Ecological and historical associations of gene flow in darters (Teleostei : Percidae)

Life history should relate to gene flow (Nm) through its influence on dispe rsal and effective population size. Comparative studies designed to elucida te this relationship must contend with historical events that can yield mis leading estimates of gene flow and statistical problems associated with inc lusion of life-history traits correlated with phylogeny. We studied the rel ationships of life-history characters and gene flow in 15 species of darter s, a monophyletic group of stream fishes. Populations of coexisting species were sampled in three geographic regions with different Pleistocene glacia tion histories. Gene flow was estimated indirectly from allozymes using two methods, a and private alleles. Isolation-by-distance was also tested usin g regression of pairwise estimates of gene flow ((M) over cap) on distance. Theta and private-alleles methods produced congruent estimates of Nm, exce pt in a study region hypothesized to have been historically fragmented and then united following Pleistocene,glaciation. A relatively weak association between life-history traits and Nm (based on theta) was observed when spec ies from the historically fragmented region were included in stepwise regre ssion analysis, because Nm was low despite life-history differences among t axa in this region. Excluding observations from this region produced strong er associations between clutch size and Nm (r(2) = 0.57), and between femal e size, egg size, and Nm (r(2) = 0.95). Additional analyses that corrected for female body size and phylogenetic nonindependence agreed that darters w ith high fecundity and small eggs exhibited high gene flow, whereas darters with small clutches and large eggs had low gene flow. The latter combinati on of life-history traits primarily is exhibited in species from headwater habitats where parental investment presumably confers survivorship on offsp ring. Reduced gene flow and genetic divergence among demes appear to be evo lutionary consequences of this strategy.