism do not get quotbetterquot over time 0 Complex traits are routinely lost or simplified over time as a result of evolution by natural selection 0 Le vestigial structures I There is No Such Thing as a Higher or Lower Organism 0 Figure 2419 pg43 0 o A human is no higher than a tapeworm parasite each is well adapted to its environment 0 All populations have evolved by natural selection based on their ability to gather resources and produce offspring All organisms are adapted to their environment and are related by common ancestor 0 Natural selection is not goal directed or progressive It simply favors individuals that happen to be better adapted to the environment existing at the time o Organisms Do Not Act for the Good of the Species o No instance of purely selfsacrificing behavior where the individual received no fitness benefit in return has ever been recorded in nature 0 Limitations of Natural Selection I NonAdaptive Traits o Vestigial traits such as the human tail goose bumps and appendix do not increase the fitness of individuals with those traits The structures exists simply because they were present in the ancestral population I Genetic Constraints 0 Selection was not able to optimize all aspects ofa trait In the case of the finches wider beaks were not the best possible beak shape for individuals living in an arid habitat Wider beaks evolved anyway due to a type of constraint called a genetic correlation o Occur because of pleiotropy in which a single allele affects multiple traits I Fitness TradeOffs o In nature selection occurs in the context of fitness tradeoffs A fitness tradeoffis a compromise between traits in terms ofhow those traits perform in the environment 0 Le during the drought medium ground finches with large bodies had an advantage because they won fights over the few remaining sources of seeds But large bodies require large amounts of food to maintain mass I Historical Constraints Cancer Adaptations are constrained by history The reason is o All traits have evolved from previously existing traits Not all traits are adaptive and even adaptive traits are constrained by genetic and historical factors In addition natural selection is not the only process that causes evolutionary change Primary cause of cancer 0 Environment I Diet I Things we are exposed to I Pollution 0 510 of cancer is hereditary o Mutations can be hereditary I Getting Xrayed in testis can cause testicular cancer Hereditary cancer most of the time show up early in life Pathway to cancer multiple mutations over a lifetime caused be environmental factors Epidemiology of cancer is the study of the factors affecting cancer as a way to infer possible trends and causes 0 Uses epidemiology methods to find the cause of cancer and to identify and develop improved treatments Cancer cluster 0 A greater of cancer cases that would be expected in a defined geographic area or group ofpeople over a certain period of time See notes for diagram of BRAC12 Hereditary breast ovarian cancer syndrome 0 Woman born with a harmful mutation of either BRAC12 genes are 5 times more likely to develop breast cancer during their lifetime BRACl is expressed in the cells ofbreast and other tissue where it helps repair damaged DNA or destroy cells if DNA cannot be repaired If BRACl itself is damaged damaged DNA is not repaired properly and this increases the risk for cancer 0 Damage it repairs is often due to radiation and crossing over BRACZ is located on chromosome 13 2272012 24800 AM Chapter 25 Evolutionary Process I 4 mechanisms that cause evolution a Natural selection increases the frequency of certain alleles the ones that contribute to reproductive success in a particular environment 0 Natural selection is the only one that acting alone can result in adaption a Genetic drift causes allele frequencies to change randomly I Gene ow occurs when individuals leave one population join another and breed Allele frequencies may change when gene ow occurs because arriving individuals introduce alleles to their new population and departing individuals remove alleles from their old population I Mutation modifies allele frequencies by continually introducing new alleles The alleles created by mutation my be beneficial or detrimental or have no affect on fitness I Natural selection is not the only agent responsible for evolution and each of the 4 evolutionary processes has different consequences 251 39 Change in Allele Frennem ie 39 The HardVWeinberg Principle 0 The Gene Pool Concept I Gene pool all gametes produced in each generation go into a single group 0 Deriving the HardyWeinberg Principle I See handout o The HardyWeinberg Model Makes Important Assumptions I For a population to conform to HW principle none of the 4 mechanisms of evolution can be acting on a population Model assumes mating is random 252 Types of Natural Selection I Natural selection occurs when individuals with certain phenotypes produce more offspring than individuals with other phenotypes do If certain alleles are associated with the favored phenotypes they increase in frequency while other alleles decrease in frequency Result is evolution I Genetic variation the and relative frequency of alleles that are present in particular population 0 Directional Selection I Directional selection natural selection has increased the frequency of drug resistant strains of the TB bacterium and caused changes in beak shape and body size in finches Called directional selection because the average phenotype ofthe populations changed in one direction 0 Directional Selection Tends to Reduce Genetic Variation I If directional selection continues over time the favored alleles will eventually approach a frequency of 10 while disadvantageous alleles will approach a frequency of 0 Alleles that reach a frequency of 10 are fixed those that reach 0 are extinct I Purifying selection when disadvantageous alleles decline in frequency 0 Directional Selection on Body Size in Cliff Swallows pg 441 I DS favored large body size occurred Larger birds survived because they had larger fat stores and did not get as cold As a result the larger birds were less likely to die of exposure to cold and more likely to avoid starvation until summer I If differences in body size among individuals were due in part to differences in their genotypes meaning heritable variation in body size existed then the population evolved IfDS occurred than the small body size would soon decline 0 Stabilizing Selection pg 442 o Favors phenotypes near the middle of the range of phenotypic variation Reduces overall genetic variation in a population I There is no change in the average value of the trait over time I Genetic variation in the population is reduced 0 Disruptive Selection pg443 I Has opposite effect of stabilize selection Instead of favoring phenotypes near the average value and eliminating extreme phenotypes it eliminates phenotypes near the average value and favors extreme phenotypes I When disruptive selection occurs the overall amount of genetic variation in the population is maintained 0 Disruptive Selection on Beak Size in BlackBellied Seed Crackers I Individuals with either very short or very long beaks survive best and that birds with intermediate phenotypes are at disadvantage I Agent causing natural selection in this case is food I Only 2 seed sizes found I Birds with small beaks crack and eat small seeds efficiently Vice versa for large beaks I Birds with intermediate have trouble with both 0 Disruptive Selection Can Lead to Formation of New Species I DS is important because it sometimes plays a part in speciation or the formation ofnew species I If small beaks began mating with other small beaked individuals their offspring would tend to be small beaked and would feed on small seeds Vice versa for large beaks o In this way selection would result in two distinct populations Under certain conditions the populations may eventually for 2 new species 0 Balancing Selection I Heterozygote advantage table 252 when selection operates in this way heterozygous individuals have higher fitness than other homozygous individuals do I Consequence of this pattern is that genetic vitiation is maintained in populations Heterozygote advantage is one mechanism responsible for more general phenomenon known as balancing selection When balancing selection occurs no single allele has a distinct advantage and increase in frequency Instead there is a balance among several alleles in term of their fitness and frequency Balancing selection also occurs when I The environment varies over time or in different geographic areas occupied by a population meaning that certain alleles are favoredby natural selection at different times or in different places As a result overall genetic variation in the population is maintained or increased 253 Genetic Drift U Certain alleles are favored when there are rare but not when they are common a pattern know as frequency development selection Ie Rare alleles responsible for coloration in guppies are favored because predators learn to recognize common color patterns Alleles for common colors get eliminated alleles for rare colors increase infrequency As a result overall genetic variation in population is maintained or increased No matter how natural selection occurs though its most fundamental attribute is the same it increases fitness and leads to adaptation Natural selection is not random It is directed by the environment and results in adaptation Genetic drift in contrast is undirected and random Genetic drift any change in allele frequencies in a population that is due to chance The process is aptly names because it causes allele frequencies to drift up and down randomly overtime When drift occurs allele frequencies change due to blind luck what is formally known as sampling error Drift occurs in every population in every generation 0 Simulation Studies of Genetic Drift Wife ATAI39I Husband ATAI39I D In this population the two alleles are each at a frequency of 05 Couple produced 5 children over lifetime U Half of the eggs produced by the wife carry allele AT and half carry allele A U Same for sperm D To simulate which sperm and which egg happen to combine to produce each of the 5 offspring you can ip a coin for each sperm and each egg with tails standing for allele AT and heads for NI figure on pg 444 0 Computer Simulations Figure 256 Genetic drift can be important even large populations U Alleles that containing silent mutations usually in the 3ml position ofa codon do not change the gene product As a result most of the alleles have little or no effect on the phenotype Yet all the alleles routinely drift to high frequency or even fixation overtime 0 Key Points About Genetic Drift Genetic drift is random with respect to fitness The allele frequency changes it produces are not adaptive Genetic drift is most pronounced in small populations In the computer simulation figure 256 allele frequencies changed much less in large population than the small population And if the couple on the deserted island had produced 50 children in stead of 5 it is almost certain that allele frequencies in the next generation would have been much closer to 05 Over time genetic drift can lead to the random loss or fixation of alleles In the computer simulation with a population of 4 it took at most 20 generations for one allele to be fixed or lost When random loss or fixation occurs genetic variation in the population declines 0 Experimental Studies of Genetic Drift Genetic marker specific allele that causes a distinctive phenotype D In this case marker was morphology or bristles 0 Fruit ies have bristles that can be straight or bent U Normal straight bristles U Forked bent bristles B Set up 96 cages o Placed 4 males and 4 females in each cage D In the F1 generation they randomly chose 4 males and 4 females at random from each of the 96 offspring populations and allowed them to produce the next generation U Repeated process until all 96 populations had undergone total of16 generations No migration occurred only genetic drift Results a After 16 generations 96 populations fell into 3 groups I Forked bristles were found on all of the individuals in 29 of the experimental populations Due to drift the forked allele had been fixed in these 29 populations and the normal allele was extinct figure 257b In 41 other populations the opposite was true I I o All individuals had normal bristles In these populations the forked allele had been lost due to change 0 Both alleles were still present in 26 of the populations I In 7096 genetic drift had reduced allelic diversity at this gene to 0 I As predicted genetic decreased genetic variation within populations and increased differences between populations 0 What Causes Genetic Drift in Natural Populations I Sampling process that occurs during fertilization occurs in every population in every generation in every species that reproduces sexually Similarly accidents that remove individuals at random occur in every population in every generation I Because drift is caused by sampling error it can occur by any process or event that involves sampling not just the sampling of gametes that occurs during fertilization or the loss of unlucky individuals due to accidents 0 Founder Effects on the Green Iguanas of Anguilla I When a group ofindividuals immigrates to a new geographic area and establishes a new population founder event occurs Founder effect a change in allele frequencies that occurs when a new population is established Each time a founder event occurs a founder effect is likely to accompany it changing allele frequencies through genetic drift 0 Genetic Bottleneck on Pingelap Atoll I Ifa large population experiences a sudden reduction is size a population bottleneck is said to occur I Disease outbreaks natural disasters or other events can cause population bottleneck I Genetic bottleneckis a sudden reduction in the of alleles in a population Drift occurs during genetic bottlenecks and causes change in allele frequencies 254 Gene Flow I Gene ow movement of alleles from one population to another Occurs when individuals leave one population join another and breed I As an evolutionary mechanism gene ow usually has 1 outcome I It equalizes allele frequencies between the source population and the recipient population When alleles move from one population to another the populations tend to become more alike 0 Gene Flow in Natural Populations 0 East and West great tit bird I Eastern laid smaller clutches groups of eggs that are incubated together than individuals hatched in west I Hatched females in eastern survive better than females in west I Birds seem to be better adapted I Why aren t west better adapted Answer is gene ow I 43 of west breeding for the first time are unbanded just arrived form the mainland I In east only 13 that breed are immigrants I Gene ow is 3 times as high in west as in east 0 How Does Gene Flow Affect Fitness I For great tits immigrants bring in alleles that have relatively low fitness in the island environment Natural selection and gene ow are working in opposition selection favors small clutch sizes and other traits that affect survival gene ow constantly introduces alleles that have high fitness in mainland habitats but low fitness on island I Gene ow is random with respect to fitness the arrival or departure of alleles can increase or decrease average fitness depending on the situation 255 Mutation D But in every case a movement of alleles between populations tends to reduce genetic differences between them How does it affect genetic variation in a population Recall that U Gene ow increases genetic diversity in a recipient population if new alleles arrive with immigrating individuals But gene ow may decrease genetic variation in the source population if alleles leave with emigrating individuals U Genetic drift tends to decrease genetic diversity over time as alleles are randomly lost or fixed U Most forms of selection favor certain alleles and lead to a decrease in overall genetic variation What restores genetic diversity Where do new alleles come from U Mutation o Mutations occur when DNA polymerase makes an error as it copies a DNA molecule resulting in a change in the sequence of deoxyribonucleotides Mutation an evolutionary mechanism that increase genetic diversity among populations Changes in the makeup of chromosomes or in specific DNA sequences do not occur in ways that tend to increase fitness or decrease fitness Mutation just happens Deleterious alleles alleles that lower fitness Beneficial alleles an allele that allows individuals to produce more offspring Should increase in frequency in the population due to natural selection 0 Mutation as an Evolutionary Mechanism Mutation is slow compared with selection genetic drift and gene ow Mutation introduces new alleles into every individual in every population in every generation 0 Experimental Studies of Mutation 0 Take Home Message 256 Nonrandom Mating o Inbreeding Mutation is the ultimate source of genetic variation Crossing over and independent assortment shuf e existing alleles into new combinations but only mutation creates new alleles If mutation did not occur evolution would eventually stop Recall that natural selection and genetic drift tend to eliminate alleles Without mutation eventually there would d be no variation for selection and drift to act on Mutation alone is usually inconsequential in changing allele frequencies at a particular gene When considered across the genome and when combined with natural selection however it becomes an important evolutionary mechanism Table 253 Each of the 4 evolutionary forces has different consequences for allele frequencies Inbreeding mating between relatives U Relatives individuals that share a recent common ancestor o How Does Inbreeding Affect Allele Frequencies and genotype Frequencies Inbreeding increase homozygosity Inbreeding takes alleles from heterozygotes and puts them into homozygotes Inbreeding does NOT cause evolution because allele frequencies do not change in the population as a whole Inbreeding and other forms ofnonrandom mating change genotype frequencies not allele frequencies 0 Why Does Inbreeding Depression Occur Inbreeding depression a decline in average fitness that takes place whenhomozygosity increases and heterzygosity decreases in a population Inbreeding depression results from two causes U Many recessive alleles rep loss of function mutations 0 Most loss of function alleles in heterozygous individuals Inbreeding increases the frequency of homozygous recessive individuals Sexual Selection 0 Many genes especially those involved in fighting disease are under intense selection for heterozygote advantage If an individual is homozygous at these genes then fitness declines Even though inbreeding does not cause evolution directly because it does not change allele frequencies it can speed the rate of evolutionary change More specifically it increases the rate at which purifying selection eliminates recessive deleterious alleles from a population I Selection based on success in courtship Occurs when individuals within a population differ intheir ability to attract mates If favors individuals with heritable traits that enhance their ability to obtain mates 0 Theory The Fundamental Asymmetry of Sex I The pattern component of their theory is that sexual selection usually acts on males much more strongly than on females As a result traits that attract members of the opposite sex are much more highly elaborated in males I The mechanism that Bateman and Trivers proposed to explain this pattern can be summarized with a quip quotEggs are expensive but sperm is cheap that is the energetic cost of creating large egg is enormous where as a sperm contains few energetic resources I In most species females invest much more in their offspring than do males fundamental asymmetry of sex I Fundamental asymmetry of sex consequences 1 I Because eggs are large and energetically expensive females produce relatively few young over the course ofa lifetime A female s fitness is limited not bythe ability to find a mate but primarily by her ability to gain the resources needed to produce more eggs Sperm are some simple to produce that a male can father an almost limitless of offspring Thus a males fitness is limited not by the ability to acquire the resources needed to produce sperm but by the of females he can mate with 0 Predictions of the BatemanTrivers Theory If male fitness is limited by access to mates then any allele that increases a male s attractiveness to females or success in male male competition should increase rapidly in the population Thus sexual selection should act more strongly on males than on females Traits that evolve due to sexual selection should be found primarily in males 0 Female Choice For Parental Care In many species females prefer to mate with males that care for young or that provide the resources required to produce eggs Females may choose mates on the basis of U Physical characteristics that signal male genetic quality U Resources or parental care provided by males U Or both listed above 0 MaleMale Competition Territory is an area that is actively defended and that provides exclusive use by the owner U Males that win in battle over other males monopolize mating with the females in their territories o Males that own beaches with large s of females father large of offspring Males that lose fights father less 0 The alleles of territoryowning males rapidly increases in frequency in population If the ability to win is determined by body size then alleles for large body size have fitness advantage leading to the evolution of large male size What are the Consequences of Sexual Selection U Because sexual selection tends to be much more intense in males than females males tend to have many more traits that function only in courtship or malemale competition U Sexual dimorphism any trait that differs between male and females figure 2517 I They range from weapons that males use to fight over females such as antlers and horns to the elaborate ornamentation and behavior used in courtship Like inbreeding and other forms ofnonrandom mating sexual selection violates the assumptions of the HW principle Unlike inbreeding however it causes certain alleles to increase or decrease in frequency as a result of evolution
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