Research

We use population genetic, traditional quantitative genetics, association genetics, and empirical studies to understand evolutionary process shaping diversity in natural populations.

At present we are working on two main projects:

Ecological and Evolutionary Limits to Species Range Expansion

Monica Geber at Cornell University, Vince Eckhart at Grinnell College, and Dave Moeller at University of Georgia. and I are using Clarkia xantiana (an annual plant native to the southern Sierra Nevada Mountains) to investigating the geographic limits to species ranges, a fundamental and long-standing problem in evolution and ecology. We are using a combination of i) demographic censuses and seed bank experiments to investigate demographic stochasticity, ii) GIS-based niche modeling to characterize suitable habitat; iii) molecular population genetics to evaluate historical population structure, effective population sizes, and gene flow, and iv) reciprocal common-garden experiments to quantify selection and genetic variance across a species range. The results from this series of experiments will provide insight into the basic processes that determine species ranges and provide an opportunity to evaluate predictions from theoretical models predicting limits to local adaptation.

Population Genomics of Cold Adaptation in Populus balsamifera

Matt Olson (Univ. of Alaska, Fairbanks) and I are examining genome-wide intra-specific nucleotide diversity and using association-genetic approaches to identify the genes underling phenotyping variation in the timing of bud-set (an important component of adaptation to light environments) in balsam poplar (Populus balsamifera). We are also examining the evolutionary history and local adaptation of candidate genes involved in timing of bud-set.

For more information please visit our project website (here).

Molecular Evolution and Population Genetics of Plant Defense Genes

We have used molecular population genetic analyses to characterizing the role selection has played in shaping nucleotide level diversity in plant defense genes. In addition to searching for evidence of selection that has affected diversity across the range of the species we have conducted extensive sampling within geographically defined subpopulations in an attempt to evaluate the how geographically-variable selection may shape diversity at the nucleotide level. This work has been motivated by a desire to integrate what we learn from experiments conducted in contemporary populations with what we can learn from population genetic methods. Integrating these approaches allows opportunities to understand temporal dynamics of selection that cannot be obtained using either approach alone. Most of this work is being done in teosintes (primarily Z. mays ssp. parviglumis), wild relatives of maize, and was done in collaboration with David Moeller.

Evolutionary impact of increased atmospheric CO₂ concentrations

We know a lot about the potential ecological effects of increasing concentrations to atmospheric CO₂; however, we know relatively little about whether increased concentrations will also affect evolution. We grew replicated populations of the model plant Arabidopsis thaliana in ambient and elevated CO2 environments (using an existing free air CO2 enrichment FACE) and a combination of traditional quantitative genetic and QTL approaches and to characterize the effect elevated CO₂ (550 ppm) on patterns of selection, responses to selection, and the genetic basis of phenotypic variation. We are also investigating how eCO2 affects plant response to herbivore damage (tolerance) and competitors effects on evolutionary trajectories. This work is a collaboration with Jennifer Lau (KBS and Michigan State University), Peter Reich (U of MN Forestry) and Ruth Shaw (U of MN EEB).

Evolutionary stability of mutualisms

Some mutualisms have been stable for millions of years. Theoretical models, however, predict that under many conditions these interactions are not stable. When Katy Heath was a graduate student in the lab we (mostly Katy) used Medicago truncatula and Sinorhizobium meliloti sampled from their native range to investigate the evolutionary forces that contribute to mutualism stability. In a greenshouse experiment, we found evidence that the forces that may maintain mutualism depend on both G x E and plant Genotype x rhizobia Genotype interactions, and the community of rhizobia to which plants are exposed. We also found evidence for partner choice – plant genotypes formed greater numbers of nodules with those rhizobia genotypes from which the plants derived greatest benefit. Both the context-dependence of selection imposed by mutualistic partners as well as partner choice may contribute to the maintenance of this mutualism.