Artificial introductions, evolutionary change and population differentiation in Trinidadian guppies (Poecilia reticulata: Poeciliidae)

Abstract

The evolutionary consequences of three artificial introductions of the guppy,Poecilia reticulata, in Trinidad were examined by comparing the allozymic structure (observed heterozygosity (Ho), and mean number of alleles (Na)) of each corresponding source (S) and transplant (T) population. In ‘Haskins’ (H) and ‘Endler's’ (E) introduction, 200 guppies (half female) were transferred to guppy-free habitats in 1957 and 1976 respectively. ‘Kenny's’ (K) introduction in 1981 involved the release of a single pregnant female into an isolated ornamental pond. Analysis of allozyme frequencies at 25 enzyme-coding loci revealed reductions in observed heterozygosity at some loci in all three transplant samples, and a marked decline in the mean number of alleles in Kenny's pond sample. Significant genetic differentiation occurred between (S) and (T) samples at some loci in all introductions, but was most marked in H(T) and K(T). Despite previous studies on rapid evolutionary changes in the life histories and morphology of Endler's transplant guppies, there was little support for any major effects of stochastic forces on allozymic diversity arising from the introduction. Selection arising from changes in predation pressure appeared to be the predominant factor causing the remarkably rapid adaptation of guppies to their new environments. Genetic divergence in some marginal or isolated natural populations was similar to, or greater than, Kenny's pond guppies (Reynolds' genetic distance,R = 0·496), indicating that chance colonization and founder effects may have contributed to the observed geographic patterns of genetic differentiation in Trinidad.