ISOLATION AND GENETIC DIVERSITY OF GAMBUSIA-HUBBSI (MOSQUITOFISH) POPULATIONS IN BLUEHOLES ON ANDROS ISLAND, BAHAMAS

Among the many freshwater habitats occupied by the mosquitofish (Gambu sia hubbsi) on Andros Island, Bahamas, are blueholes: vertical caves t hat filled with water as sea levels rose during the past 15 000 years. At the present time, many of the blueholes on Andros Island appear to be highly isolated habitats analogous to islands. However, geographic al distance among blueholes and the geological history of bluehole for mation may also affect the genetic structure of G. hubbsi populations in blueholes. Thirty-two isozyme loci in 14 G. hubbsi populations inha biting blueholes and three inhabiting surface-water habitats were assa yed to gain insight into the effects of geological history and geograp hical distribution on population structure. Genetic structure among po pulations is high and significant (F-ST = 0.38-0.4) and Nei's genetic distance (D) is low among all populations. Although G. hubbsi populati ons are closely related, bluehole geology imposes significant barriers to gene flow. A pattern of isolation-by-distance was not evident in t he genetic data. Furthermore, none of the bluehole populations appears to be a source from which other bluehole populations were founded. Ra ther, they appear to be 'sinks' into which migration may occur rarely. Genetic drift appears to have had a profound effect on allozyme allel e frequencies in the G. hubbsi populations that were sampled. Local po pulation structure measured by allozymes appears to be more heavily in fluenced by historical population dynamics and stochastic migration th an by geographical locality or age of the blueholes.