Substitute the fitnesses (wii) in condition I above into the expression ∆p = pt+1- pt and prove for yourself that the equations on page 101 (eqn. 5.5) is related to the expression for pt+1shown above. First three are directional in that selection stops only when allele is eliminated. In I the elimination process slows down because as q becomes small the a alleles are usually in heterozygote state and there is no phenotypic variance. In II selection is slow at first because with q small most genotypes are AA so there is low phenotypic variance; as selection eliminates A alleles q increases and the frequency of the favored genotype (aa) increases so selection accelerates. III is like the worked example run to fixation/loss. IV is known as balancing selectiondue to overdominance(heterozygotes are "more" than either homozygote). Both alleles maintained in population by selection. This is an example of a polymorphic equilibrium(fixation/loss is also an equilibrium condition but it is not polymorphic). The frequencies of the alleles at equilibrium will be:
This is the end of the preview.
access the rest of the document.