Lecture 5 - Chapter 23, Sections 1 and 2 - Evolutionary Mechanisms

Lecture 5 - Chapter 23, Sections 1 and 2 - Evolutionary Mechanisms

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Evolutionary Mechanisms I 2 Populations evolve, individuals do not In case you didn’t get that Populations evolve, individuals do not 3 The Modern Synthesis • Established in the 1930s and 1940s • Connected the units of evolution (genes) with the mechanism of evolution (selection on populations) • Integrated Darwinian evolution with Mendelian genetics + 4 Architects of the modern synthesis JBS Haldane G Ledyard Stebbins Theodosius Dobzhansky Sewall Wright
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5 Hardy Weinberg principle Given a series of assumptions, allele frequencies and genotype frequencies will not change between generations • The assumptions are: No selection No mutation No migration Infinitely large population Random mating First the simple approach: 6 AA Aa aa AA Aa aa aa Generation 1, “Gene pool” Generation 2, allele freq p and q allele freq p and q AA Aa aa AA Aa aa aa A a a A A a a A a If these assumptions are met, allele and genotype frequencies will not change between generations 7 Some terminology Imagine a populations made up of 100 diploid individuals: 40AA, 40Aa and 20aa The genotype frequencies are? ……… . • The allele frequencies are?……. The genotype numbers are? …… . The allele numbers are .. 8 Remember the multiplication and addition rules? If you just roll one die, what s the chance of getting a 4? What’s the chance of getting 4 twice in a row? The combined probability of independent events occurring together is the product of their individual probabilities
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9 If you just roll one die, what s the probability of getting either a 4 or a 6? The combined probability of 2 mutually exclusive events is the sum of their individual probabilities 10 Inheritance governed by laws of probability • Alleles of one gene segregate into gametes independently of another gene’s alleles • The probability of independent events A and B both happening = P(A) x P(B) • (=the multiplication rule) • The probability that either • A or B happens = P(A) + P(B) (= the addition rule) 11 Under Hardy-Weinberg equilibrium, allele frequencies predict genotype frequencies Here’s a numerical example using Punnett squares: 0.5 0.5 0.5 0.5 0.25 AA 0.50 Aa 0.25 aa Freq A = p = 0.5 Freq a = q = 0.5 .25 AA .25 aA .25 Aa .25 aa A A a a Eggs Sperm Aa x Aa 12 A A a a p q Eggs Sperm Here’s the same thing using symbols. .. p
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Lecture 5 - Chapter 23, Sections 1 and 2 - Evolutionary Mechanisms

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