Gene flow and population differentiation in two species of goodeid fishes (Cyprinidontiformes: Goodeidae)

The role of gene flow in population differentiation was examined by electrophoretic analysis of populations of two species of goodeid fishes, Goodea atripinnis and Chapalichthys encaustus, from lakes and streams on the Mesa Central of Mexico. Microgeographic differentiation was observed among continuous stream populations of Goodea. Highly significant genic heterogeneity was exhibited among continuous lacustrine populations of both species. Levels of differentiation (based on a genetic distance coefficient) among populations of G. atripinnis in Lake Chapala were similar to levels among populations from a number of isolated drainages.

These results suggested that population continuity and gene flow do not necessarily imply genetic continuity and allele frequency homogeneity. Neighborhood effects (population subdivisions due to behavioral constraints such as homing or low vagility) were proposed as contributing to reductions in gene flow among populations from lakes and streams, but at least in the case of Goodea were not of major importance. Data from Lake Chapala for both species lended support to intralacustrine or sympatric models of lacustrine species flock evolution.

Population comparisons of Goodea from a number of drainage systems suggested that a simple time-since-divergence model was insufficient to explain the observed patterns of genetic variation. Local effects (drift, bottlenecks, selection) were proposed as important mediators of genetic variation and population differentiation. It is suggested that levels of gene flow much greater than the “one migrant” rule would still permit differentiation of populations in the absence of selection.