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Chapter 23

Chapter 23. Evolution of Populations. Question? . Is the unit of evolution the individual or the population? Answer = while evolution effects individuals, it can only be tracked through time by looking at populations. So what do we study? . We need to study populations , not individuals

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Chapter 23

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  1. Chapter 23 Evolution of Populations

  2. Question? • Is the unit of evolution the individual or the population? • Answer = while evolution effects individuals, it can only be tracked through time by looking at populations

  3. So what do we study? • We need to study populations, not individuals • We need a method to track the changes in populations over time • This is the area of Biology called ‘population genetics’

  4. Population Genetics • The study of genetic variation in populations. • Represents the reconciliation of Mendelism and Darwinism. • Modern Synthesis uses population genetics as the means to track and study evolution • Looks at the genetic basis of variation and natural selection

  5. Population • A localized group of individuals of the same species.

  6. Species • A group of similar organisms. • A group of populations that could interbreed.

  7. Gene Pool • The total aggregate of genes in a population. • If evolution is occurring, then changes must occur in the gene pool of the population over time.

  8. Microevolution • Changes in the relative frequencies of alleles in the gene pool.

  9. Overview • A common misconception is that organisms evolve, in the Darwinian sense, during their lifetimes • Natural selection acts on individuals, but only populations evolve • Individuals are selected; populations evolve!

  10. The Smallest Unit of Evolution • Genetic variations in populations contribute to evolution • Microevolution is a change in allele frequencies in a population over generations

  11. Is this finch evolving by natural selection?

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

  13. 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

  14. Nonheritablevariation?

  15. Nonheritablevariation?

  16. 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

  17. 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

  18. Variation Between Populations • Most species exhibit geographic variation,differences between gene pools of separate populations or population subgroups Geographic variation in isolated mouse populations on Madeira

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

  20. 1.0 0.8 Maine Cold (6°C) 0.6 Ldh-Bb allele frequency 0.4 Georgia Warm (21°C) 0.2 0 44 42 40 34 46 38 36 32 30 Latitude (°N) A cline determined by temperature

  21. Mutation • Mutations are 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

  22. 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 in a gene might not affect protein production because of redundancy in the genetic code

  23. Point mutations • 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

  24. 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

  25. Mutations That Alter Gene Number or Sequence • 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

  26. Hardy-Weinburg

  27. Hardy-Weinberg Theorem • Developed in 1908. • Mathematical model of gene pool changes over time.

  28. 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

  29. 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

  30. Basic Equation • p + q = 1 • p = % dominant allele • q = % recessive allele

  31. Expanded Equation • p + q = 1 • (p + q)2 = (1)2 • p2 + 2pq + q2 = 1

  32. Genotypes • p2 = Homozygous Dominants2pq = Heterozygousq2 = Homozygous Recessives

  33. 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

  34. The Hardy-Weinberg principle states 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

  35. 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 Selecting alleles at random from a gene pool

  36. 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 • -where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

  37. Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies do change over time • Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

  38. 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

  39. Example Calculation • Let’s look at a population where: • A = red flowers • a = white flowers

  40. Starting Population • N = 500 • Red = 480 (320 AA+ 160 Aa) • White = 20 • Total alleles = 2 x 500 = 1000

  41. Dominant Allele • A = (320 x 2) + (160 x 1) = 800 = 800/1000 A = 80%

  42. Recessive Allele • a = (160 x 1) + (20 x 2) = 200/1000 = .20 a = 20%

  43. A and a in HW equation • Cross: Aa X Aa • Result = AA + 2Aa + aa • Remember: A = p, a = q

  44. Substitute the values for A and a • p2 + 2pq + q2 = 1 (.8)2 + 2(.8)(.2) + (.2)2 = 1 .64 + .32 + .04 = 1

  45. Dominant Allele • A = p2 + pq = .64 + .16 = .80 = 80%

  46. Recessive Allele • a = pq + q2 = .16 + .04 = .20 = 20%

  47. Importance of Hardy-Weinberg • Yardstick to measure rates of evolution. • Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold (no evolution should occur). • Way to calculate gene frequencies through time.

  48. Example • What is the frequency of the PKU allele? • PKU is expressed only if the individual is homozygous recessive (aa).

  49. 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 • Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

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