Kim T. Scribner 1993 <p><span>Results from two experiments are presented that contrast differences in life-history traits and population dynamics between two species of live bearing fishes (<i>Gambusia affinis</i> and <i>G. holbrooki</i>) that hybridize across portions of the southeastern United States. Progeny from parental <i>holbrooki</i> and <i>holbrooki</i>-<i>affinis</i> F1 crosses exhibited larger lengths at birth, at 15 days, and matured earlier, and at larger size than did progeny from parental <i>affinis</i> and <i>affinis-holbrooki</i> F1 crosses. Comparisons of experimental populations of <i>affinis</i>, <i>holbrooki</i>, and mixed (<i>affinis</i> + <i>holbrooki</i>) species composition followed over two years revealed that <i>affinis</i> populations consistently exhibited smaller population size, lower carrying capacity, lower recruitment, and larger over-winter mortality than did <i>holbrooki</i> or mixed populations. Evidence for density-dependent reductions in fecundity and concomitant increases in juvenile mortality rates were observed in all populations, but were most pronounced for <i>affinis</i> populations. Genotype-specific differences in life-history traits appear to confer differential advantage to offspring of parental <i>holbrooki</i> origin and F1 progeny of <i>holbrooki</i> maternal parentage given the resource availability and the age structure and densities experienced during these experiments. Results have direct implications regarding the rate and direction of evolution within hybrid zones formed by these two species.</span></p> application/pdf 10.2307/2410075 en Society for the Study of Evolution Hybrid zone dynamics are influenced by genotype-specific variation in life-history traits: Experimental evidence from hybridizing <i>Gambusia</i> species article