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Hardy-Weinberg • The Hardy-Weinberg theorem (p2+2pq+q2 = 1) describes gene frequencies in a stable population that are well adapted to the environment. It assumes the following: – – – – – Extremely large populations No gene flow between populations Random mating No natural selection No mutation • As you can see the chance for a these conditions leading to a stable population is entirely improbable. Any departure in these conditions leads to evolution. Why it doesn’t work Application of Hardy Weinberg • An investigator has determined by inspection that 16% of a human population has a recessive trait (tt). Complete all the genotype and allele frequencies for this population, assuming that it is in Hardy-Weinberg equilibrium. • p2+2pq+q2 = 1 • • • • & p+q=1 Since the trait is expressed in the homozygous form (tt) .16 = q2 q: t2 = 0.16, √t2 = √16 = 0.4 so p + q = 1, p + 0.4 = 1, p = 0.6 p2(TT)= .62 = 0.36 2pq (Tt) = 2(0.6)(0.4) = 0.48 • Genetic Drift - deviations from expected (Hardy-Weinberg) frequencies – bottle-neck effect • caused by a disaster suddenly altering the environmental pressures causing the genes of a few survivors to dominate – founder effect • when a few individuals move and start a new population allowing their genes to dominate – gene flow • changes to a gene pool as individuals move out and into a breeding population • tends to reduce differences between populations Agents of Evolution Stabilization of a gene pool • Directional selection – selection toward a phenotype that is best adapted to the environment – most common type of selection • Disruptive selection – occurs when environmental factors favor individuals on both extremes of the phenotypic range – often to facilitate different food sources Stabilization of a gene pool • Stabilizing selection – selects against the extremes of phenotypes – humans • Balancing Selection – occurs when nature allows 2 distinctly different polymorphisms to exist in relatively equal frequencies (balanced polymorphism) – leads to heterozygous advantage and frequency dependent selection • heterozygous advantage - heterozygous individuals exhibit less genetic disorders • frequency dependent selection frequency of the dominant phenotype becomes less fit over time (prey species)