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Evolution of Populations: Natural Selection and Genetic Variation

Explore the evolutionary processes of natural selection, genetic variation, and allele frequencies in populations. Learn about the Hardy-Weinberg equilibrium, genetic drift, gene flow, and the different modes of selection.

jamesmwilliams
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Evolution of Populations: Natural Selection and Genetic Variation

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  1. 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

  2. Mutation and sexual reproductionproduce 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 (b) (a)

  3. 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

  4. 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

  5. Fig. 23-5 Porcupine herd MAP AREA CANADA ALASKA Beaufort Sea One species, two populations NORTHWEST TERRITORIES Porcupine herd range Fortymile herd range YUKON ALASKA Fortymile herd

  6. 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

  7. 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 • p2 + 2pq + q2 = 1 Alleles in the population Frequencies of alleles Gametes produced p = frequency of Each egg: Each sperm: CR allele = 0.8 q = frequency of 80% chance 80% chance 20% chance 20% chance CW allele = 0.2

  8. Fig. 23-7-4 20% CW(q = 0.2) 80% CR(p = 0.8) Sperm CW (20%) CR (80%) CR (80%) Eggs 16% (pq) CR CW 64% (p2) CR CR 4% (q2) CW CW 16% (qp) CR CW CW (20%) 64% CR CR,32% CR CW, and4% CW CW Gametes of this generation: 64% CR +16% CR=   80% CR = 0.8 = p 4% CW+16% CW=20% CW= 0.2 = q Genotypes in the next generation: 64% CR CR,32% CR CW, and4% CW CW plants

  9. 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

  10. Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population • 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

  11. Genetic Drift • Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next (reduces variation) • significant in small populations • causes allele frequencies to change at random • can lead to a loss of genetic variation within populations • can cause harmful alleles to become fixed • Founder effect - occurs when a few individuals become isolated from a larger population

  12. Fig. 23-9 Bottleneck effect - sudden reduction in population size due to environmental change Bottlenecking event Surviving population Original population

  13. 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

  14. 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

  15. 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

  16. Fig. 23-13 Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population (b) Disruptive selection (c) Stabilizing selection (a) Directional selection

  17. 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)

  18. Fig. 23-15

  19. Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population • 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 • 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

  20. 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

  21. Why Natural Selection Cannot Fashion Perfect Organisms • Selection can act only on existing variations • Evolution is limited by historical constraints • Adaptations are often compromises • Chance, natural selection, and the environment interact

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