Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 23 The Evolution of Populations Natural selection acts on individuals, but only populations evolve Genetic variations in populations contribute to evolution Microevolution - change in allele frequencies in a population over generations

(a) (b) Mutation and sexual reproduction produce variation in gene pools contributing to differences among individuals Variation in individual genotype leads to variation in individual phenotype which is not always heritable Natural selection can only act on variation with a genetic component

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Variation Between Populations Average heterozygosity measures the average percent of loci that are heterozygous in a population Geographic variation - differences between gene pools of separate populations or population subgroups Cline - a graded change in a trait along a geographic axis

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Pools and Allele Frequencies Population - localized group of individuals capable of interbreeding and producing fertile offspring Gene pool - all the alleles for all loci in a population

Fig Porcupine herd Porcupine herd range Beaufort Sea NORTHWEST TERRITORIES MAP AREA ALASKA CANADA Fortymile herd range Fortymile herd ALASKA YUKON One species, two populations

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Hardy-Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy- Weinberg principle, it can be concluded that the population is evolving Hardy-Weinberg principle - frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population

Frequencies of alleles Alleles in the population Gametes produced Each egg:Each sperm: 80% chance 80% chance 20% chance 20% chance q = frequency of p = frequency of C R allele = 0.8 C W allele = 0.2 Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p 2 + 2pq + q 2 = 1

Fig Gametes of this generation: 64% C R C R, 32% C R C W, and 4% C W C W 64% C R + 16% C R = 80% C R = 0.8 = p 4% C W + 16% C W = 20% C W = 0.2 = q 64% C R C R, 32% C R C W, and 4% C W C W plants Genotypes in the next generation: Sperm C R (80%) C W (20%) 80% C R ( p = 0.8) C W (20%) 20% C W ( q = 0.2) 16% ( pq ) C R C W 4% ( q 2 ) C W C W C R (80%) 64% ( p 2 ) C R C R 16% ( qp ) C R C W Eggs

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The five conditions for nonevolving populations are rarely met in nature: – No mutations – Random mating – No natural selection – Extremely large population size – No gene flow

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Three major factors alter allele frequencies and bring about most evolutionary change: – Natural selection – Genetic drift – Gene flow Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genetic Drift Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next (reduces variation) 1.significant in small populations 2.causes allele frequencies to change at random 3.can lead to a loss of genetic variation within populations 4.can cause harmful alleles to become fixed Founder effect - occurs when a few individuals become isolated from a larger population

Fig Original population Bottlenecking event Surviving population Bottleneck effect - sudden reduction in population size due to environmental change

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Flow Gene flow consists of the movement of alleles among populations – Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) – Reduces differences between populations over time – Is more likely than mutation to alter allele frequencies directly – Can Increase / Decrease the fitness of a population – The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Relative Fitness The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals Reproductive success is generally more subtle and depends on many factors – Relative fitness - contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Directional, Disruptive, and Stabilizing Selection Three modes of selection: – 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

Fig Original population (c) Stabilizing selection (b) Disruptive selection (a) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Selection Sexual selection - natural selection for mating success – can result in sexual dimorphism - marked differences between the sexes in secondary sexual characteristics Intrasexual selection - competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection, (AKA mate choice), occurs when individuals of one sex (usually females) are choosy in selecting their mates (why? male showiness)

Fig

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population 1.Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes Ex. The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance 2.Frequency-dependent selection - fitness of a phenotype declines if it becomes too common in the population Selection can favor whichever phenotype is less common in a population

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Neutral Variation Neutral variation - appears to confer no selective advantage or disadvantage For example, variation in noncoding regions of DNA and variation in proteins that have little effect on protein function or reproductive fitness

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Why Natural Selection Cannot Fashion Perfect Organisms 1.Selection can act only on existing variations 2.Evolution is limited by historical constraints 3.Adaptations are often compromises 4.Chance, natural selection, and the environment interact