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Transcript
Chapter 11: Evolution and Natural Selection 11.1 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 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 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 • 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 • 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 11.2 The Evidence For Evolution Evidence for evolution comes from the following Fossil record Molecular record Anatomical record 1 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 Fossils have been found linking all the major groups •The forms linking mammals to reptiles are particularly well known • 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- • 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 Note: Different proteins evolve at different rates • Anatomical Record •All vertebrates share a basic set of developmental instructions •Homologous structures Have different structure and function but are all derived from the same part of a common ancestor •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 •Vestigial organs Structures that are no longer in use The human appendix Apes have a much larger appendix that is involved in digestion 11.3 Evolution’s Critics •Critics of evolution raise seven principal objections 1. Evolution is not solidly demonstrated 2. There are no fossil intermediates 3. The intelligent design argument 4. Evolution violates the 2nd law of thermodynamics 5. Proteins are too improbable 6. Natural selection does not imply evolution 7. The irreducible complexity argument All of these objections are without scientific merit. 2 11.4 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 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 Number of individuals falling within a category Frequency = 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 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 (p + q)2 = p2 + 2pq + q2 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 p2= (0.6)2 = 0.36 (36 out of 100 cats are black (BB) Frequency of the heterozygous genotype is 2pq = 2(0.6)(0.4) = 0.48 (48 out of 100 cats are black (Bb) Hardy-Weinberg Assumptions •The Hardy-Weinberg equation is true only if the following five assumptions are met 1. Large population size 2. Random mating 3. No mutation 4. No migration 5. No natural selection 3 11.5 Why Allele Frequencies Change Five evolutionary forces can significantly alter the allele frequencies of a population 1. Mutation 2. Migration 3. Genetic drift 4. Nonrandom mating 5. Selection Mutation Errors in DNA replication The ultimate source of new variation Mutation rates are too low to significantly alter allele frequencies on their own Migration 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 Genetic Drift Random loss of alleles - 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 Nonrandom Mating Mating that occurs more or less frequently than expected by chance Inbreeding: Mating with relatives, Increases homozygosity Outbreeding: Mating with non-relatives, Increases heterozygosity Selection Some individuals leave behind more offspring than others Artificial selection - Breeder selects for desired characteristics Natural selection - Environment selects for adapted characteristics 11.6 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 Stabilizing Selection - In humans, infants with intermediate weight at birth have the highest survival rate In chicken, eggs of intermediate weight have the highest hatching success 4 Disruptive Selection - In the African seed-cracker finch, large- and small-beaked birds predominate Intermediate-beaked birds are at a disadvantage Unable to open large seeds Too clumsy to open small seeds 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 11.7 Sickle-Cell Anemia Sickle-cell anemia is a hereditary disease affecting hemoglobin molecules in the blood It was first detected on December 31st, 1904 The sickle-cell mutation changes the 6th -hemoglobin 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 • 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 • Stabilizing selection is thus acting on the sickle-cell allele It occurs because malarial resistance counterbalances lethal anemia • 11.8 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 In high-predation pools, males Exhibit drab coloration Reproduce younger Attain relatively small adult sizes The evolution of these differences in guppies was experimentally tested in laboratory greenhouses Website for simulation: http://www.pbs.org/wgbh/evolution/educators/lessons/lesson4/act2.html 11.9 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 5 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 11.10 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 Prezygotic Isolating Mechanisms Geographic isolation Ecological isolation Hybridization has been successful in captivity But it does not occur in the wild Temporal isolation Behavioral isolation Mechanical isolation Prevention of gamete fusion 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 Hybrids between them produced defective embryos in the lab 11.11 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 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 6 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 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 11.12 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) • 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 • 7