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Hardy-Weinberg equilibrium

Hardy-Weinberg equilibrium. if p = frequency of allele A q = frequency of allele a p + q = 1, ( p + q ) 2 = 1 p 2 + 2pq + q 2 = 1 if only law of probability affects the frequency w/ which gametes combine to form new individuals. Bisexual population

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Hardy-Weinberg equilibrium

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  1. Hardy-Weinberg equilibrium • if p = frequency of allele A q = frequency of allele a • p + q = 1, ( p + q )2 = 1 • p2 + 2pq + q2 = 1 • if only law of probability affects the frequency w/ which gametes combine to form new individuals

  2. Bisexual population • Large population • Random mating • No mutation • Migration ~ 0 • Natural selection does not affect the locus

  3. A population that is in Hardy-Weinberg equilibrium will experience no change in either genotype frequency or allele frequency • If one or more of the conditions is violated, genotype frequency and allele frequency will change

  4. Example • If only 6% of the population displays pale eyes (recessive gene e). What is the frequency of genotype Ee in this population? q2 = 0.06 ---> q = 0.24 p + q = 1 ---> p = 0.76 Ee = 2pq = 2(0.76)(0.24) = 0.36

  5. Genetic variation • Cline – a measurable, gradual change over a geographical region in the average of some phenotype character • Ecotype – abrupt changes in the phenotype characters within a species, which often reflect abrupt changes in local environment

  6. Geographic isolates – semi-isolated populations prevented by some extrinsic barriers from a free flow of genes • Genetic polymorphism – the existence within a species or population of different forms of individuals

  7. Maintenance of balanced polymorphism (vs. transitional or directional polymorphism) • heterosis • diversifying evolution • frequency-dependent selection • selective forces operating in different directions within different patches of a fine mosaic in the population

  8. Stabilizing, directional, diversifying or disruptive evolution • Speciation • Allopatric speciation • Sympatric speciation

  9. Isolation mechanism • Pre-zygotic: habitat, temporal, ethological, mechanical • Post-zygotic • hybrid inviability or weakness • hybrid sterility • F2 breakdown

  10. Reduction in variation • inbreeding, bottlenecks, founder effect, genetic drift • genetic drift - random shifts in allele frequencies

  11. Effect of small populations • More demographic variation, inbreeding depression, genetic drift → higher risk of extinction Minimum viable population size • the threshold # of individuals that will ensure the persistence of subpopulation in a viable state for a given time interval

  12. Effective population size (Ne) • the size of a genetically idealized population with which an actual population can be equated genetically, Ne = N , if • equal sex ratio • equal probability of mating • constant dispersal rate • progeny per family randomly distributed

  13. unequal sex ratio Ne = 4 Nm˙Nf / (Nm + Nf ) • population fluctuation 1 / Ne = (1 / t )(1/N1 + 1/N2 + … + 1/Nt) • non-random progeny distribution Nk Ne = ----------------------------------------- (N/N-1)˙Vk/k˙(1+F) + (1-F)

  14. Effect of continental drift • Distribution/radiation of monotremes and marsupials

  15. Extinction and its causes • Natural causes: climatic changes and stochastic event • Human disturbance • habitat alteration • over-exploitation • exotic species • diseases and other factors

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