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Chapter 23

Chapter 23. The Evolution of Populations. 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

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Chapter 23

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  1. Chapter 23 The Evolution of Populations

  2. 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 Microevolutionis a change in allele frequencies in a population over generations Overview: The Smallest Unit of Evolution

  3. Fig. 23-1 Is this finch evolving by natural selection?

  4. Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals Evolution

  5. 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 Genetic Variation

  6. Fig. 23-2 Diet related appearances (b) (a) Feed on the flowers Feed on the leaves

  7. Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis (colors of pea flowers) Quantitative characters vary along a continuum within a population (hair color) Variation Within a Population

  8. Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels Average heterozygositymeasures the average percent of loci that are heterozygous in a population Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

  9. Fig. 23-3 Variation Between Populations • Most species exhibit geographic variation • *fused chromosomes haven't caused a genetic change • *therefore the mice look the same Island of Madeira 8.11 1 2.4 3.14 5.18 6 7.13 8.11 9.12 10.16 19 13.17 XX Mus musculus 1 2.19 3.8 6.7 4.16 5.14 15.18 XX 13.17 9.10 11.12

  10. Fig. 23-4 1.0 0.8 Cline is a graded change in a trait along a geographic axis 0.6 Ldh-Bb allele frequency 0.4 0.2 0 44 42 40 34 46 38 36 32 30 Latitude (°N) Maine Cold (6°C) Georgia Warm (21°C)

  11. Mutationsare changes in the nucleotide sequence of DNA Mutations cause new genes and alleles to arise Only mutations in cells that produce gametes can be passed to offspring Mutation

  12. A point mutation is a change in one base in a gene The effects of point mutations can vary: 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 Point Mutations

  13. 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 Mutations That Alter Gene Number or Sequence

  14. 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 Mutation Rates

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

  16. A populationis a localized group of individuals capable of interbreeding and producing fertile offspring A gene poolconsists of all the alleles for all loci in a population A locus is fixed if all individuals in a population are homozygous for the same allele Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving

  17. Fig. 23-5 Porcupine herd MAP AREA CANADA ALASKA Beaufort Sea NORTHWEST TERRITORIES Porcupine herd range Shared area Fortymile herd range YUKON ALASKA Fortymile herd

  18. The frequency of an allele in a population can be calculated For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

  19. By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies The frequency of all alleles in a population will add up to 1 For example, p + q = 1

  20. 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 The Hardy-Weinberg Principle

  21. The Hardy-Weinberg principlestates that 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 Hardy-Weinberg Equilibrium

  22. Fig. 23-6 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

  23. Hardy-Weinberg equilibriumdescribes 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 where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

  24. 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, and 4% 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, and 4% CW CW plants

  25. The Hardy-Weinberg theorem describes a hypothetical population, not always occurs in nature In real populations, allele and genotype frequencies do change over time Conditions for Hardy-Weinberg Equilibrium

  26. 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

  27. Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: The PKU gene mutation rate is low Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele Applying the Hardy-Weinberg Principle

  28. 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

  29. The occurrence of PKU is 1 per 10,000 births q2 = 0.0001 q = 0.01 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

  30. Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

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

  32. The smaller a sample, the greater the chance of deviation from a predicted result (Tay-Sachs) Genetic driftdescribes how allele frequencies fluctuate unpredictably from one generation to the next Genetic drift tends to reduce genetic variation through losses of alleles Genetic Drift

  33. Fig. 23-8-3 CW CW CR CR CR CR CR CR CR CR CR CW CR CW CR CR CR CR CR CR CR CR CR CR CW CW CW CW CR CR CR CW CR CR CR CR CR CW CR CR CW CW CR CR CR CW CR CR CR CR CR CR CR CW CR CW CR CR CR CW Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 p = 1.0 p = 0.5 q (frequency of CW) = 0.3 q = 0.0 q = 0.5

  34. The founder effectoccurs 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 The Founder Effect

  35. The bottleneck effectis a sudden reduction in population size due to a change in the environment The resulting gene pool may no longer be reflective of the original population’s gene pool If the population remains small, it may be further affected by genetic drift Gene flow is a change in the allele frequency due to transfer of alleles into or out of the pool The Bottleneck Effect

  36. The Bottleneck Effect

  37. Example of Bottleneck effect: Attwater Chicken From 1,000,000 in 1900s to less than 100 today Lack of grassland, environmental pressure (predators, fireants) Low genetic pool

  38. Fig. 23-11 Gene flow

  39. Gene flow can decrease the fitness of a population In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils Windblown pollen moves these alleles between populations The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

  40. Gene flow can increase the fitness of a population Insecticides have been used to target mosquitoes that carry West Nile virus and malaria Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes The flow of insecticide resistance alleles into a population can cause an increase in fitness

  41. Natural selection brings about adaptive evolution by acting on an organism’s phenotype 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 Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

  42. Relative fitnessis 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

  43. Three modes of selection: Directional selectionfavors individuals at one end of the phenotypic range Disruptive selectionfavors individuals at both extremes of the phenotypic range Stabilizing selectionfavors intermediate variants and acts against extreme phenotypes Directional, Disruptive, and Stabilizing Selection

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

  45. 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 The Key Role of Natural Selection in Adaptive Evolution

  46. Fig. 23-14 Enables it to hide from predators and surprise prey (a) Color-changing ability in cuttlefish Movable bones Allows them to swallow food much larger than the mouth (b) Movable jaw bones in snakes

  47. 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

  48. Sexual selectionis natural selection for mating success It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Sexual Selection

  49. Intrasexual selectionis 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

  50. Various mechanisms help to preserve genetic variation in a population Diploidy maintains genetic variation in the form of hidden recessive alleles The Preservation of Genetic Variation

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