Habitat Affiliation Determines Population Structure and Genetic Diversity in Costa Rican Amphibians and Reptiles

The genetic diversity held within populations can indicate much about the past and present state of a species. Population genetic structure can be an indicator of current or past dispersal barriers that are disrupting gene flow. Documenting and understanding barriers to gene flow allows us to determine what ecological factors play a role in halting genetic dispersal. Habitat affiliation can directly affect population structure and interact with ecological factors to determine whether a species experiences certain barriers as limiting to genetic dispersal. In addition, understanding gene flow and genetic diversity can help us predict how such factors might vary between species with different habitat affiliations, and allow us to answer questions about species’ life histories. For a herptile community in tropical Costa Rica, I asked three main questions. First, I asked for which amphibian and reptile species does elevation act as dispersal barrier and restrict gene flow over evolutionary time? Second, I asked what ecological traits do these species share which discriminate them from species that were able to experience undisrupted gene flow across elevational barriers? Lastly, I asked how habitat affiliation of species affects their population structure. To answer these questions I sampled three species of amphibians, Dendropsophus ebraccatus, D. microcephalus, Centrolene prosoblepon, and one species of lizard, Norops polylepis. Exhibiting an array of habitat affiliations in southern Costa Rica, from ponds to streams to terrestrial habitats, and an array of macrohabitat requirements, from warmer, low elevations to cool, higher elevations, they allowed for good models to understand the intersection of gene flow, dispersal barriers, and habitat affiliation. Dendropsophus ebraccatus and D. microcephalus, two pond-affiliated species that prefer warmer, lower elevation habitats, were found to have very low levels of genetic diversity. These results supported evidence for evolutionarily low population size in both species. Another possible explanation is a population bottleneck and then subsequent recent migration in both species. Norops polylepis, a terrestrial species was found to have very high genetic diversity that was organized spatially. This allowed me to surmise that N. polylepis might be experiencing a barrier to dispersal, and that this barrier was likely either fixed or organized spatially. However, large population size could also explain these results. Lastly, Centrolene prosoblepon, a stream-affiliated species with a higher elevation tolerance was found to have high levels of genetic diversity, that was not organized spatially. These results indicated that C. prosoblepon experienced a barrier to dispersal and that this barrier was either widespread or mobile due to the lack of spatial organization. Environmental barriers to dispersal are extremely hard to test, however genetic diversity held within each species, and why these levels of diversity are so different from one another can give us possible insight into population structure and dispersal barriers.

Keywords:  Department of Biology
dispersal barriers
amphibians
reptiles
habitat affiliation
gene flow
population structure
Stanford University Department of Biology
Thesis
Advisor: Daily Gretchen
Frishkoff Luke
Hadly Elizabeth