Ellen Williamson

Background

My undergraduate degree was in mathematics with an emphasis in applications in physics. In graduate school, I became interested in evolution, in particular, population and quantitative genetics. My Ph.D. work focussed on evolution in the teleost species Fundulus heteroclitus. Here in the Slatkin lab I have -become interested in applications population genetics theory to problems in conservation genetics.

Slatkin Lab

In the Slatkin lab I have become interested in applications of population genetics theory to problems in conservation biology. Populations targeted for conservation are typically small and are rarely in equilibrium with respect to common population genetic parameters such as migration, genetic drift, and mutation. For this reason, many of the classical population genetics theoretical models may not always be useful for conservation.

For single isolated populations, Monty Slatkin and I developed a maximum likelihood framework for using temporal changes in allele frequencies to estimate the number of breeding individuals in a population. This method does not require any knowledge of the recent history of the population and can be applied to non-equilibrium populations. We compared this estimator to an F-statistic estimator of variance effective population size using simulations. In these comparisons the maximum likelihood estimator typically had a lower variance and smaller bias. Taking advantage of the flexibility of the likelihood framework, we extended the model to include exponential growth and showed that temporal allele frequency data can be used to test for population growth.

Dissertation - Fundulus heteroclitus

My dissertation involved studying the evolution of a quantitative character, developmental rate, using a simulation model that incorporated ecological and physiological data from the species Fundulus heteroclitus. F. heteroclitus spawns with a semi-lunar rhythm in the intertidal zone during spring tides. The eggs are placed at elevations reached only by the highest tides, thus the timing of hatching is critical to survival for F. heteroclitus larvae. To quantify and clarify the relationship between developmental rate, hatching, and survival, I developed a computer model that simulated the reproductive strategy in F. heteroclitus. The model estimated hatching success for F. heteroclitus embryos as a function of developmental rate by simulating spawning, embryo development, and the hatching of F. heteroclitus embryos in Delaware Bay.

The simulation results suggest that there is normalizing selection acting on embryonic developmental rate in this species. This large scale is on the order of differences in developmental rate found between different populations of F. heteroclitus along the species range. Countergradient (counter to a sharp latitudinal temperature gradient) variation in developmental rate has recently been documented in this species. Although embryos from different populations develop at different rates under the same laboratory conditions (i.e. same temperature), they develop at very similar rates in their respective habitats. The model predicts that although the developmental rate of any individual is strongly influenced by temperature, selection should be acting to maintain similar developmental rates in situ in the different populations of F. heteroclitus along the Atlantic coast of the United States. This model is therefore consistent with the observed countergradient in developmental rate.

My Ph.D. supervisor, Leonard DiMichele, and his collaborators, have demonstrated functional non-equivalence between the allozyme products from several polymorphic loci in F. heteroclitus. The enzyme products from these loci are key metabolic enzymes and appear to be functionally related to developmental rate. Several of these enzyme loci are clinal along the species range. Allozyme heterozygotes are frequently intermediate to alternate homozogotes in terms of both enzyme activity and developmental rate. This has led some researchers to hypothesize that in the intermediate environmental conditions of the middle of the species range, the intermediate heterozygotes may have a selective advantage. I tested this hypothesis by computing marginal "fitness" coefficients for each locus known to influence developmental rate using the simulation results described above. I found that an intermediate genotype was favored in an intermediate environment. However, this genotype was a homozygous combination of "northern" and "southern" genotypes. I found no evidence for overdominance at any of the loci.