Worldwide habitat loss and fragmentation remain serious threats to population persistence, as reduced dispersal affects population dynamics and reduced gene flow impacts genetic integrity of populations. While increased isolation of populations and reduction in genetic diversity can negatively impact individual and population fitness, increased isolation may also be beneficial as it can allow populations to reach their adaptive optima. Here, I investigate the causes and consequences of population connectivity using an integrative approach, combining molecular tools, experimental data, field surveys, and geographic information systems data, in the wood frog, Rana sylvatica. This species occupies two ecologically divergent habitats (open and closed canopy ponds) over very small spatial scales, where gene flow is likely to play an important role in the divergence of populations. I first assessed the effects of habitat fragmentation on population connectivity of wood frogs by comparing historical and current landscape structure to contemporary genetic structure across 51 populations. Wood frog populations showed rapid neutral genetic divergence following habitat fragmentation. Additionally, I assessed how gene flow affects the local adaptation of populations, using a common-garden experiment to compare trait differences among 16 populations from open- and closed-canopy ponds across a gradient of pond isolation. Overall, wood frog larvae showed similar levels of divergence among open- and closed-canopy ponds at both low and high levels of population connectivity, suggesting that selection is strong enough that divergence can occur despite gene flow. To determine the consequences of the combined effects of selection and gene flow on fitness, I compared population-level fitness correlates across populations ranging from outbred to inbred. Populations with low levels of inbreeding had higher levels of larval survivorship in a common garden experiment and larger population sizes compared to more inbred and outbred populations. The reduced survivorship of outbred populations with the pattern of divergence with gene flow points to disruption of local adaptation as a mechanism for outbreeding depression. Together, these results elucidate the fine balance between strong divergent selection and population connectivity. I discuss the implications for ecology and evolutionary biology, provide suggestions for conservation and land management, and outline areas for future research.
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