My primary research focuses on developing statistical methods for parameter estimation and hypothesis testing in population genetics and molecular phylogenetics. I am particularly interested in developing methods for estimating the relative contributions of demographic forces (e.g., population structure, population size expansion / contraction) and selective forces on the history of natural populations using data from standing genetic variation as well fixed differences between populations. My group's research has been heavily focused on using Poisson Random Field models for gene frequencies under selection, as well as coalescent based approaches for inference in population genetics. For example, in Williamson et al. (2005), we developed the first model for how natural selection affects the distribution of mutation frequencies in a population that had recently changed in size. We demonstrated that human populations show strong evidence of very recent growth and that many amino acid replacement SNPs are likely slightly deleterious. We are currently expanding this approach so as to deal with bottlenecks as well as migration among subpopulations.