Though seldom considered, the numerous impacts of roads can mediate natural selection and other evolutionary forces acting in contemporary time. Thus, road effects can modify trait distributions and population fitness across small spatial scales over handfuls of generations. Indeed, roads and runoff likely act as novel sources of selection for many wild populations. I investigated local population responses to roads in two species of pool-breeding amphibians—the wood frog (Rana sylvatica) and spotted salamander (Ambystoma maculatum). I asked whether populations breeding and dwelling in ponds adjacent to roads show evidence for local adaptation. I used a combination of field transplants, common gardens, and field surveys to evaluate potential divergence.
Results/Conclusions
Local population responses to road proximity varied considerably between these two species. Whereas spotted salamanders showed clear evidence for local adaptation to roads, wood frogs in these same habitats showed mixed effects. Compared to populations away from roads, aquatic stage wood frogs from roadside populations survived at lower rates in common garden and transplant experiments, and surveys of adults revealed high rates of edema (presumably induced by road salt). However, adult wood frogs from roadside populations also showed increased locomotor performance and fecundity, and these populations tended be older than those away from roads. We lack comprehensive estimates of fitness for these populations and are therefore unable to resolved these contrasting outcomes. Thus, whether or not roadside wood frogs are locally maladapted or adapted remains to be determined. Regardless of exact outcomes and mechanisms, roadside amphibian populations are clearly differentiated from neighboring populations located away from roads. Evolutionary perspectives therefore offer important insights into the complexity of biological responses to roads and runoff.