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The Living World Fourth Edition GEORGE B. JOHNSON 13 Evolution and Natural Selection PowerPoint® Lectures prepared by Johnny El-Rady Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.1 Evolution: Getting from There to Here The word “evolution” refers to how an entity changes through time Darwin initially used the phrase “descent with modification” to explain the concept of evolution The concept of evolution helps explain the great paradox of biology: In life there exists both unity and diversity Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.1 Evolution: Getting from There to Here Natural selection, the process that leads to evolution, occurs in steps 1. Gene variation exists among individuals in a population 2. This variation is often passed to offspring 3. All populations overproduce offspring 4. Individuals with traits that aid survival and reproduction have a better chance of contributing to the next generation 5. Over time, the population changes such that the traits of the more successful reproducers are more prevalent Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.1 Evolution: Getting from There to Here Macroevolution Evolutionary change on a grand scale Encompasses the origins of new species and major episodes of extinction Microevolution Evolutionary change on a small scale Encompasses the genetic changes that occur within a species over time These changes are the result of changes in gene frequencies Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.1 Evolution: Getting from There to Here Jean-Baptiste Lamarck proposed that evolution occurred by inheritance of acquired characteristics Individuals passed on to offspring body and behavior changes acquired during their lives In Darwin’s theory, by contrast, variation is not created by experience It already exists when selection acts on it Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.1 How did long necks evolve in giraffe Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The Rate of Evolution Different kinds of organisms evolve at different rates Bacteria evolve much faster than eukaryotes The rate of evolution also differs within the same group of species In punctuated equilibrium, evolution occurs in spurts In gradualism, evolution occurs in a gradual, uniform way Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.2 a) Punctuated equilibrium b) Gradualism Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.2 The Evidence For Evolution Evidence for evolution comes from the following Fossil record Molecular record Anatomical record Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fossil Record Provides the most direct evidence for macroevolution Fossils are the preserved remains, tracks, or traces of once-living organisms They form when organisms become buried in sediment and calcium in hard surfaces mineralizes Arraying fossils according to age often provides evidence of successive evolutionary change Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Large blunt horns Small bony protuberance Fig. 13.3 Evolution in the titanotheres Hoofed mammals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fossils have been found linking all the major groups The forms linking mammals to reptiles are particularly well known Fig. 13.4 Whale “missing links” Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Molecular Record New alleles arise by mutations and they come to predominance through favorable selection Thus, evolutionary changes involve a continual accumulation of genetic changes Distantly-related organisms accumulate a greater number of evolutionary differences than closelyrelated ones This divergence is seen among vertebrates in the 146-amino acid hemoglobin b chain Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The greater the evolutionary distance Fig. 13.5 Molecules reflect evolutionary divergence The greater the number of amino acid differences Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display This same pattern of divergence is seen with DNA sequences, such as that of the cytochrome c gene The changes appear to accumulate at a constant rate This phenomenon is referred to as a molecular clock Fig. 13.6 Note: Different proteins evolve at different rates Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Anatomical Record All vertebrates share a basic set of developmental instructions Fig. 13.7 Relict developmental forms Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Anatomical Record Homologous structures Have different structure and function but are all derived from the same part of a common ancestor The same basic bones are present in each forelimb Fig. 13.8 Homology among vertebrate limbs Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Anatomical Record Analogous structures Resemble each other as a result of parallel evolutionary adaptations to similar environments They are the result of convergent evolution Different animals often adapt in similar fashion when challenged by similar opportunities Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.9 Convergent evolution: many paths to one goal Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.9 Convergent evolution: many paths to one goal Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Anatomical Record Vestigial organs Structures that are no longer in use The human appendix Apes have a much larger appendix that is involved in digestion Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.3 Evolution’s Critics Critics of evolution raise seven principal objections 1. 2. 3. 4. 5. 6. 7. Evolution is not solidly demonstrated There are no fossil intermediates The intelligent design argument Evolution violates the 2nd law of thermodynamics Proteins are too improbable Natural selection does not imply evolution The irreducible complexity argument All of these objections are without scientific merit! Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.4 Genetic Change Within Populations: The Hardy-Weinberg Rule Population genetics is the study of the properties of genes in a population Genetic variation in populations puzzled scientists Dominant alleles were believed to drive recessive alleles out of populations In 1908, G. Hardy and W. Weinberg pointed out that in large populations with random mating, allele frequencies remain constant Dominant alleles do not, in fact, replace recessive ones Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A population that is in Hardy-Weinberg equilibrium is not evolving Hardy and Weinberg came to their conclusion by analyzing allele frequencies in successive generations Frequency = Number of individuals falling within a category Total number of individuals being considered If a population of 100 cats has 84 black and 16 white Then the frequencies of black and white phenotypes are 0.84 and 0.16, respectively Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display B allele Black color b allele White color By convention The more common allele (B) is designated p The less common allele (b) is designated q p+q=1 The Hardy-Weinberg equilibrium can be written as an equation Individuals homozygous (p + q)2 = p2 + 2pq + q2 for allele b Individuals homozygous for allele B Individuals heterozygous for alleles B and b Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The equation allows calculation of allele frequencies Frequency of white (bb) cats = 16/100 = 0.16 => q2 = 0.16 => q = √ 0.16 = 0.4 p + q =1 => p = 1 – q = 1 – 0.4 = 0.6 What about genotype frequencies? Frequency of the homozygous dominant genotype is 36 out of 100 cats are black (BB) p2 = (0.6)2 = 0.36 Frequency of the heterozygous genotype is 48 out of 100 cats are black (Bb) 2pq = 2(0.6)(0.4) = 0.48 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.10 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hardy-Weinberg Assumptions The Hardy-Weinberg equation is true only if the following five assumptions are met 1. 2. 3. 4. 5. Large population size Random mating No mutation No migration No natural selection Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.5 Why Allele Frequencies Change Five evolutionary forces can significantly alter the allele frequencies of a population 1. 2. 3. 4. 5. Mutation Migration Genetic drift Nonrandom mating Selection Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mutation Table 13.1 Errors in DNA replication The ultimate source of new variation Mutation rates are too low to significantly alter allele frequencies on their own Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Migration Table 13.1 Movement of individuals from one population to another Immigration: movement into a population Emigration: movement out of a population A very potent agent of change Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Genetic Drift Random loss of alleles Table 13.1 More likely to occur in smaller population Founder effect Small group of individuals establishes a population in a new location Bottleneck effect A sudden decrease in population size to natural forces Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Nonrandom Mating Mating that occurs more or less frequently than expected by chance Table 13.1 Inbreeding Mating with relatives Increases homozygosity Outbreeding Mating with non-relatives Increases heterozygosity Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Selection Some individuals leave behind more offspring than others Table 13.1 Artificial selection Breeder selects for desired characteristics Natural selection Environment selects for adapted characteristics Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.6 Forms of Selection Selection is a statistical concept One cannot predict the fate of any single individual But it is possible to predict which kind of individual will tend to become more common in a population Three types of natural selection have been identified Stabilizing selection Acts to eliminate both extreme phenotypes Disruptive selection Acts to eliminate intermediate phenotypes Directional selection Acts to eliminate a single extreme phenotype Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.12 Three kinds of natural selection Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Stabilizing Selection Increase in the frequency of the intermediate phenotype In humans, infants with intermediate weight at birth have the highest survival rate In chicken, eggs of intermediate weight have the highest hatching success Fig. 13.13 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Disruptive Selection In the African seedcracker finch, largeand small-beaked birds predominate Can open tough shells of large seeds Fig. 13.14 Intermediate-beaked birds are at a disadvantage Unable to open large seeds Too clumsy to open small seeds More adept at handling small seeds Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Directional Selection Drosophila flies that flew toward light were eliminated from the population The remaining flies were mated and the experiment repeated for 20 generations Fig. 13.15 Phototropic flies are far less frequent in the population Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.7 Sickle-Cell Anemia Sickle-cell anemia is a hereditary disease affecting hemoglobin molecules in the blood It was first detected on December 31st, 1904 Fig. 13.16 Sickled RBCs Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The sickle-cell mutation changes the 6th amino acid in the bhemoglobin chain from glutamic acid to valine This causes hemoglobin molecules to clump The result is sickled red blood cells In normal RBCs, the hemoglobin chains do not clump Fig. 13.17a Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sickle-cell homozygosity leads to a reduced life span Heterozygosity produces enough hemoglobin to keep RBCs healthy The disease originated in Central Africa It affects 1 in 500 African Americans But it is almost unknown in other racial groups Why is the defective allele still around? People who are heterozygous for the sickle-cell allele have less susceptibility to malaria This is an example of heterozygote advantage Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.18 Stabilizing selection is thus acting on the sickle-cell allele It occurs because malarial resistance counterbalances lethal anemia Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.8 Selection on Color in Guppies Poecilia reticulata (guppy) is a popular aquarium fish In nature, it is found in small streams in NE South America and in mountainous streams in Trinidad Due to dispersal barriers, guppies can be found in two different pool environments Below waterfalls, where risk of predation is high Above waterfalls, where risk of predation is low Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.19 The evolution of protective coloration in guppies A voracious predator of guppies Rarely preys on guppies Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In low-predation pools, males Display gaudy colors and spots Reproduce at a late age Attain larger adult sizes In high-predation pools, males Exhibit drab coloration Reproduce younger Attain relatively small adult sizes In the absence of predators, larger more colorful fish may produce more offspring In the presence of predators, smaller and less colorful fish are likely favored by selection Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The evolution of these differences in guppies was experimentally tested in laboratory greenhouses Fig. 13.20 Indistinguishable from low-predation controls Same results were obtained in field experiments Smaller and drab in color Thus, natural selection can lead to rapid evolutionary change Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.9 The Biological Species Concept Speciation is the species-forming process It involves progressive change 1. Local populations become increasingly specialized 2. Natural selection acts to keep them different enough Ernst Mayr coined the biological species concept “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” Reproductively isolated populations Populations whose members do not mate with each other or who cannot produce fertile offspring Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.10 Isolating Mechanisms Reproductive isolating mechanisms are the barriers that prevent genetic exchange between species Prezygotic isolating mechanisms Prevent the formation of zygotes Postzygotic isolating mechanisms Prevent the proper functioning of zygotes after they have formed Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Prezygotic Isolating Mechanisms Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.21 Lions and tigers are ecologically isolated Live in forest Live in open grassland Tiglon Hybridization has been successful in captivity But it does not occur in the wild Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Prezygotic Isolating Mechanisms Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Prezygotic Isolating Mechanisms Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Postzygotic Isolating Mechanisms Examples: Hybridization between sheep and goats produces embryos that die in the earliest embryonic stages Leopard frogs of the United States are a group of similar species, and NOT a single species as was long assumed Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.22 Postzygotic isolation in leopard frogs Rana pipiens Rana blairi Rana sphenocephala Rana berlandieri Hybrids between them produced defective embryos in the lab Their mating calls also differ substantially Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.11 Working with the Biological Species Concept Speciation is a two-part process 1. Identical populations must diverge 2. Reproductive isolation must evolve to maintain these differences Speciation occurs much more readily in the absence of gene flow This much more likely in geographically isolated populations Populations can become geographically isolated for several reasons Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 13.23 New colonization Barriers to movement Extinction of intermediate populations Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Allopatric speciation The differentiation of geographically isolated populations into distinct species Sympatric speciation The differentiation of populations within a common geographical area into distinct species Instantaneous sympatric speciation may occur through polyploidy More than two sets of chromosomes Autopolyploidy All chromosomes from one species Allopolyploidy Chromosomes derived from two species, via hybridization Much more common in plants than animals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Problems with the Biological Species Concept The biological species concept has been criticized for several reasons 1. The extent to which all species are truly are reproductively isolated It is becoming increasingly evident that hybridization is not that uncommon in plants and animals 2. It can be difficult to apply the concept to populations that do not occur together in nature It is not possible to observe whether they would interbreed naturally 3. The concept is more limited than its name would imply Many organisms are asexual and reproduce without mating Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Problems with the Biological Species Concept For these reasons, other concepts have been proposed to define a species However, none has universal applicability Indeed, because of the diversity of organisms, it may be that there is no single definition of a species Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.12 The Pace of Evolution The standard view since Darwin was that evolutionary change occurred extremely slowly Imperceptible changes accumulate such that, over thousands or millions of years, major changes could occur This is termed gradualism In 1972, Niles Eldredge and Stephen Jay Gould proposed the Punctuated Equilibrium hypothesis Evolutionary change occurs in bursts separated by long periods of little or no evolutionary change (termed stasis) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.12 The Pace of Evolution Examples of both gradualism and punctuated equilibrium exist So speciation clearly occurs in different ways However, the idea that speciation is necessarily linked to phenotypic change has not been supported Speciation can occur without phenotypic change Phenotypic change can occur within a species in the absence of speciation Copyright ©The McGraw-Hill Companies, Inc. 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