My research focuses on complex morphologies, which are structures that comprise multiple traits whose relationships matter to fitness. The central question posed by complex morphologies is: How can the component parts be coordinated in their changes so that those changes improve rather than impair the performance of the whole? One answer is that developmental processes bias the structure of variation, coordinating the variation of functionally interdependent parts, which, in turn, coordinates their evolution. Another is that natural selection correlates the evolution of developmentally independent traits. The mammalian jaw is a classic example of a complex morphology and using it as a model system, I examine how it changes over the course of an individual’s lifetime, how its components covary within populations and how the shape of the complex evolves. The more specific questions that I address concern the rodent mandible: (1) how are ontogenetic changes in jaw shape related to feeding function and life-history? (2) Is variation biased and does the structure of variation evolve? (3) Does the structure of variation either limit or enhance the ability of populations to adapt rapidly to environmental change? and (4) how are diversification and morphological evolution related throughout the adaptive radiation of rodents?
The major focus of my current research is on adaptation to the unprecedented rates of ongoing climate change. Among the indirect effects of climate change are altered biotic interactions because whole communities do not track their thermal environment in concert. My studies focus on the extensive collections of chipmunks and ground squirrels made in the early 20th century by Joseph Grinnell and colleagues, and the several populations that were sampled again in the mid 20th century plus the exceptional collections made when a thorough resurvey was undertaken by the Grinnell Resurvey project. In the Sierra Nevadas, some chipmunk species are moving upwards to higher elevations but their ranges are dramatically contracting because the upper limit of their ranges is also moving downwards. Other species, especially those at low or mid-elevations are maintaining stable range sizes. By comparing jaw morphologies over time, transects and species, I am quantifying patterns of variation and measuring rates and directions of evolutionary change. Through these analyses, I am finding that jaw morphology evolves at the highest rate that populations can sustain, that directions of change are not strongly biased by the structure of variation, and that species differ significantly in their directions of change, suggesting that they are rapidly adapting to local biotic interactions rather than to more general changes in climate.