SPECIATION - FOUNDER EVENTS AND THEIR EFFECTS ON X-LINKED AND AUTOSOMAL GENES

We analyze the role that random genetic drift and founder events might play in speciation and the evolution of reproductive isolation by exa mining the relative probabilities that X-linked and autosomal genes af fecting reproductive isolation will be involved in a drift-mediated pe ak shift. X chromosomes are more likely than autosomes to be affected by drift. However, the smaller effective population size and increased drift variance of X-linked genes are exactly offset by their lower, w ithin-population, equilibrium genetic variance at mutation-selection b alance. Thus, the among-deme variance of X-linked genes is equivalent to that of autosomal genes. However, the third and higher moments of t he distribution of gene frequencies among populations do differentiall y affect the relative probabilities of peak shifts by founder event pr ocesses, such that traits controlled by a single additive X-linked gen e are more likely to shift than with a similar autosomal gene. Genes a ffecting only one sex are more likely to undergo peak shifts than gene s affecting both sexes because of higher gene frequencies at mutation- selection balance. Sex specificity may make genes affecting fertility more likely to undergo peak shifts than genes affecting viability. We conclude that the hypothesis of speciation by drift-mediated peak shif ts requires special assumptions concerning the genetic conditions that obtain before as opposed to after a founder event and depend on the n ature of genetic variation for reproductive isolation.