| The study of island endemism offers an unparalleled opportunity to identify fundamental genetic and environmental mechanisms that underlie the speciation process. Here we examine the evolution of the Drosophila dunni subgroup, a radiation of nine taxa that are endemic to the islands of the Eastern Caribbean. The subgroup exhibits a remarkable interspecific cline in abdominal pigmentation that increases in intensity with decreasing latitude. Our goals are to elucidate the phylogenetic relationships among these species, to characterize demographic and biogeographic patterns that may have played a role in speciation, and to identify the genetic basis of the cline in abdominal pigmentation.;In order to estimate the divergence between these species, we have created a library of microsatellites that can be amplified across the subgroup. We have further explored the molecular evolution of these microsatellites by comparing their sequence basis among species, and describing their relationship with a previously unidentified mobile element. Although the patterns revealed by this study are interesting, we find these microsatellites to be unstable among even very closely related species, making them inappropriate for our phylogenetic study.;Instead, we have used DNA sequence data from four unlinked genes in order to explore patterns of intra- and interspecific variation across the D. dunni subgroup. Our results show that many members of the subgroup have diverged remarkably recently, in perhaps as little as the last 23,000 years, with no evidence that population bottlenecks have contributed to these speciation events.;The recent divergence of these species makes the cline in pigmentation even more intriguing. We used the candidate gene approach to identify possible footprints of natural selection at two additional loci, yellow and Dopa decarboxylase. We have found an excess of amino acid substitutions occurring at the 5' end of the yellow gene between two lineages of the D. dunni subgroup, indicating that natural selection has acted on this locus. This variation occurs in the signal peptide and has implications for the post-translational regulation of the yellow protein. Post-translational regulatory mechanisms have not previously been explored as a means through which selection can act to cause adaptive changes. |
