Gene Flow in Scrub Jays: Frequency and Direction of Movement

Based on strong differences in plumage coloration between coastal California (californica subspecies group) and Great Basin (woodhouseii subspecies group) populations of Scrub Jays (Aphelocoma coerulescens), museum specimens representing gene flow between the two forms are identified. A total of 27 examples of apparent genetic exchange between two forms (individuals of one subspecies group taken within the range of the other) is documented. Immigration rates are on the order of one per hundred or one per thousand individuals, a rate sufficient to prevent differentiation by genetic drift alone if effective population sizes are in the range of 100-550 individuals. Gene flow east-to-west across the Mojave Desert is two to seven times stronger than west-to-east movement. This directional bias has theoretical implications because an important assumption (symmetry of gene flow patterns) of most theoretical treatments of the effects of gene flow is violated. If effective population sizes are comparable in the two forms, then the bias in gene flow should lead to an overall greater rate of differentiation in the genetically more isolated woodhouseii populations.