Population genetic patterns in sister species pairs sharing a single speciation event: a study of two species pairs of freshwater fishes

Abstract

This study asked whether population genetic structure of daughter species shows a pattern indicative of change caused by the speciation process. Fresh water fishes composing two geminate species pairs from a vicariant event in the southeastern United States were examined: Ammocrypta beanii/A. bifascia (sand darters) and Fundulus escambiae/F. nottii (starhead topminnows). AFLP data was used to derive estimates of genetic distance and population genetic parameters, and cytochrome b sequence data was used to test phylogenetic affinities and estimate genetic distance between species. AFLP sampling was: A. beanii, 123 individuals, 5 drainages; A. bifascia, 115 individuals, 5 drainages; F. nottii, 88 individuals, 5 drainages; F. escambiae, 77 individuals, 4 drainages. Representatives were used from multiple drainages for each species in the sequence analysis. Population genetic parameters estimated were average heterozygosity, θB (a Bayesian analog of Fst), average genetic distance between drainages, and isolation by distance. Findings were: (1) A. beanii contains a distinct genetic subgroup within the Mobile Bay Drainage; (2) population subdivision was the same in the two Ammocrypta species and could be inferred to have been similar in the ancestor; (3) heterozygosity and isolation by distance differed between the Ammocrypta species, so had changed from ancestral values in at least one descendant; (4) isolation by distance, heterozygosity, and possibly subdivision differed between the Fundulus species, so had changed from ancestral values in at least one descendant; (5) genetic distance between the Fundulus species was several times greater than that between the Ammocrypta species; (6) genetic distance between populations and genetic subdivision was generally much greater in the F. escambiae/F. nottii species pair than in A. beanii/A. bifascia, perhaps because of life history differences between the species pairs; (7) of the five population genetic parameters estimated, only heterozygosity showed the pattern identified as supporting the hypothesis that population genetic change was caused by the speciation event. The results of this study suggest that microevolutionary parameters (allele frequencies and their distributions) are not directly shaped my macroevolutionary dynamics (the processes involved in speciation), although more species pairs need to be examined before this result can be stated with confidence.