COS 88-3 - Does demographic variation drive local adaptation across a species range?

Background/Question/Methods

Population ecologists broadly acknowledge that environmental factors like climate contribute to population dynamics. Similarly, evolutionary ecologists recognize that environmental drivers of individual and population performance often cause selection for local adaptation. Thus, the underlying drivers of demographic dynamics and the evolution of local adaptation may often be shared. Furthermore, the extent to which populations achieve local adaptation may feed back to influence their dynamics over time. Despite these intuitive links, few studies have directly assessed the relationships between adaptation and demography among populations across a range of environments.

In this experiment, we interrogated the relationship between demography and local adaptation in Clarkia xantiana (Onagraceae), an annual wildflower endemic to California. By monitoring wild C. xantiana populations for 10 years, we characterized demographic patterns of 10 focal populations, including the population growth rate and its elasticity to underlying vital rates and life-history loops. We used these data to estimate the pairwise “demographic distances” between populations and identify population pairs that were demographically similar (“demographic pairs”). We then carried out two large-scale field reciprocal transplant experiments to assess the magnitude of local adaptation across source-site combinations for the five demographic pairs.

Results/Conclusions

When transplanted into each other’s environments, demographic pairs showed, on average, less evidence of local adaptation than demographically unpaired populations. In other words, the difference in lifetime fitness in home vs. away sites was lower between demographically paired than unpaired populations. This suggests that demographic similarity among populations may be linked to local adaptation.

We also used multiple regression to assess the contribution of demographic similarity between population pairs to local adaptation, relative to other putative drivers: environmental similarity and geographic distance among sites. In contrast to the above finding, when considered jointly with environmental distance and geographic distance, demographic distance among populations was not strongly related to local adaptation. Instead, geographic distance was the best predictor of local adaptation.

The prevailing effect of geographic distance on local adaptation, accounting for effects of environment and demography, suggests that isolation by distance may drive population performance more than does selection by the environment or past evolution of similar life-history strategies. This may indicate that gene flow constrains or swamps local adaptation at the spatial scale of this species’ range, and/or that geographic patterns in performance are driven more by common plastic responses across sites than by local adaptation.