Is the CFTR allele maintained by mutation/selection balance? In some populations: Freq of Cystic Fibrosis: 2% Under mutation/selection balance, need: m = 4 x 10-4 However the actual rate is 6.7 x 10-7 too low! Cultured mouse cells with CFTR genotypes The fitness cost of dF508 / dF508 with respect to Pseudomonas is balanced by the fitness advantage of dF508 / + with respect to Salmonella Incorporating Migration Island Geneflow Continent Migration can alter allele and genotype frequencies. Migration is a homogenizing force; it prevents divergence of populations Lake Erie Water Snakes Banded vs Unbanded Natural Selection for Unbanded Forms on Islands Banded Unbanded Incorporating Migration Banded alleles Island Opposed by Natural Selection Continent It is possible to calculate the change in frequency of the banded allele (q) as a function of q. Change in frequency of the banded allele (q) as a function of q. Equilibrium points Incorporating Genetic Drift In populations of finite size, sampling of gametes Probability of Maintaining the Same Initial Allele Frequency The Ultimate Fate of Random Genetic Drift The Effects of Drift are More Pronounced in Smaller Populations 8 pops The frequency of heterozygotes decreases under drift. Hg+1 = Hg[1-1/2N] N=16 N=9 Ne = 4NmNf / (Nm + Nf) Effective population size N=16 Rate of Evolution by Genetic Drift Rate of evolution equals rate that an allele is fixed at a locus. Depends upon: (2Nu) number of mutations arising at locus per generation, and initial frequency of new allele (1/2N) K = 2Nm x 1/2N = m rate of substitution = rate of mutation! Neutralist view: allele substitution and polymorphism are determined by the same evolutionary process. • Mutation provides a continual supply of new alleles. Because many alleles are neutral or effectively neutral, alleles becomes fixed or lost from a population as a result of genetic drift. • Polymorphism is simply a snapshot of a continuous process of mutational input and subsequent random extinction or fixation of alleles. Mootoo Kimura’s concept of neutralism is illustrated in the following diagram from his original paper. Selectionist view: allele substitution and polymorphism are determined by different, selective processes. • Mutation yields advantageous alleles that are driven to fixation by positive natural selection. • Two or more alleles are maintained at a locus in a population by over dominance.