15The nascent stages of speciation start with the emergence of sexual isolation. 16 Understanding the influence of reproductive barriers in this evolutionary process is an 17 ongoing effort. We present a study of Drosophila melanogaster populations from the 18 southeast United States and Caribbean islands undergoing incipient sexual isolation.

Santa Catalina Island is a small island off the coast of southern California and for its modest size harbors several species of flies from the Drosophila genus. We performed an island-wide survey of Drosophila species to ascertain which species were endemic to the island and where they were most abundant. In doing so, we have assembled useful sampling information for researchers who wish to conduct field studies on Santa Catalina Island. From this survey, we determined that Drosophila hamatofila, Drosophila mainlandi, and Drosophila mettleri were the prominent repleta species on the island. Other repleta species encountered included Drosophila mojavensis and Drosophila wheeleri. Non-repleta species sighted on the island include Drosophila melanogaster, Drosophila pseudoobscura, Drosophila simulans, and an unknown species not seen before on the island. Additionally, we performed seasonal collections at two locations on the island and observed that species abundance and composition at these two sites vary between seasons. One of the seasonal sites was sampled in two consecutive summer seasons, which revealed that species composition had shifted between years, but relative species abundances were approximately the same.