Adaptation at a range limit in the mussel Mytilus californianus

Background/Question/Methods

Range limits are poorly understood ecological and evolutionary phenomena. A primary question related to range limits and their enforcement is whether and how often gene flow induced maladaptation contributes to their determination. Theoreticians have put forth numerous models of range dynamics which include gene flow induced maladaptation, but no empirical examples exist of range limits being set or reinforced by this mechanism. In order to test theory empirically, this project investigates a population likely to be maladapted at a range limit due to gene flow from another part of the range, using the phenotype of thermal tolerance in the species Mytilus californianus. The adaptive state of multiple mussel populations along a shallow gradient of thermal change is compared to the adaptive state of a small population at the hot end of a steep thermal gradient which receives immigrants from cold adapted populations. A common garden experiment between the small hot population and the nearby large cold adapted population was used to explore the mechanisms underlying the observed adaptive state of the small population.

Results/Conclusions

The small population did not exhibit maladaptation relative to the relationship found for the populations existing along shallower portions of the gradient, providing no evidence for gene flow induced maladaptation and enforcement of a range limit. The common garden experiment supported the effectiveness of local adaptation and provided evidence that genetic variance not developmental plasticity allowed for the adaptedness of the small marginal population. These results indicate that models of range limits in Mytilus californianus and similar species probably need not include gene flow dynamics to be accurate. More generally the lack of maladaptation in this situation reduces the plausibility of the hypothesis that gene flow induced maladaptation frequently enforces range limits, because many species natural history makes them less likely than Mytilus californianus to exhibit such behavior. Specifically species who disperse over relatively short distances and exhibit population structure at small scales unlike Mytilus californianus, are less likely to be affected by the gene flow range limit hypothesis because swamping of local selection is less likely to occur.