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Chapter 23

Chapter 23. The Evolution of Populations. Overview: The Smallest Unit of Evolution. Microevolution is a change in allele frequencies (gene pool) in a population over generations (time) One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes

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Chapter 23

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  1. Chapter 23 The Evolution of Populations

  2. Overview: The Smallest Unit of Evolution • Microevolution is a change in allele frequencies (gene pool) in a population over generations (time) • 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

  3. Genetic Variation • Variation (of alleles) in individual genotype leads to variation in individual phenotype • Not all phenotypic variation is heritable (fig 23.2) • Body builders do not pass their huge muscles to the next generation • Natural selection can only act on variation with a genetic component

  4. Fig. 23-2 Nonheritable Variation (b) (a) • Caterpillars raised on a diet of oak flowers resembled the flowers • Siblings raised on oak leaves resembled oak twigs

  5. Variation Within a Population • Both discrete and quantitative characters contribute to variation within a population • Discrete characters can be classified on an either-or basis • Mendel’s flowers: Purple or white • Quantitative characters vary along a continuum within a population • Skin color

  6. Variation Between Populations • Most species exhibit geographic variation,differences between gene pools of separate populations or population subgroups

  7. Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Two processes that produce the variation in gene pools that contributes to differences among individuals • mutation • Point Mutations • Chromosome mutations • sexual reproduction • Crossing Over • Independent Assortment • Fertilization

  8. Mutation • Mutations are changes in the nucleotide sequence of DNA • Mutations cause new genes and alleles to arise • Original source of new alleles • Only mutations in cells that produce gametes can be passed to offspring Animation: Genetic Variation from Sexual Recombination

  9. Point Mutations • A point mutation is a change in one base in a gene • The effects of point mutations can vary: • Mutations in noncoding regions of DNA are often harmless • Mutations that result in a change in protein production are often harmful • Mutations that result in a change in protein production can sometimes increase the fit between organism and environment

  10. Mutations That Alter Gene Number or Sequence • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Duplication of large chromosome segments is usually harmful • Duplication of small pieces of DNA is sometimes less harmful and increases the genome size • Duplicated genes can take on new functions by further mutation

  11. Sexual Reproduction • Sexual reproduction can shuffle existing alleles into new combinations in every generation. • Crossing over during Prophase I of meiosis • Independent Assortment of chromosomes during meiosis • Fertilization

  12. Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving • The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

  13. Gene Pools and Allele Frequencies • A population is a localized group of individuals capable of interbreeding and producing fertile offspring • A gene pool consists of all the alleles for all loci in a population • In diploid species each individual has two alleles for a particular gene and the individual may be either heterozygous or homozygous • If all the members of a population are homozygous for the same allele , the allele is fixed. • The greater the number of fixed alleles the lower the species diversity

  14. Hardy-Weinberg Equilibrium • The Hardy-Weinberg principle describes a population that is not evolving • The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation unless they are acted upon by forces other than Mendelian segregation and the recombination of alleles.

  15. 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 p+q=1 • where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

  16. 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 p2 + 2pq + q2 = 1 p+q=1 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 With random mating, these gametes will result in the same mix of genotypes in the next generation Genotypes in the next generation: 64% CR CR,32% CR CW, and4% CW CW plants

  17. Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies do change over time

  18. 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

  19. Applying the Hardy-Weinberg Principle • We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: • The PKU gene mutation rate is low (1) • Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele (2) • Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions (3) • The population is large (4) • Migration has no effect as many other populations have similar allele frequencies (5)

  20. The occurrence of PKU is 1 per 10,000 births • q2 = 0.0001 • q = 0.01 p + q = 1 • The frequency of normal alleles is • p = 1 – q = 1 – 0.01 = 0.99 • The frequency of carriers is • 2pq = 2 x 0.99 x 0.01 = 0.0198 • or approximately 2% of the U.S. population p2 + 2pq + q2 = 1

  21. 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

  22. Natural Selection • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

  23. 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 • Founder Effect • Bottleneck Effect Less green alleles available Animation: Causes of Evolutionary Change

  24. The Founder Effect • The founder effect occurs when a few individuals become isolated from a larger population • Allele frequencies in the small founder population can be different from those in the larger parent population

  25. The Bottleneck Effect • The bottleneck effect is a sudden reduction in population size due to a change in the environment (fire or flood) • The resulting gene pool may no longer be reflective of the original population’s gene pool

  26. Effects of Genetic Drift: A Summary • Genetic drift is significant in small populations • Genetic drift causes allele frequencies to change at random • Genetic drift can lead to a loss of genetic variation within populations • Genetic drift can cause harmful alleles to become fixed

  27. Gene Flow • Gene flow occurs when a population gains or loses alleles by genetic additions or subtractions from the population. • Gene flow tends to reduce differences between populations over time • Gene flow occurs at a higher rate than mutations and is more likely to alter allele frequencies directly • Once introduced to a population, natural selection may then cause the new allele to increase or decrease in frequency

  28. Fig. 23-11 Gene flow and human evolution

  29. Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution • Only natural selection consistently results in adaptive evolution (organism is becomes better suited to its environment) by acting on an organism’s phenotype

  30. 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 other than battle • Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Selection favors certain genotypes by acting on the phenotypes of certain organisms

  31. 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

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

  33. The Key Role of Natural Selection in Adaptive Evolution • Natural selection increases the frequencies of alleles that enhance survival and reproduction • Adaptive evolution occurs as the match between an organism and its environment increases

  34. Because the environment can change, adaptive evolution is a continuous process • Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

  35. Sexual Selection • Sexual selection is natural selection for mating success. • Individuals with certain characteristics are more likely than other individuals to obtain mates. • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

  36. Fig. 23-15 Sexual dimorphism and sexual selection

  37. Types of Sexual Selection • Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex • Intersexual selection, often called mate choice,occurs when individuals of one sex (usually females) are choosy in selecting their mates • Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

  38. The Preservation of Genetic Variation • What prevents natural selection from reducing genetic variation by culling all unfavorable genotypes? • Various mechanisms help to preserve genetic variation in a population

  39. Diploidy • Diploidy maintains genetic variation in the form of hidden recessive alleles

  40. Balancing Selection • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population • Heterozygote Advantage • Frequency Dependent Selection

  41. Heterozygote Advantage • Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes • Natural selection will tend to maintain two or more alleles at that locus • The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

  42. Fig. 23-17 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 parasitic unicellular eukaryote) 7.5–10.0% 10.0–12.5% >12.5%

  43. Frequency-Dependent Selection • In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population • Selection can favor whichever phenotype is less common in a population

  44. Fig. 23-18 “Right-mouthed” 1.0 “Left-mouthed” Frequency of “left-mouthed” individuals 0.5 0 ’87 1981 ’82 ’84 ’86 ’89 ’90 ’83 ’85 ’88 Sample year

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

  46. Fig. 23-19

  47. Fig. 23-UN1 Evolved population Original population Disruptive selection Directional selection Stabilizing selection

  48. Fig. 23-UN2 Sampling sites (1–8 represent pairs of sites) 3 4 2 6 11 1 5 9 10 8 7 Allele frequencies lap94 alleles Other lap alleles Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation, American Scientist 75:134–141 (1987). Salinity increases toward the open ocean 8 7 6 5 4 3 2 Long Island Sound 1 9 N 10 Atlantic Ocean W E 11 S

  49. Fig. 23-UN3

  50. You should now be able to: • Explain why the majority of point mutations are harmless • Explain how sexual recombination generates genetic variability • Define the terms population, species, gene pool, relative fitness, and neutral variation • List the five conditions of Hardy-Weinberg equilibrium

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