BIOLOGY 1407 CHAPTER 23 THE EVOLUTION OF POPULATIONS

5. Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible

6. Altering Gene Number or Position Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Duplication of small pieces of DNA increases genome size and is usually less harmful Duplicated genes can take on new functions by further mutation An ancestral odor-detecting gene has been duplicated many times: humans have 1,000 copies of the gene, mice have 1,300

7. Rapid Reproduction Mutation rates are low in animals and plants The average is about one mutation in every 100,000 genes per generation Mutations rates are often lower in prokaryotes and higher in viruses

8. Sexual Reproduction Sexual reproduction can shuffle existing alleles into new combinations In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

10. Figure 23.6 Figure 23.6 One species, two populations.Figure 23.6 One species, two populations.

11. 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 Quantitative characters vary along a continuum within a population

12. Genetic variation can be measured as gene variability or nucleotide variability For gene variability, average heterozygosity measures the average percent of loci that are heterozygous in a population Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

13. Variation Between Populations Most species exhibit geographic variation, differences between gene pools of separate populations For example, Madeira is home to several isolated populations of mice Chromosomal variation among populations is due to drift, not natural selection

14. Figure 23.4 Figure 23.4 Geographic variation in isolated mouse populations on Madeira.Figure 23.4 Geographic variation in isolated mouse populations on Madeira.

15. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis For example, mummichog fish vary in a cold-adaptive allele along a temperature gradient This variation results from natural selection

16. Figure 23.5 Figure 23.5 A cline determined by temperature.Figure 23.5 A cline determined by temperature.

18. 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 Microevolution is a change in allele frequencies in a population over generations

19. Figure 23.1 Is this finch evolving by natural selection?Figure 23.1 Is this finch evolving by natural selection?

20. Genetic Variation Variation in individual genotype leads to variation in individual phenotype Not all phenotypic variation is heritable Natural selection can only act on variation with a genetic component

21. Figure 23.2 Nonheritable variationFigure 23.2 Nonheritable variation

23. Figure 23.7 Figure 23.7 Selecting alleles at random from a gene pool.Figure 23.7 Selecting alleles at random from a gene pool.

24. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool Consider, for example, the same population of 500 wildflowers and 100 alleles where p ? freq CR ? 0.8 q ? freq CW ? 0.2

25. Figure 23.8 Figure 23.8 The Hardy-Weinberg principle.Figure 23.8 The Hardy-Weinberg principle.

28. Applying the Hardy-Weinberg Principle We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:

29. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions The population is large Migration has no effect as many other populations have similar allele frequencies

31. 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 Changes in allele frequencies result from random events Genetic drift tends to reduce genetic variation through losses of alleles

32. Figure 23.8 Genetic driftFigure 23.8 Genetic drift

33. The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population Allele frequencies in the small founder population is likely to be different from those in the larger parent population What about here in Hawaii? What do you know about native populations?

34. The Bottleneck Effect The bottleneck effect is a sudden and drastic reduction in population size due to a change in the environment The resulting gene pool does not reflect the original population�s gene pool If the population remains small, it may be further affected by genetic drift and inbreeding

35. Figure 23.9 The bottleneck effectFigure 23.9 The bottleneck effect

36. Figure 23.10 Bottleneck effect and reduction of genetic variationFigure 23.10 Bottleneck effect and reduction of genetic variation

37. Gene Flow Gene flow is the movement of alleles between populations Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) Gene flow tends to reduce differences between populations over time Gene flow is more likely than mutation to alter variations in allele frequencies of different populations

38. Figure 23.11 Gene flow and human evolutionFigure 23.11 Gene flow and human evolution

39. Gene flow can decrease the fitness of a population Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland Mating causes gene flow between the central and eastern populations Immigration from the mainland introduces alleles that decrease fitness Natural selection selects for alleles that increase fitness Birds in the central region with high immigration have a lower fitness; birds in the east with low immigration have a higher fitness

40. Figure 23.12 Figure 23.12 Gene flow and local adaptation.Figure 23.12 Gene flow and local adaptation.

41. Assortive Mating Nonrandom mating does not change gene frequencies but does tend to increase the number of homozygous loci This can lead to genetic problems in small populations where inbreeding is a common occurrence

43. 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

44. Figure 23.14 Examples of adaptationsFigure 23.14 Examples of adaptations

45. Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels Phenotypicly heterozygosity is expressed by variation in phenotypic expression.

48. Genetic Variation

49. Figure 23.17 Mapping malaria and the sickle-cell allele Figure 23.17 Mapping malaria and the sickle-cell allele

52. 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

53. 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

54. 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

55. Figure 23.13 Modes of selectionFigure 23.13 Modes of selection

56. 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

57. Figure 23.15 Sexual dimorphism and sexual selectionFigure 23.15 Sexual dimorphism and sexual selection

58. 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

59. How do female preferences evolve? The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be favorably selected.

60. Figure 23.16 Do females select mates based on traits indicative of �good genes�? Figure 23.16 Do females select mates based on traits indicative of �good genes�?

61. Figure 23.19 Figure 23.19 Evolutionary compromise.Figure 23.19 Evolutionary compromise.