Office: 213 Pharmacy Biology Building
|B.S.||Biology||2004||Univerisity of Virginia|
|Ph.D.||Forestry & Env. Studies||2012||Yale University|
In short, my research is focused on understanding how the landscape can affect the movement of wildlife and, consequently, how various degrees of landscape connectivity can alter population dynamics. The fields of ecology and evolutionary biology have both focused on this issue, however their predictions regarding the role of movement and gene flow on the fate of populations do not always converge. On one hand, immigration increases the size of the recipient population, effectively bolstering demographic prospects over the short term. On the other hand, if populations occupy heterogeneous habitats that impart divergent selection pressures, gene flow could disrupt localized adaptation and move the recipient population away from some theoretical trait optimum. And then there are additional considerations dealing with disease transfer, inbreeding depression and minimum levels of genetic variation required (for selection to act upon). As evidence accumulates that species and populations can evolve in response to natural selection over times scales relevant for ecological processes, it’s becoming clear that the movement of individuals between populations needs to be considered from both an ecological and evolutionary perspective.
To approach some of these questions, I am studying populations of two amphibian species widespread in eastern and northern North America – the wood frog (Rana sylvatica) and the spotted salamander (Ambystoma maculatum). Both species typically breed in forested vernal pond habitats. These ephemeral habitats provide both great opportunity (temporary flood of resources, few permanent predators) and substantial challenges (more stressful abiotic conditions, an impermanent hydroperiod available to support the larval stage) for species that breed in them. There is also considerable heterogeneity among vernal ponds in terms of habitat variables that can affect local conditions within the ponds. This heterogeneity can generate varying selection pressures on populations inhabiting these habitats. I use tree canopy cover over a pond as a proxy for several habitat variables known to impart selection pressures on pond-breeding amphibians, including water temperature and hydroperiod. With my dissertation research, I am trying to address (1) whether adaptive divergence can occur in response to these selection pressures that differ among natural populations of amphibians, (2) at what geographic scale this divergence is possible, and (3) how this divergence is influenced by migration and gene flow among pond populations. Below are brief summaries of some projects that I’m working on as part of my dissertation research.
Comparative population genetic structure of two amphibian species
One primary goal of conservation is to understand what features on the landscape serve as barriers to movement, and to mitigate the effects of these barriers. To this end, I am using a landscape genetics approach to identify these potential barriers among populations of wood frogs and spotted salamanders. The focal populations I am studying are scattered throughout the New England region of the U.S. Across this broad geographic scale, there are many discrete landscape features of interest, which provide ample opportunity to address specific hypotheses about the impact of particular landscape elements, both of anthropogenic and natural origin.
Above: Land cover (top) and resistance surface (bottom) maps used to model dispersal and gene flow across the landscape.
Adaptive divergence of wood frog populations
As mentioned above, canopy cover can differ considerably among vernal pond habitats, creating very different conditions (and selection pressures) for the growth and development of amphibian larvae within ponds. I am conducting a set of reciprocal transplant experiments to address whether adaptive divergence occurs in response to canopy cover. I select ponds that differ in canopy cover, collect eggs and then raise them either within their natal pond, or transplant them into the paired pond of opposite canopy type. Experiments occur in mesocosm cages within the focal ponds. This approach allows us to assess the effects of true adaptive divergence of populations while accounting for any plasticity seen in the phenotypic differences in the field.
Maternal effects and local adaptation
Maternal effects are any maternal influences on offspring phenotype, irrespective of offspring genotype or environment. Female wood frogs could potentially allocate different resources to eggs based on what habitat type they decide to lay in, and this could explain any prospective patterns of adaptive divergence of populations based on canopy cover. To account for this, I am estimating several proxies of wood frog maternal investment into eggs for my experimental populations. These include egg mass, egg numbers per clutch deposited, and nutrient content of eggs. I am collaborating with Dror Hawlena here at Yale for this project.
Adaptive divergence of spotted salamanders in response to predation
I am working on a project to estimate the genetic divergence among populations of spotted salamanders that are known to diverge phenotypically in response to predation pressures within vernal ponds. Given the evidence for phenotypic divergence in this species, estimating gene flow among populations can tell us something about the strength of selection and rate of evolution in this system. This data can also help us understand the relative influences of gene flow and selection on the establishment of localized adaptation among populations of a species inhabiting heterogeneous habitats.
Effective population size estimation
Wood frog breeding aggregations provide in interesting system to answer questions that lie at the intersection of population demography and population genetics. Adults arrive at vernal ponds to breed shortly after ponds have thawed, and breeding concludes within a week or two. During this frenetic period, females deposit a single egg mass, which allows us to accurately estimate the number of breeders at that pond during that year. These census estimates of population size can be combined with genetic estimates of effective population size (a critical parameter in conservation biology) to get a more comprehensive picture of the long-term population dynamics and viability of pond-breeding amphibians.