Metapopulation theory predicts that reduced dispersal may lead to genetic and phenotypic differentiation of populations through selection and drift. For populations remaining after severe habitat fragmentation, both the size of and distance between populations may vary substantially. Highly isolated populations likely have a higher probability of inbreeding, genetic drift and reduced fitness relative to more highly connected populations due to small population sizes and a lack of gene flow. The Midwest is one region where serious losses of wetland habitat have occurred over the last 150 years. I used ponds in three Midwestern states to test the hypothesis that there is an inverse relationship between isolation and average fitness within a population for temporary pond Daphnia. I collected individuals from 13 populations and cultured them in a laboratory common garden. I then measured juvenile growth rates for individuals from each population, a metric that is highly correlated with lifetime fitness. Average growth rates were compared to the relative isolation of each population and other relevant ecological variables.
Results/Conclusions
Fitness estimates were obtained for 145 clones from 13 populations. I found a significant difference among the populations in their average juvenile growth rate along with a large variance within populations. However, there was no effect of spatial isolation on the mean fitness in the populations. Additionally, no ecological variable significantly correlated with fitness or increased the explanatory power of the statistical model (all p > 0.05). Two potential alternatives for variation in average fitness among populations are 1) the phenotypes of local populations are randomly drifting apart and 2) natural selection is driving local adaptation of populations. An experiment testing for local adaptation to resources and microsatellite
