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Chapter 23 The Evolution of Populations PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: The Smallest Unit of Evolution • One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes • Natural selection acts on individuals, but only populations evolve • Genetic variations in populations contribute to evolution Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.1: Population genetics provides a foundation for studying evolution • Microevolution is change in the genetic makeup of a population from generation to generation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Modern Synthesis • Population genetics is the study of how populations change genetically over time • Population genetics integrates Mendelian genetics with the Darwinian theory of evolution by natural selection • This modern synthesis focuses on populations as units of evolution Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Pools and Allele Frequencies • A population is a localized group of individuals capable of interbreeding and producing fertile offspring • The gene pool is the total aggregate of genes in a population at any one time • The gene pool consists of all gene loci in all individuals of the population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MAP AREA CANADA ALASKA LE 23-3 Beaufort Sea Porcupine herd range Fairbanks Fortymile herd range Whitehorse The Hardy-Weinberg Theorem • The Hardy-Weinberg theorem describes a population that is not evolving • It states that frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work • Mendelian inheritance preserves genetic variation in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-4 Generation 1 X CRCR genotype Generation 2 Plants mate CWCW genotype All CRCW (all pink flowers) 50% CW gametes 50% CR gametes come together at random Generation 3 25% CRCR 50% CRCW 50% CR gametes 25% CWCW 50% CW gametes come together at random Generation 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions Preservation of Allele Frequencies • In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hardy-Weinberg Equilibrium • Hardy-Weinberg equilibrium describes a population in which random mating occurs • It describes a population where allele frequencies do not change Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p2 + 2pq + q2 = 1 – And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR CW (20%) p2 pq 64% CRCR 16% CRCW (20%) CR (80%) CW Eggs (80%) qp 4% CWCW 16% CRCW q2 Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies do change over time Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The five conditions for non-evolving populations are rarely met in nature: – Extremely large population size – No gene flow – No mutations – Random mating – No natural selection Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population Genetics and Human Health • We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible • Two processes, mutation and sexual recombination, produce the variation in gene pools that contributes to differences among individuals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation • Mutations are changes in the nucleotide sequence of DNA • Mutations cause new genes and alleles to arise Animation: Genetic Variation from Sexual Recombination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Point Mutations • A point mutation is a change in one base in a gene • It is usually harmless but may have significant impact on phenotype Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutations That Alter Gene Number or Sequence • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Gene duplication is nearly always harmful Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Rates • Mutation rates are low in animals and plants • The average is about one mutation in every 100,000 genes per generation • Mutations are more rapid in microorganisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Recombination • Sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition • Three major factors alter allele frequencies and bring about most evolutionary change: – Natural selection – Genetic drift – Gene flow Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Natural Selection • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Drift • The smaller a sample, the greater the chance of deviation from a predicted result • Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next • Genetic drift tends to reduce genetic variation through losses of alleles Animation: Causes of Evolutionary Change Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-7 CWCW CRCR CRCR CRCW Only 5 of 10 plants leave offspring CRCR CWCW CRCW CWCW CRCR CRCW CRCW CRCR CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCR Only 2 of 10 plants leave offspring CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCW CRCW Generation 2 p = 0.5 q = 0.5 CRCR CRCR Generation 3 p = 1.0 q = 0.0 The Bottleneck Effect • The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population • The resulting gene pool may no longer be reflective of the original population’s gene pool Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-8 Original population Bottlenecking event Surviving population • Understanding the bottleneck effect can increase understanding of how human activity affects other species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Founder Effect • The founder effect occurs when a few individuals become isolated from a larger population • It can affect allele frequencies in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Flow • Gene flow consists of genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes • Gene flow causes a population to gain or lose alleles • It tends to reduce differences between populations over time Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.4: Natural selection is the primary mechanism of adaptive evolution • Natural selection accumulates and maintains favorable genotypes in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Variation • Genetic variation occurs in individuals in populations of all species • It is not always heritable Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-9 Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white Variation Within a Population • Both discrete and quantitative characters contribute to variation within a population • Discrete characters can be classified on an eitheror basis • Quantitative characters vary along a continuum within a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polymorphism • Phenotypic polymorphism describes a population in which two or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable • Genetic polymorphisms are the heritable components of characters that occur along a continuum in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Measuring Genetic Variation • Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels • Average heterozygosity measures the average percent of loci that are heterozygous in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Variation Between Populations • Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-10 1 2.4 3.14 8.11 9.12 10.16 5.18 6 13.17 19 1 2.19 3.8 4.16 9.10 11.12 13.17 15.18 5.14 7.15 XX 6.7 XX • Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-11 Heights of yarrow plants grown in common garden Mean height (cm) 100 50 0 3,000 2,000 1,000 Sierra Nevada Range 0 Seed collection sites Great Basin Plateau A Closer Look at Natural Selection • From the range of variations available in a population, natural selection increases frequencies of certain genotypes, fitting organisms to their environment over generations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Fitness • The phrases “struggle for existence” and “survival of the fittest” are commonly used to describe natural selection but can be misleading • Reproductive success is generally more subtle and depends on many factors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Relative fitness is the contribution of a genotype to the next generation, compared with contributions of alternative genotypes for the same locus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Directional, Disruptive, and Stabilizing Selection • Selection favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection: – Directional – Disruptive – Stabilizing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Directional selection favors individuals at one end of the phenotypic range • Disruptive selection favors individuals at both extremes of the phenotypic range • Stabilizing selection favors intermediate variants and acts against extreme phenotypes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Frequency of individuals LE 23-12 Original population Evolved population Directional selection Original population Phenotypes (fur color) Disruptive selection Stabilizing selection The Preservation of Genetic Variation • Various mechanisms help to preserve genetic variation in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diploidy • Diploidy maintains genetic variation in the form of hidden recessive alleles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Balancing Selection • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population • Balancing selection leads to a state called balanced polymorphism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heterozygote Advantage • Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes • Natural selection will tend to maintain two or more alleles at that locus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance • It exemplifies the heterozygote advantage Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan) 7.5–10.0% 10.0–12.5% >12.5% Frequency-Dependent Selection • In frequency-dependent selection, the fitness of any morph declines if it becomes too common in the population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-14 On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue a new set of images (a new feeding opportunity). Parental population sample 0.6 Phenotypic variation Experimental group sample 0.5 0.4 Frequencyindependent control 0.3 0.2 0 Plain background Patterned background 20 40 60 Generation number 80 100 Neutral Variation • Neutral variation is genetic variation that appears to confer no selective advantage Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Selection • Sexual selection is natural selection for mating success • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Intrasexual selection is competition among individuals of one sex for mates of the opposite sex Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Intersexual selection occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex • Selection may depend on the showiness of the male’s appearance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Evolutionary Enigma of Sexual Reproduction • Sexual reproduction produces fewer reproductive offspring than asexual reproduction, a so-called “reproductive handicap” Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 23-16 Asexual reproduction Female Sexual reproduction Generation 1 Female Generation 2 Male Generation 3 Generation 4 • Sexual reproduction produces genetic variation that may aid in disease resistance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises • Chance and natural selection interact • Selection can only edit existing variations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings