Rapid response to perturbation of chromosome frequencies in natural populations of Drosophila robusta.

Perturbation of gene or chromosome frequencies in natural populations is one of the most powerful ways of demonstrating whether natural selection maintains genetic polymorphism or if other evolutionary forces are at work. Gene arrangement frequencies in two natural populations of Drosophila robusta were perturbed multiple times by releasing adult flies with contrasting karyotypes and carefully monitoring post-perturbation presence of hybrids and chromosome frequencies. In all cases, frequencies quickly returned to pre-perturbation levels, and in the following sampling periods, no evidence of the introduced chromosomes was apparent. Analysis of post-perturbation frequency changes included tests for heterogeneity among chromosome arrangements in rates of return to equilibrium values using population admixture analysis. In several cases, significant heterogeneity was detected indicating some form of natural selection was operating. Technical challenges to carrying out perturbation experiments in the wild are also discussed.