1. ¹Department of Zoology, 1735 Neil Avenue, The Ohio State University, Columbus, Ohio 43210, USA
2. ²Department of Biology, George Mason University, Fairfax, Virginia 22030, USA
3. ³Department of Molecular Genetics, 484 West 12th Avenue, The Ohio State University, Columbus, OH 43210, USA

Correspondence: Malcolm D Schug, Section of Genetics and Development, Biotechnology Building, Cornell University, Ithaca, NY 14853, USA

⁴Present address: Section of Genetics and Development, Biotechnology Building, Cornell University, Ithaca, NY 14853, USA

Received 6 February 1997.

Abstract

Among the many freshwater habitats occupied by the mosquitofish (Gambusia hubbsi) on Andros Island, Bahamas, are blueholes: vertical caves that 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, geographical distance among blueholes and the geological history of bluehole formation may also affect the genetic structure of G. hubbsi populations in blueholes. Thirty-two isozyme loci in 14 G. hubbsi populations inhabiting blueholes and three inhabiting surface-water habitats were assayed to gain insight into the effects of geological history and geographical distribution on population structure. Genetic structure among populations is high and significant (F_ST=0.38- 0.4) and Nei's genetic distance (D) is low among all populations. Although G. hubbsi populations are closely related, bluehole geology imposes significant barriers to gene flow. A pattern of isolation-by-distance was not evident in the genetic data. Furthermore, none of the bluehole populations appears to be a source from which other bluehole populations were founded. Rather, they appear to be 'sinks' into which migration may occur rarely. Genetic drift appears to have had a profound effect on allozyme allele frequencies in the G. hubbsi populations that were sampled. Local population structure measured by allozymes appears to be more heavily influenced by historical population dynamics and stochastic migration than by geographical locality or age of the blueholes.