Marked genetic divergence revealed by allozymes among populations of the guppy Poecilia reticulata (Poeciliidae), in Trinidad

Populations of the guppy Poecilia reticulata from six locations in N. Trinidad were examined by starch gel electrophoresis to estimate their degree of genetic divergence. Variability at seven enzyme‐coding loci demonstrated that populations differed markedly in allele frequencies with some allelic substitution between sites (for 23 loci, Nei's mean genetic identity, Ī ranged from 0.869–1.00; the coefficient of gene differentiation, G_ST= 0.086; and the absolute differentiation between populations, D̄_m= 0.044). There was a good correspondence between degree of physical isolation and extent of genetic differentiation, as exemplified by the partitioning of gene diversity (Nei's gene diversity analysis = 66% between river basins; 32% within river basins; 2% within rivers), though there was considerable variability in the contribution of individual loci. Most populations were in Hardy‐Weinberg equilibrium, and any significant deviations were due exclusively to heterozygote deficiencies. The patterns of population differentiation are discussed in relation to probable past geological and historical events, and present‐day evolutionary forces. Notable among these are previous continental land connections, periodic immigration from N.E. South America, and post‐colonization events including differential predation, sexual selection, apparent restricted vagility and small effective population sizes.