Population differentiation in Trinidadian guppies (Poecilia reticulata): patterns and problems

Populations of the guppy, Poecilia reficulata, in N. Trinidad exhibit marked population differentiation in allozyme frequencies. Here we investigate six further populations electrophoretically at 25 enzyme‐coding loci to examine patterns in geographical structuring, genotypic distributions and genetic diversity. With one exception, possibly related to an experimental introduction, populations divided broadly (dendrogram of Nei's mean genetic identity, Ī) in accordance with proposed ancestral colonization. Most populations were in Hardy Weinberg equilibrium, though some significant deficits in heterozygotes were detected. Incorporating information from published data, markedly hjgher levels of genetic diversity (mean observed heterozygosity, H̄_o) were recorded in lowland [H̄_o= 0.0382 ± k0406 (s.e.), n= 9] compared with upstream populations [H̄_o= 0.0112 ± 00034 (S.E.), n = 9]. Patterns are discussed in relation to historical and present‐day evolutionary forces.