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IF: genotypes differ in average fitness THEN:

Selection can change population genetic structure. Selection – nonrandom survival or reproductive success (RS) of different phenotypes – differential reproductive success of different phenotypes. When can selection lead to evolution?

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IF: genotypes differ in average fitness THEN:

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  1. Selectioncan change population genetic structure Selection – nonrandom survival or reproductive success (RS) of different phenotypes – differential reproductive success of different phenotypes When can selection lead to evolution? When different phenotypes represent different genotypes (i.e., when phenotypic differences are heritable). R = S x h2 IF: genotypes differ in average fitness THEN: some genotypes will contribute more alleles to future generations

  2. sperm 0.5 A 0.5 a 0.5 A eggs 0.5 a Howdoes selection cause change in allele frequencies? 0.25 AA 0.25 Aa Parental allele freq: 0.5A 0.5a 0.25 Aa 0.25 aa 1000 zygotes: 250 AA 500Aa 250 aa Survival to repro: 100% for AA 75% for Aa 50% for aa Survivors: 250 AA 375Aa 125 aa 750 indv’s Offspring allele freq: 250 x 2 + 375 = 875 / 1500 = 0.58 A 125 x 2 + 375 = 625 / 1500 = 0.42 a Did allele frequencies change? Why?

  3. sperm 0.5 A 0.5 a 0.5 A eggs 0.5 a How does selection cause change in allele frequencies? 0.25 AA 0.25 Aa Parental allele freq: 0.5A 0.5a 0.25 Aa 0.25 aa 1000 zygotes: 250 AA 500Aa 250 aa Survival to repro: 100% for AA 75% for Aa 50% for aa Survivors: 250 AA 375Aa 125 aa 750 indv’s Offspring allele freq: 250 x 2 + 375 = 875 / 1500 = 0.58 A 125 x 2 + 375 = 625 / 1500 = 0.42 a What if selection continued over many generations? Use AlleleA1 to find out.

  4. 1000 zygotes: 250 AA 500Aa 250 aa Survival to repro: 100% for AA 75% for Aa 50% for aa Survivors: 250 AA 375Aa 125 aa 750 indv’s sperm 0.5 A 0.5 a 0.5 A eggs 0.5 a How does selection cause change in allele frequencies? 0.25 AA 0.25 Aa Parental allele freq: 0.5A 0.5a Genotype freq: 250 / 750 = 0.333 375 / 750 = 0.500 125 / 750 = 0.167 0.25 Aa 0.25 aa H-W genotype freq: p2 = (0.582)2 = 0.340 2pq = 2(0.582)(0.417) = 0.486 q2 = (0.417)2 = 0.174 Offspring allele freq: 250 x 2 + 375 = 875 / 1500 = 0.58 A 125 x 2 + 375 = 625 / 1500 = 0.42 a Is the population in Hardy-Weinberg equilibrium?

  5. How does selection cause change in allele frequencies? What if ais recessive lethal at birth? Parental allele freq: 0.5A 0.5a 1000 zygotes: 250 AA 500Aa 250 aa Survival: 100% for AA 100% for Aa 0% for aa Survivors: 250 AA 500Aa 0 aa 750 indv’s Genotype freq: 250 / 750 = 0.333 500 / 750 = 0.667 0 / 750 = 0.0 Offspring allele freq: 250 x 2 + 500 = 1000 / 1500 = 0.667 A 0 x 2 + 500 = 500 / 1500 = 0.333 a H-W genotype freq: p2 = (0.667)2 = 0.444 2pq = 2(0.667)(0.333) = 0.444 q2 = (0.333)2 = 0.111 What if selection continued over many generations? Use AlleleA1 to find out.

  6. How does selection cause change in allele frequencies? Dominance and allele frequency interact to determine rate of evolution. Recessive allele is COMMON: 0.1A 0.9a 1000 zygotes: 10 AA 180Aa 810 aa Survival: 100% for AA 100% for Aa 0% for aa Survivors: 10 AA 180Aa 0 aa 190 indv’s When a deleterious recessive allele is COMMON, evolution by natural selection is RAPID Offspring allele freq: 10 x 2 + 180 = 200 / 380 = 0.526 A 0 x 2 + 180 = 180 / 380 = 0.474 a Change in frequency of A? Change in frequency of a? 0.526 – 0.1 = 0.426 0.474 – 0.9 = - 0.426 What % of a alleles are exposed to selection? (810 x 2) / (810 x 2 + 180) = 1620 / 1800 = 90%

  7. How does selection cause change in allele frequencies? Dominance and allele frequency interact to determine rate of evolution. Recessive allele is RARE: 0.9A 0.1a 1000 zygotes: 810 AA 180Aa 10 aa Survival: 100% for AA 100% for Aa 0% for aa Survivors: 810 AA 180Aa 0 aa 990 indv’s When a deleterious recessive allele is RARE, evolution by natural selection is SLOW Offspring allele freq: 810 x 2 + 180 = 1800 / 1980 = 0.909 A 0 x 2 + 180 = 180 / 1980 = 0.091 a Change in frequency of A? Change in frequency of a? 0.909 – 0.9 = 0.009 0.091 – 0.1 = - 0.009 What % of a alleles are exposed to selection? (10 x 2) / (10 x 2 + 180) = 20 / 200 = 10%

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