Response to selection and evolvability of invasive populations
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While natural selection might in some cases facilitate invasions into novel habitats, few direct measurements of selection response exist for invasive populations. This study examined selection response to changes in salinity using the copepod Eurytemora affinis. This copepod has invaded fresh water from saline habitats multiple times independently throughout the Northern Hemisphere. Selection response to a constant intermediate salinity (5 PSU) was measured in the laboratory for saline source and freshwater invading populations from the St. Lawrence drainage (North America). These populations were reared under three conditions: (1) native salinities (0 or 15 PSU) for at least two generations, (2) 5 PSU for two generations, and (3) 5 PSU for six generations. Full-sib clutches taken from populations reared under these three conditions were split across four salinities (0, 5, 15, and 25 PSU) to determine reaction norms for survival and development time. Contrasts in survival and development time across the three rearing conditions were treated as the selection response. Selection at 5 PSU resulted in a significant decline in freshwater (0 PSU) tolerance for both the saline and freshwater populations. Yet, evolutionary differences in freshwater tolerance persisted between the saline and freshwater populations. The saline and freshwater populations converged in their high-salinity (25 PSU) tolerance, with an increase in the freshwater population and decline in the saline population. Development time did not shift greatly in response to selection at 5 PSU. For all three rearing conditions, the freshwater population exhibited retarded larval development and accelerated juvenile development relative to the saline population. Results from this study indicate that both the saline and freshwater populations exhibit significant responses to selection for a fitness-related trait critical for invasions into a novel habitat.
KeywordsBiological invasion Selection experiment Genetic variation Physiological evolution Rapid evolution Evolvability Exotic Nonindigenous Great Lakes
The paper constitutes a portion of the material presented for the symposium entitled, “All Stressed Out and Nowhere to Go: Does Evolvability Limit Adaptation in Invasive Species?” at the 2004 SSE/SSB/ASN conference in Fort Collins, CO, organized by Carol Lee and George Gilchrist. Funding for this study was provided by NSF DEB-0130543 to C. E. Lee. Stephane Plourde, Marc Ringuette, and Gesche Winkler collected saline samples from the Baie de L’Isle Verte in Quebec, Canada. Heather Free, Meghan Olson, Dan Skelly, and Greg Gelembiuk collected freshwater samples from Racine Harbor, Lake Michigan, Wisconsin and helped maintain the copepod and algal cultures. Greg Gelembiuk made useful suggestions on the statistical analyses. Lee Lab members provided editorial comments. Two reviewers provided useful suggestions on organization and structure of the paper.
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