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

Agents of Evolution 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

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)