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

Life history should relate to gene flow (Nm) through its influence on dispersal and effective population size. Comparative studies designed to elucidate this relationship must contend with historical events that can yield misleading estimates of gene flow and statistical problems associated with inclusion of life-history traits correlated with phylogeny. We studied the relationships of life-history characters and gene flow in 15 species of darters, a monophyletic group of stream fishes. Populations of coexisting species were sampled in three geographic regions with different Pleistocene glaciation histories. Gene flow was estimated indirectly from allozymes using two methods, a and private alleles. Isolation-by-distance was also tested using regression of pairwise estimates of gene flow ((M) over cap) on distance. Theta and private-alleles methods produced congruent estimates of Nm, except 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 species from the historically fragmented region were included in stepwise regression analysis, because Nm was low despite life-history differences among taxa in this region. Excluding observations from this region produced stronger associations between clutch size and Nm (r(2) = 0.57), and between female size, egg size, and Nm (r(2) = 0.95). Additional analyses that corrected for female body size and phylogenetic nonindependence agreed that darters with high fecundity and small eggs exhibited high gene flow, whereas darters with small clutches and large eggs had low gene flow. The latter combination of life-history traits primarily is exhibited in species from headwater habitats where parental investment presumably confers survivorship on offspring. Reduced gene flow and genetic divergence among demes appear to be evolutionary consequences of this strategy.