Direct and correlated responses to artificial selection on acute thermal stress tolerance in a livebearing fish

Tradeoffs in performance or fitness across environments have important impl ications regarding the nature of evolutionary constraints. It remains contr oversial whether tradeoffs such as these reflect genetic correlations that are genuine evolutionary constraints. However, if such long-term genetic co nstraints do exist, they must be due to underlying pleiotropy such that all eles that confer high performance in one environment invariably confer low performance in another. The distribution of genetic correlations within and among populations can provide insight about the existence of such pleiotro pic tradeoffs. The long-term association of certain teleost fish taxa with particular abio tic environments suggests that tradeoffs in performance across environments have constrained the geographic distribution of those taxa. Here we report the results of an experiment in which we artificially selected on acute he at- and cold-stress tolerance in two stocks of the poeciliid fish Heterandr ia formosa from source populations with different thermal histories. Unexpe ctedly, we observed no direct responses to selection. Under certain conditi ons, fish from the different source populations differed significantly in c old tolerance, but not in heat tolerance. The results suggest there are no strong pleiotropic tradeoffs between heat- and cold-stress tolerance in the se populations.