My primary interest is in theoretical population genetics. Most of the things I work on can be described as population genetics theory loosely related to plant mating system evolution, or plant biology in general. I'm now in my second year as a graduate student in the Slatkin lab-- here are some things I'm working on now, as well as some things I've done in the past.

Balancing selection

Theoretical work has shown that at equilibrium, the coalescent tree for loci under balancing selection is identical to the neutral gene tree, except that it is "stretched out" by a known scaling factor. Interpretation of these trees is, however, problematic, particularly when they deviate from the expected neutral shape. Do such deviations mean that the theory is invalid, and if so, why? Is the model of selection wrong, or has the assumption of equilibrium conditions been violated in some important way? A more flexible theoretical approach is needed to distinguish between these possibilities. In addition, we are often specifically interested in what happens in non-equilibrium cases, but it is not clear that the mapping between equilibrium and non-equilibrium conditions that exists for neutral genes has a corresponding map under balancing selection, making it difficult to apply neutrality-derived results. The goal of this project is to examine balancing selection under non-equilibrium conditions in more detail, in order to characterize the effects of deviations from equilibrium on allele dynamics and on the gene tree.

Overdominance/MHC

As part of this project, Monty Slatkin developed a model of overdominance (based on the SSWM limit of John Gillespie and a Markov chain approach) that allows time-dependence of parameters, and thus analysis of non-equilibrium populations. We have been using this model framework to examine questions about the effect of population bottlenecks on alleles under overdominant selection. It is also possible, using this model, to consider the effects of selective differences among allelic classes, and to consider the effects of this type of selection on linked neutral loci. All of these modifications have applications to existing HLA and MHC data sets.

Self-incompatibility

Gametophytic self-incompatibility is a genetic system for inbreeding avoidance in plants. It works through an extreme form of balancing selection and shows some similarities to MHC/HLA systems, although the frequency dependence introduces some peculiarities, particularly away from equilibrium. I am now extending the Markov chain approach to SI, in order to address the data and questions specific to that system.

Inbreeding depression

Dynamics of inbreeding depression

Inbreeding depression is the empirical observation that often inbred (particularly selfed) progeny are less fit than outbred progeny. There are some long-standing controversies about its genetic basis and importance in plant mating system evolution. With Russ Lande, I studied the dynamics of inbreeding depression (due to recurrent deleterious mutations) under a mating system that included partial asexuality as well as selfing and outcrossing. We found that under partial selfing, substantial inbreeding depression could be maintained at equilibrium even with high (measured) selfing rates, providing a potential reconciliation of the basic theory with some apparently anomalous empirical results.

Inbreeding depression as background

Inbreeding depression is a widespread phenomenon. For a partially selfing plant, it can be regarded as the genetic context in which all other evolution takes place. I studied the evolution of advantageous alleles in such a context. I found that with the associations between loci that result from partial selfing (identity disequilibrium), the background of inbreeding depression could influence other loci in a manner somewhat analogous to background selection at (physically linked) loci.