Geographic range limits and adaptation to changing climates: the effects of phenotypic lag, selection, and genetic variance

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Every species has a limited geographic distribution. In other words, every species occupies a finite area on the landscape. Evolution has allowed life on earth to take many forms and occupy many habitats, so what prevents any particular species from adapting to areas outside their range and spreading indefinitely? As the climate changes, what new evolutionary challenges will populations face, and how will these affect their geographic distributions? These questions are central to research in my lab. In the work proposed here we will test several hypotheses for what constrains adaptation at range edges and under climate change, using an annual wildflower that grows in western North America as our focal species. One possibility is that range limits arise because populations fail to evolve the traits necessary to thrive at and beyond the range edge. If this is the case, we expect phenotypes to deviate further from the optimum near range edges than they do at the range centre. We also expect selection to be strong and directional at the range edge, but weak and stabilizing at the range centre. However, these patterns are rarely measured across geographic ranges. I propose to characterize them in the range centre, at the northern range edge, and beyond, with greenhouse crosses and field transplant experiments in Clarkia pulchella. An additional hypothesis for why range edges occur is that populations at range edges lack the genetic variance in key traits that is necessary to adapt. However, fitness depends upon more traits than can be measured, so I propose to directly measure genetic variance in fitness, again using greenhouse crosses and transplant experiments. My proposed design will allow us to decouple the contributions of population qualities (edge vs. centre) from site qualities (edge vs. centre). We will also examine whether populations are able to adapt to new combinations of environmental factors that they are likely to experience under climate change, or whether genetic correlations constrain the evolution of novel combinations of traits. This type of research is fundamentally interesting, but also important to society. Species of economic and cultural importance are experiencing the effects of climate change. By understanding what has limited distributions in the recent past and what is likely to be limiting in the future, we will be better able to plan conservation and cultivation of important species.