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General Zoology

by: Laura Drury

General Zoology ZOO1010C

Laura Drury

GPA 3.8

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These notes cover general zoology ideas as well as the process of evolution.
General Zoology
Dr. Peter Sleszynski
Class Notes
Zoology, darwin, evolution, scientific method, natural selection, population, Genes. Genetics
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This 20 page Class Notes was uploaded by Laura Drury on Friday September 23, 2016. The Class Notes belongs to ZOO1010C at Hillsborough Community College taught by Dr. Peter Sleszynski in Fall 2016. Since its upload, it has received 13 views. For similar materials see General Zoology in Animal Science and Zoology at Hillsborough Community College.

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Date Created: 09/23/16
Zoology Chapter 1: Science of Zoology and Evolution of Animal Diversity  Zoology is the scientific study of animals  Evolution is the extensive and ongoing change of animals, from the earliest animals to the millions of animal species living today  Phylogeny (also called a phylogenetic tree) is the depiction of the history of animal live as a branching genealogical tree  2 major goals of the scientific study of animal diversity: o To reconstruct a phylogeny of animal life and to find where in evolutionary history we can locate the origins of major characteristics (multicellularity, a coelom, spiral cleavage, vertebrae, homeothermy) and all other dimensions of animal diversity as we know it o To understand historical processes that generate and maintain diverse species and adaptations throughout evolutionary history Principles of Science  Essential characteristics of science: o Guided by natural law o Must be explanatory by reference to natural law o Its conjectures are testable against the empirical world o Its conclusions are tentative and not necessarily the final word o It is falsifiable Scientific Method  Compromised of 6 steps: o Observation o Question o Hypothesis o Empirical test o Conclusions o Publication  Hypotheses are potential explanations of a phenomenon of nature Experimental Vs Comparative Methods  We test hypotheses of proximal causes using the experimental method  3 steps to experimental method o Predicting how a system being studied would respond to a treatment o Making the treatment o Comparing observed results to predicted ones  Controls eliminate any unperceived conditions that might bias an experiment's outcome  Controls are repetitions of an experimental procedure that lack the treatment  Test of hypotheses of ultimate casualty require the comparative method o Characteristics of molecular biology, cell biology, organismal structure, development, and ecology are compared among species to identify patterns of variation o Scientists then use patterns of similarity and dissimilarity to test hypotheses of relatedness to reconstruct the phylogenic tree that relates the species being compared  Systematics is the ordering of organisms according to their inferred evolutionary relationships for comparative study o Recent advances in DNA sequencing allows precise testing of evolutionary relationships among species o Comparative studies also serve to test hypotheses of evolutionary processes that have molded diverse species  Comparative method often relies on results of experimental sciences to reveal the characteristics being compared among animals Origins of Darwinian Evolution Theory  Darwin and Alfred Russel Wallace were the first to establish evolution as a powerful scientific theory  Darwinian theory allows us to explain both the genetics of populations and long-term trends in the fossil record. Pre-Darwinian Evolutionary Ideas  Early Greek philosophers (like Aristotle) recorded idea that life has a long history of evolutionary change  Recognized fossils as evidence for former life  Ancient Greeks failed to establish an evolutionary concept that could guide a meaningful study of life's history Lamarckism: The First Scientific Hypothesis for Evolution  Jean Baptiste de Lamarck o In 1809, authored first complete hypothesis for evolution o Stated that fossils were made of extinct animals o "Inheritance of acquired characteristics", his evolutionary mechanism that answered the question of how evolution could construct biological characteristics that seemed designed to serve animals' needs o Organisms would acquire adaptations and pass them by hereditary to their offspring  Lamarck's concept of evolution transformational because as individual organisms transform their characteristics through use and disuse of body parts, heredity makes corresponding adjustments to produce evolution o Now rejected because genetic studies show that traits acquired during an organism's lifetime are not transmitted to offspring  Darwin's theory differs in that it's a variational theory. Evolution occurs at the level of the population, and it includes changes across generations  Darwin argued that organisms whose hereditary characteristics provided an advantage for survival or reproduction would contribute the greatest numbers of offspring to future generations Charles Lyell and Uniformitarianism  Sir Charles Lyell o Principles of Geology o 1830-1833 o Established principle of uniformitarianism o Encompasses 2 important assumptions  That the laws of physics and chemistry have not changed throughout earth's history  That past geological events occurred by natural processes similar to those that we observe in action today o Showed that natural forces acting over long periods of time could explain the formation of fossil-bearing rocks o Also stressed the gradual nature of geological changes that occur through time, and that such changes have no inherent directionality Darwin's Great Voyage of Discovery  The Voyage of the Beagle o Charles Darwin's account of the historic 5 year voyage of the Beagle around the world o Was a extensive surveying voyage to South America and the Pacific o 1831-1836 o Made extensive collections and observations of the faunas and floras of these regions o Unearth numerous fossils of animals long extinct o Noted a resemblance between fossils of South America and known fossils of north America o Strengthened Darwin's conviction that natural forces could explain geological features of the earth o Galapagos Islands  Discovered that although Galapagos and Cape Verde islands were similar in topography and climate, Galapagos plants and animals resembled those of South American mainland  Galapagos life must have originated in continental South America, colonized islands in rare events of trans-oceanic dispersal, and then undergone modification in various environmental conditions of different islands  Concluded that these species were products of a long history of evolutionary change Darwin's Theory of Evolution  Darwinism should be viewed as 5 major theories o Perpetual change  Basic theory of evolution on which the others are based  States that the living world has a long history of ongoing change, with hereditary continuity from past to present life  Did not gain widespread acceptance until Darwin advocated it in the context of his 4 other theories  Documented by fossil record o Common Descent  States that all life forms propagated from a common ancestor through a branching of lineages  Studies of organismal form, cellular structure, and macromolecular structures (including those of DNA) confirm the theory that life's history has the structure of a branching evolutionary tree-phylogeny  Resulting phylogeny provides basis for our taxonomic classification of animals o Multiplication of Species  States that evolution produces new species by splitting and transforming older ones  Adds a spatial dimension to evolutionary processes  When population of a species becomes isolated from each other by geographical barriers, the isolated populations undergo separate evolutionary change and can diverge from each other  Species are reproductively distinct populations of organisms that usually but not always differ from each other in organismal form o Gradualism  States that large differences in anatomical traits among species originate by accumulation of many small incremental changes of very long periods of time  Theory is important because genetic changes having very large effects on organismal form are usually harmful to an organism o Natural Selection  Explains why organisms are constructed to meet the demands of their environments, a phenomenon called adaptation  Darwin's Observations  Observation 1: Organisms have great potential fertility  All populations produce large numbers of gametes and potentially large numbers of offspring each generation  Population size would increase exponentially at an enormous rate if all the individuals produced each generation survived and reproduced  Observation 2: Natural populations normally remain constant in size, except for minor fluctuations  Natural populations fluctuate in size across generations, sometimes going extinct, but no natural populations show the exponential growth their reproductive capacity could theoretically sustain  Observation 3: Natural resources are limited  Exponential growth of a population would require unlimited natural resources for food and habitat, but natural resources are finite  Inference 1 - A continuing struggle for existence occurs among members of a population  Survivors represent a small portion of all individuals produced each generation  Struggle for food, shelter, and space becomes increasingly severe with overpopulation  Observation 4: All populations show organismal variation  No two individuals are exactly alike  They differ in size, color, physiology, behavior, and many other ways  Observation 5: Variation is heritable  Darwin noted that offspring tend to resemble their parents  Many years later hereditary mechanism discovered by George Mandel would be applied  Inference 2 - Varying organisms show differential survival and reproduction favoring advantageous traits (=natural selection)  Survival in a struggle for existence is not random with respect to the hereditary variation in a population  Some traits give an advantage in using environmental resources for effective survival and reproduction  Survivors transmit favored traits to offspring, causing said traits to accumulate in the population  Inference 3 - Over many generations, natural selection gradually produces new adaptations and new species  Differential reproduction gradually transforms species and causes their long-term "improvement"  Darwin knew that people used hereditary variation to produce new useful livestock and plants  Natural selection acting over million of years should be more effective in producing new types than artificial selection imposed during a human lifetime  Natural selection can be considered a 2-step process with a random component and a nonrandom component  Production of variation among organisms is the random part  Mutational processes have no inherent tendency to generate traits that are favorable to an organism  The nonrandom part is differential persistence among traits  Differential survival and reproduction among varying organisms is called sorting, and should not be equated with natural selection.  Darwin's theory of natural selection states that sorting occurs because certain traits give their possessors advantages in survival and reproduction relative to other that lack those traits. Evidence for Darwin's 5 Theories of Evolution  A fossil is a remnant of past life uncovered from the earth's crust  Some fossils constitute complete remains, actual hard parts, or petrified skeletal parts infiltrated with silica or other minerals  Other fossils include molds, casts, impressions, and fossil excrement  Fossils reveal profound changes in the earth's physical environments, including major changes in the locations of lands and seas.  Evolutionary study of the fossil record is called paleontology Interpreting the Fossil Record  Fossil record is biased because preservation is selective o Vertebrate skeletal parts and invertebrates with shells and other hard structures left the best record o Soft-bodied animals, like jellyfish and most worms, are fossilized only under unusual circumstances  Fossil deposits form stratified layers, with new deposits forming over older ones  Stratigraphy is the study of fossil-bearing rocks Geological Time  Geologists divided the earth's history into a table of succeeding events based on ordered layers of sedimentary rock  "Law of Stratigraphy" produced a relative dating, with oldest layers at the bottom and most recent at the top of a sequence  Time was divided into eons, eras, periods, and epochs  In the 1940s, radiometric dating methods were developed for determining the absolute ages (in years) of rock formations  These "radioactive clocks" are independent of pressure and temperature changes and not affected by often violent earth-building activities  Several isotopes exist for dating purposes, some of which help to determine the age of the earth itself  Precambrian era occupies 85% of all geological time o Receives much less attention than later eras, partly because oil, which provides a commercial incentive for much geological work, seldom exists in Precambrian formations Evolutionary Trends  The fossil record reveals evolutionary change across the broadest scale of time  Throughout the geological history recorded by fossil record, millions of species have arise and almost as many have gone extinct  Animal species typically survive approximately 1-10 million years, although their durations are highly variable  When we study patterns of species or taxon replacement thorough time, we observe trends  Trends are directional changes in characteristic features or patterns of diversity in a group of organisms  Fossil trends clearly demonstrate Darwin's principle of perpetual change  Evolutionary trends in diversity are observed in fossils of many different groups of animals Common Descent  Darwin proposed that all plants and animals have descended from some one form  Life's history forms a branching tree (phylogeny) that gives all of life a unified evolutionary history  Pre-Darwinian evolutionists (including Lamarck) advocated multiple independent origins of life  Common descent makes several important predictions that can be tested and potentially used to reject it  According to this theory, one should be able to trace the genealogies of all modern species backward until they converge on ancestral lineages shared with other species, both living and extinct Homology and Reconstruction of Phylogeny  Darwin recognized a major source of evidence for common descent in the concept of homology.  Richard Owen used this term to denote "the same organ in different organisms under every variety of form and function"  The Descent of Man and Selection in Relation to Sex o Darwin's book devoted to the idea that humans share common descent with apes and other animals o To Darwin, the close resemblances could only be explained by common descent Evolutionary processes generate new characteristics that are transmitted across generations   Every time a new feature becomes established in a lineage destined to be ancestral to others, a new homology originates  Pattern formed by these homologies provides evidence for common descent and allows us to reconstruct a branching evolutionary history of life  Branches of an evolutionary tree combine species into a nested hierarchy of groups within groups o Smaller groups (species grouped near terminal branches) are contained within larger ones (species grouped by basal branches, including the trunk of the tree)  Evolutionists test the theory of common descent by observing patterns of homology present in all groups of organisms  The nested hierarchical structure of homology is so pervasive in animals that it forms the basis for our systematic groupings o Genera grouped into families o Families grouped into orders o Orders into classes o Classes into phyla o Phyla into the animal kingdom  Hierarchical classification preceded Darwin because the pattern was so evident, but was not explained scientifically before Darwin Ontogeny, Phylogeny, and Recapitulation  Ontogeny is the history of an organism's development through it's entire life  Early developmental and embryological features contribute greatly to our knowledge of homology and common descent  Studies of ontogeny show how evolutionary alteration of developmental timing generates new phenotypes, thereby causing evolutionary divergence among lineages  Phenotypes are expressed characteristics, or the appearance of an organism  Homeotic genes are mutations of such genes  Homeobox is a sequence of 180 base pairs of genes o Encodes a protein sequence that binds to other genes, thereby altering their expression  Ernst Haeckel o German zoologist o Contemporary of Darwin o Proposed the influential hypothesis that each successive stage in an organism's development representing an adult form present in its evolutionary history o His generalization: ontogeny (individual development) repeats phylogeny (evolutionary descent) o Notion later called recapitulation, or biogenetic law o Based his biogenetic law on the premise that evolutionary change often occurs by successively adding stages onto the end of an unaltered ancestral ontogeny, condensing the ancestral ontogeny into earlier developmental stages  K.E. von Baer o 19th century embryologist o Gave an alternative explanation of the relationship between ontogeny and phylogeny o Argued that early developmental features were simply more widely shared among different animal groups that were later ones  We now know many parallels between ontogeny and phylogeny  Evolutionary change in the timing of development is called heterochrony  Darwin's common descent is strengthened by many homologies found among developmental stages of organisms belonging to different species Multiplication of Species  Multiplication of species through time is a logical corollary to Darwin's theory of common descent  A branch point in a phylogenetic tree means that an ancestral species has split into 2 different ones  Darwin's theory postulates that variation present within a species, especially variation between geographically separated populations, provides material from which new species are produced  Total number of species produced by evolution increases through time, although most of these species eventually go extinct without leaving descendant species  Major challenge for evolutionists is to discover the processes by which an ancestral species "branches" to form two or more descendant species  3 important criteria for recognizing a species o Individuals of the same species descend from a common ancestral population and form an unbranched lineage of ancestral-descendant populations o Individuals of the same species exhibit reproductive compatibility (ability to interbreed) among individuals and reproductive incompatibility between species (for sexually reproducing organisms) o Individuals within the same species maintain genotypic and phenotypic cohesion (lack of abrupt differences among populations)  Speciation is the studies of species formation  Biological factors that prevent different species from interbreeding are called reproductive barriers  Reproductive barriers between populations usually evolve gradually o Evolution of reproductive barriers requires that diverging populations be kept physically separate for long periods of time o If diverging populations were reunited before reproductive barriers were completely formed, interbreeding would occur between the populations and they would merge o Usually requires 10,000 to 100,000 years o Geographic isolation followed by gradual divergence is most effective way for barriers to evolve o Many evolutionists consider geographical separation a prerequisite for branching speciation o Speciation that results from evolution of reproductive barriers between geographically separated populations is called allopatric speciation, or geographical speciation  Production of many ecologically diverse species from a common ancestral species is called adaptive radiation, especially when these species arise within a short interval of geological time (a few million years) Gradualism  Darwin's theory of gradualism opposed arguments for a sudden origin of species  Small differences, resembling those that we observe among organisms within populations today, are the raw material from which different major forms of life evolved  Phenotypic gradualism is controversial o Not all phenotypic changes are small, incremental ones o Some mutations that appear during artificial breeding (called "sports") change a phenotype substantially in a single mutational step o Some argue sports have negative side effects that would prevent affected organisms from surviving in a natural population  Phyletic gradualism is a long series of intermediate forms bridging phenotypes of ancestral and descendant populations o Darwin recognized that phyletic gradualism is not often revealed by fossil record o Studies conducted since Darwin's time have failed to produce a continuous series of fossils as predicted by phyletic gradualism  Is the theory of gradualism refuted by fossil record? o No, because the fossil record is too imperfect to preserve each minor change in animal form that occurs across generations in a species lineage  Although evolution is a slow process by our standarts, it is rapid relative to the rate and which good fossil deposits accumulate  Others have argued that abrupt origins and extinctions of species in the fossil record force us to conclude phyletic gradualism is rare  Niles Eldredge & Stephen Jay Gould o Proposed a model called punctuated equilibrium o 1972 o Explained discontinuous evolutionary changes observed throughout geological time o Punctuated equilibrium states that phenotypic evolution is concentrated in relatively brief events branching speciation, followed by much longer intervals of morphological evolutionary stasis Natural Selection  Sometimes natural selection can proceed very rapidly as happened, for example, when flies and mosquitoes quickly developed high resistance to insecticides  A recurring criticism is that it can't generate new structures or species but can only modify existing ones o To anwer this, darwin proposed that many structures evolved initially for purposes different from the uses that they have today  Exaptation denotes the utility of a structure for a biological role that was not part of the structure's evolutionary origin o Contrasts with adaptation, which implies that a structure arose by natural selection for the utility in question Revisions of Darwinian Evolutionary Theory Neo-Darwinism  Most serious weakness in Darwin's argument was his failure to identify correctly a mechanism for inheritance  Saw hereditary as a blending phenomenon in which the hereditary factors of parents melded in their offspring  Also invoked Lamarck hypothesis that an organism could alter its hereditary through use and disuse of body parts and through direct environmental influence  Did not understand that hereditary factors could be discreet and nonblending, and that a new genetic variant could persist unaltered from one generation to the next  August Weismann o 1834-1914 o Rejected Lamarckian inheritance by showing experimentally that modifications of an organism during its lifetime do not change its hereditary o Revised Darwinian evolutionary accordingly o Neo-Darwinian is the term used to denote Darwinian evolutionary theory as revised by Weismann o Genetic basis of neo-Darwinism eventually became what we know call the chromosomal theory of inheritance  A synthesis of Mendelian genetics and cytological studies of the segregation of chromosomes into gametes Emergence of Modern Darwinism: A Synthetic Theory  In the 1930s a new generation of geneticists reevaluated Darwinian evolutionary theory from a mathematical perspective  They were population geneticists (scientists who studied variation in natural populations of animals and plants and who had a sound knowledge of statistics)  New comprehensive theory emerged o Brought together population genetics, paleontology, biogeography, embryology, systematics, and animal behavior in a Darwinian framework  Population geneticists study evolution as change in the genetic composition of populations  With establishment of population genetics, evolutionary biology became 2 subfields o Microevolution pertains to evolutionary changes in frequencies of variant forms of genes within populations o Macroevolution refers to evolution on a grand scale  encompassing origins of new organismal structures and designs, evolutionary trends, adaptive radiation, phylogenetic relationships of species, and mass extinction  Both marco- and microevolution have operated firmly within a tradition of neo-Darwinism, and both have expanded Darwinian theory in important ways Microevolution: Genetic Variation and Change Within Species  Microevolution is the study of genetic change occurring within natural populations  A population is a reproductively cohesive group of organisms of the same species o Populations of sexually reproducing forms show interbreeding among taheir members  Variant forms of a single gene are called alleles o Example: in humans 3 different allelic forms occur for the gene encoding ABO blood types  I I , i o A person has 2 copies of this gene, one from each parent  A person's genotype denotes the pair of allelic forms present for a particular gene o For the ABO blood type, the possible genotypes are A A A B A B B B  I I , I I , I i , I I , I i , ii o A genotype is homozygous if it contains two copies of one allele  I I , I I , ii o A genotype is heterozygous if it contains copies of two alleles  I I , I i , I i o An organismal trait associated with a particular genotype is called the phenotypic effect of that genotype  A capital letter in the allelic designation indicates that an allele is genetically dominant, being expressed in both homozygous and heterozygous genotypes  A lower-case letter indicates that an allele is genetically recessive, revealing it's characteristics only in its homozygous genotype o Therefore, two genotypes specify blood types A and B, but blood types AB and O have single genotypes. o Many genetic traits show this standard pattern of inheritance, called Mendelian inheritance  The flow of information from the sequence of bases in a gene to the sequence of amino acids in a protein, sometimes called the "central dogma" of molecular biology, explains the relationship between genotype and phenotype  A protein-coding gene is expressed when a molecule of messenger RNA is copied from one of its 2 DNA strands, a process called transcription  Occurrence of different alleles of a gene in a population is called polymorphism  All alleles of all genes possessed by members of a population collectively form its gene pool o Polymorphism potentially enormous in large populations because at observed mutation rates, many different alleles are expected for all genes  Population geneticists study polymorphism by identifying allelic forms of a gene present in a population and then measuring their relative frequencies in that population o Relative frequency of a particular allele of a gene in a population is called its allelic frequency o Because each individual genotype contains two copies of this gene, the total number of copies present in a population is twice the number of individuals  Dominance describes the phenotypic effect of an allele in heterozygous individuals, not its relative abundance in a population of individuals o In many human populations, genetically recessive traits are very common  O blood type  Blonde hair  Blue eyes Protein Polymorphism  Beginning in 1960s, studies of protein variation provided evidence that animal populations typically contain large amounts of genetic variation  DNA studies reveal even larger amounts of variation than studies of protein  Protein polymorphism is the phenomenon of different allelic forms of genes encoding proteins that differ slightly in their amino acid sequence o If these differences affect the protein's net electric charge, the different allelic forms can be separated using protein electrophoresis  We can identify the genotypes of particular individuals for protein-coding genes and measure allelic frequencies in a population  Over last 45 years, geneticists using this approach have discovered far more variation than was previously expected  Despite high levels of polymorphism discovered using protein electrophoresis, these studies underestimate both protein polymorphism and the total genetic variation present in a population  Example: protein polymorphism that does not involve charge differences is not detected. Furthermore, because the genetic code is degenerate, protein polymorphism does not reveal all the genetic variation present  Genetic changes that do not alter protein structure sometimes alter patterns of protein synthesis during development and can be very important to organism  When all kinds of variations are considered, it is evident that most species have enormous potential for further evolutionary change Genetic Equilibrium  Hardy-Weinberg equilibrium o Mathematical theorem that permits us to estimate the relationship between the frequency of al allele in a population and the frequency of a phenotype influenced by that allele o We apply theorem to one locus at a time, tracing the alleles through the formation of gametes followed by fertilization of gametes to produce individuals of the next generation o Frequency of an allele in the gametes produced by a population equals its frequency in the gene pool o Random mating consists mathematically of drawing pairs of gametes at random from the gene pool, uniting each random pair, and then quantifying the frequencies of the resulting organismal genotypes o Mating is usually random with respect to most molecular genetic traits (such as blood type) because they are unlikely to influence mate choice or recognition o We can estimate from theorem the ratios of genotypes and phenotypes that should occur in a generation Process of Evolution: How Genetic Equilibrium is Upset  Recurring mutation is the ultimate source of variability in all populations, but upsetting genetic equilibrium usually requires interaction with one or more other factors  Genetic equilibrium is upset in natural population by 4 ways: o Random genetic drift  Some species, such as the cheetah, contain very little genetic variation, probably because their ancestral lineage were sometimes restricted to small populations  Small population clearly cannot contain large amounts of genetic variation  Each individual has at most 2 different allelic forms of each gene, therefore a single breeding pair has at most, 4  Mendelian genetics states that chance decides with allelic forms of a gene get passed to offspring  Possible by chance alone that 1 or 2 parental alleles are not passed down  Highly unlikely that the different alleles present in small ancestral population are all passed to descendants without any change in allelic frequency  This chance fluctuation in allelic frequency from one generation to the next, including loss of alleles from a population, is called genetic drift  Genetic drift occurs to some degree in all populations of finite size  Perfect consistency of allelic frequencies occurs in only infinitely large populations, and such populations only occur in mathematical models  All populations of animals are finite and experience some sort of genetic drift, which becomes greater as population size declines  Genetic drift erodes the genetic variability of a population  If population size remains small for many generations in a row, genetic variation can be greatly depleted  This loss is harmful to evolutionary success of a species because it restricts potential genetic responses to environmental change  Term population bottleneck denotes a large reduction in size of a population, causing loss of genetic variation, by genetic drift, followed by an increase on population size  Loss of variation is proportional to the number of generations that population size remains small before the population expands  A bottleneck associated with the founding of a new geographic population is called a founder effect and sometimes initiates the formation of a new species o Nonrandom mating  If mating is nonrandom, genotypic frequencies deviate from Hardy-Weinberg expectations  In positive assortative mating, individuals mate preferentially with others of the same genotype, as when albinos mate with other albinos, for example Mating among homozygous parents generate offspring that are homozygous  like themselves  Mating among heterozygous parents produce on average 50% heterozygous offspring and 50% homozygous offspring each generation  Positive assortative mating increases frequency of homozygous genotypes and decreases frequency of heterozygous genotypes in a population but doesn't change allelic frequencies  Preferential mating among close relatives also increases homozygosity and is called inbreeding  Whereas positive assortative mating usually affects one or a few traits, inbreeding simultaneously affects all variable traits  Strong inbreeding greatly increases chances that rare recessive alleles become homozygous and are thereby expressed o Migration  Migration prevents different populations of a species from becoming genetically dissimilar  If a species is divided into many small populations, genetic drift and selection acting separately in different populations can produce evolutionary divergence among them  A small amount of migration among populations each generation keeps different populations from diverging strongly o Natural Selection  Can change both allelic frequencies and genotypic frequencies in a population  Organism that possesses a superior combination of traits is favored  An animal may have some traits that confer no advantage or even a disadvantage, but the animal is successful overall if combination of traits is favorable  When we claim that a particular genotype has a higher relative fitness than do alternative genotypes, we state that the genotype on average confers an advantage in survival and reproduction in a population  Using genetic theory of natural selection, one can measure relative fitness values associated with different genotypes in a population  From measured fitness values and knowledge of the frequencies of alleles in a population and its system of mating, one can calculate the average effect that an allele has on the phenotype of relative fitness in that population  Some traits and combinations of traits are advantageous for certain aspects of an organism's survival or reproduction and disadvantageous for others  Darwin used term sexual selection to denote selection of traits that are advantageous for obtaining mates but perhaps harmful for survival  Environmental changes can alter the selective values of different traits  Selection is often studied using quantitative traits, those that show continuous variation with no obvious pattern of Mendelian segregation in their inheritance  Values of a trait in offspring often are intermediate between the values of their parents  Such traits are influenced by variation at many genes, each of which follows Mendelian inheritance and contributes a small, incremental amount to a phenotype  When values are graphed with respect to frequency distribution, they often approximate a normal, bell-shaped curve  Most individuals fall near the average, fewer fall well above or below the average, and extreme values form "tails" of the frequency curve with increasing rarity  Selection can act on quantitative traits to produce 3 different kinds of evolutionary response  Stabilizing selection favors average values of a trait and disfavors extreme ones  Directional selection favors an extreme value of a phenotype and causes a population average to shift toward it  When we think about natural selection in terms of producing change, we usually have this selection in mind  Disruptive selection, in which two different extreme phenotypes are favored simultaneously, but their average is disfavored  If inbreeding or positive assortative mating accompanies disruptive selection this can cause a population to become bimodal, meaning that 2 very different types of phenotypes predominate  Interaction of Selection, Drift, and Migration o Subdivision of a species into small populations that exchange migrants is an optimal way to promote rapid adaptive evolution of a species o Interactions of genetic drift and selection in different populations permit many different genetic combinations of many polymorphic genes to be tested against natural selection o Migration among populations permits particularly favorable new genetic combinations to spread throughout the species as a whole o Interactions of selection, genetic drift, and migration in this example produce evolutionary change qualitatively different from what would result if any of these three factors acted alone  Geneticist Sewall Wright called this interaction shifting balance because it permits a population to switch from an initial adaptive combination of traits to a new and possibly better one  Genetic drift, nonrandom mating, migration, and natural selection interact in natural populations to create an enormous opportunity for evolutionary change; the perpetual stability predicted by Hardy-Weinberg equilibrium almost never lasts across any substantial amount of evolutionary time Macroevolution: Major Evolutionary Events and Processes  Describes large-scale events and processes in organic evolution  Speciation links microevolution and macroevolution Speciation and Extinction Through Geological Time  Evolutionary change at the macroevolutionary level provides a new perspective on Darwin's theory of natural selection  Although a species can persist for many millions of years, it ultimately has 2 possible evolutionary fates: o It may give rise to new species o It may become extinct without leaving descendants  Rates of speciation and extinction vary among lineages, and lineages that have the highest speciation rates and the lowest extinction rates produce the greatest diversity of living forms  Characteristics of a species might make it more or less likely than others to undergo speciation or extinction  Because many characteristics are passed from ancestral to descendant species analogous to hereditary at the organismal level, lineages whose properties enhance the probability of speciation and confer resistance to extinction should dominate the living world  This species-level process that produces differential rates of speciation and extinction among lineages is analogous in many ways to natural selection  It represents an expansion of Darwin's theory of natural selection  Species selection denotes the differential survival and multiplication of species through geological time based on variation among lineages in species-level properties o Includes mating rituals, social structuring, migration patterns, and geographic distribution o Descendant species usually resemble their ancestors for these properties  We expect speciation rates to be enhanced by social systems that promote the founding of new populations by small numbers of individuals  Certain social systems might increase the probability that a species survives environmental challenges through cooperative action  Such properties would be favored over geological time by species selection Mass Extinctions  When we study evolutionary change on an even larger time-scale, we observe episodic events in which large numbers of taxa become extinct nearly simultaneously. These events are called mass extinctions o Most cataclysmic of these extinction episodes (Permian extinction) happened 250 million years ago, when at least half of the families of shallow-water marine invertebrates and fully 90% of marine invertebrate species disappeared within a few million years o The Cretaceous extinction, which occurred about 65 million years ago, marked the end of dinosaurs as well as numerous marine invertebrates and many small reptilian species  Causes of mass extinctions and their occurrence at intervals of approximately 26 million years throughout the past 250 million years are difficult to explain o Some researchers have proposed biological explanations for these mass extinctions, and others consider them artifacts of our statistical and taxonomic analyses o Walter Alvarez proposed that the earth was occasionally bombarded by asteroids, causing the mass extinction at the end of the Cretaceous and perhaps others  Drastic effects of such a bombardment were observed when fragments of a comet bombarded Jupiter in 1994  Most violent event in recorded history of our solar system  Bombardments by asteroids or comets could change the earth's climate drastically, sending debris into the atmosphere and blocking sunlight  Temperature changes would challenge ecological tolerances of many species  So far, the end-Cretaceous mass extinction is the only one that coincides with strong geological evidence for asteroid bombardment  Sometimes lineages favored by species selection are unusually at risk during a mass extinction  Climatic changes produced by hypothesized asteroid bombardments could produce selective challenges very different from those encountered at other times in the earth's history  Selective discrimination of particular biological traits by mass extinction events is termed catastrophic species selection o For example, mammals survived the end-Cretaceous mass extinction that destroyed dinosaurs and other prominent vertebrate and invertebrate groups o Ability of small mammals to occupy underground burrows might have permitted them to withstand climatic stress that killed more exposed animals o Following this event, mammals were able to use environmental resources that previously had been denied then, leading to their adaptive radiation  Natural selection, species selection, and catastrophic species selection interact to produce macroevolutionary trends that we see in fossil record


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