For organisms with complex life cycles (CLC) that are being affected by environmental change, different life stages may be differentially impacted by shifts in habitat conditions. CLC species may also have different demographic processes regulating vital rates during different life stages. For example, the tadpole stage of many frog species face strong negative density dependent survival and very low rates of dispersal. In contrast, adult population size is not typically regulated by density dependence and is the only life stage in which dispersal across a landscape can occur. Consequently, adaptive responses to environmental change in CLC species may be affected by these demographic constraints. In this study we use individual based models to explore how stage-specific selection pressures and immigration affect adaptive potential in stage-structured populations across a range of demographic and selection scenarios. We constructed and parameterized our model based on our previous work suggesting that barrier island populations of a common anuran amphibian, Hyla cinerea, has developed heightened tolerance to elevated salinities relative to freshwater, inland populations.
We examine how enhanced stage-specific selection pressures interact with demographic processes (e.g. density-dependence and immigration) at different life stages. For example, strong selection on egg stage embryo survival can facilitate more rapid rates of adaptation by modifying the population level implications of strong negative density dependence during the larval phase. However, when the strength of density dependence or selective pressure on the egg stage is relaxed, adaptation proceeds more slowly. Similarly, we show that the effect of immigration (e.g., gene flow) is dependent upon stage-specific selection pressures as well as demographic and vital rates in other life history stages. Since most organisms have complex life cycles, developing our understanding of how adaptive evolution occurs in stage-structured populations provides useful insights into our understanding of rapid adaptation generally, and informs effective management of stage-structured populations in the face of climate change.