Ph.D., Brown University, 1999
evolutionary ecology, ecological and evolutionary genetics
My primary interest is examining the effects of variation in the abiotic and biotic environment on the evolutionary dynamics of specific phenotypes and genotypes in natural populations. This research is broad in scope, incorporating methodologies from diverse disciplines ranging from molecular and classical genetics to experimental field ecology. Such a multidisciplinary approach can yield a more comprehensive understanding of the interactions between genetic and environmental variation and is readily applied to a variety of organisms and ecological communities. Members of the laboratory are encouraged to work in any organismal system that is well suited to their particular interests. Currently our lab is working in both the rocky intertidal in New England and fruit orchards along the east coast of the U.S., “natural” habitats of the introduced dipteran Drosophila melanogaster.
environmental heterogeneity and selection in the intertidal zone
The rocky intertidal community in the northwest Atlantic has been intensively studied over the last 50 years; the wealth of natural history and ecological data available provide an excellent context for evolutionary studies. For example, variation in the degree of physical stress (e.g., temperature and desiccation) experienced by organisms is well known as one of the primary determinants of the distribution and abundance of various members of the intertidal community. Such environmental heterogeneity may also create habitat-specific selection regimes that influence patterns of genetic variation in marine invertebrate populations. This can be illustrated by the dynamics of the mannose-6-phosphate isomerase (MPI) polymorphism in the northern acorn barnacle, Semibalanus balanoides. In this species, the two common MPI alleles appear to be actively maintained by differential selection for alternative homozygous genotypes in both high-stress (e.g., exposed substrate in the high intertidal zone) and low-stress (e.g., beneath the algal canopy in the low intertidal) microhabitats. The mechanism of selection at this locus appears to be governed by the presence of mannose-containing compounds in the barnacle diet coupled with exposure to varying degrees of thermal/desiccation stress.
I am also investigating patterns of genetic and phenotypic variation in four species of intertidal snail and their common introduced predator, the green crab Carcinus maenas. As with barnacles, these snails experience predictable variation in thermal stress and predation intensity across various spatial scales. Furthermore, the various snail taxa exhibit different life histories and have distinct modes of larval dispersal, making very different predictions regarding the genetic structure of populations and the potential evolutionary responses to predation and thermal stress at small vs. large spatial scales. Current and future projects in this system include: 1) using hypervariable genetic markers to evaluate genetic structure of crab and snail populations; 2) examining the effects of variation in thermal stress and predation on the distribution, abundance, and fitness of various shell phenotypes and enzyme genotypes in the four snail species; and 3) the effects of life history characteristics on the potential for local adaptation to specific environmental regimes.
reproductive diapause in Drosophila melanogaster
D. melanogaster is well known as a model organism in biology but there is a surprising lack of information regarding its natural history and ecology. The species is native to tropical Africa and colonized the New World in the last several hundred years. In the eastern United States, populations exist across a relatively continuous environmental gradient. Conditions at the southern extent of this range, such as in southern Florida, are generally conducive to year-round reproductive activity, but in the north reproduction and population growth are confined to the summer months. For a tropical insect, temperate habitats such as apple orchards in northern New England represent a novel environment; survivorship over the winter season may present a major challenge.
In many insect taxa, survivorship over extended and stressful time periods is conferred by the expression of diapause (a neuroendocrine mediated syndrome that is cued by token environmental stimuli, causes reproductive arrest and is associated with increased longevity and stress resistance). D. melanogaster was long believed to have no seasonal response, but it is now known that adult females can a) survive the winter season, b) exhibit reproductive diapause, and c) that the expression of this dichotomous, fitness-related trait is highly variable within and among natural populations. The frequency of diapause incidence is strikingly clinal across the latitudinal gradient in the eastern U.S., varying from approximately 35% in southern Florida to greater than 80% in New England populations. Even in the absence of diapause-inducing cues, diapause and nondiapause genotypes are phenotypically distinct for a variety of traits including patterns of resource allocation, lipid content and starvation resistance.
The “decision” to either diapause and postpone reproduction or develop directly clearly has an impact on organismal fitness, and variation for the diapause response may be of critical importance to population dynamics and the evolution of life histories. I am interested in examining whether diapause is a central component of a series of traits involved in the adaptation of Drosophila to temperate climates, and whether diapause variation reflects a larger set of underlying pleiotropic and fitness correlated traits associated with the same causal pathway. This series of projects includes: 1) gathering basic ecological and natural history data on resident D. melanogaster populations in temperate and sub-tropical orchards as well as human-commensal populations in urban settings; 2) using a combination of wild-caught and inbred, laboratory evolved lines to evaluate what types of traits are genetically correlated with both the ability to diapause (i.e., genotype) and the expression of diapause itself (i.e., the diapause syndrome); 3) examining the dynamics of selection on and potential adaptive significance of diapause in this species, using experimental manipulations in both the field and laboratory; 4) evaluating whether variation in diapause expression is associated with life history tradeoffs and different patterns of resource allocation/energy budgets; and 5) utilizing the tools of Drosophila genetics to dissect the trait at the molecular level, evaluate the contribution of candidate genes to phenotypic expression, and determine generalized expression profiles of diapause and nondiapause lines under various environmental regimes.
Schmidt, P.S. 2001. The effects of diet and physiological stress on the evolutionary dynamics of an enzyme polymorphism. Proc. R. Soc. Lond. B: 268, 9-14.
Schmidt, P.S. and D.M. Rand 2001. Adaptive maintenance of genetic polymorphism in an intertidal barnacle: Habitat- and life-stage-specific survivorship of Mpi genotypes. Evolution 55 (7): 1336-1344.
Schmidt P.S., Bertness, M.D. and D.M. Rand. 2000. Environmental heterogeneity and balancing selection in the acorn barnacle. Semibalanus balanoides. Proc. R. Soc. Lond. B: 267 (1441): 379-384.
Schmidt, P.S., Duvernell, D.D. and W.F. Eanes. 2000. Adaptive evolution of a candidate gene for aging in Drosophila. Proc. Natl. Acad. Sci. USA 97: 10861-10865.
Bertness M.D., Leonard, G.H., Levine J.M., Schmidt P.R. and A.O. Ingraham. 1999. Testing the relative contribution of positive and negative interactions in rocky intertidal communities. Ecology 80 (8): 2711-2726.
Schmidt, P.S. and D.M. Rand. 1999. Intertidal microhabitat and selection at Mpi: Interlocus contrasts in the northern acorn barnacle, Semibalanus balanoides. Evolution 53 (1): 135-146.
Leonard G.H., Levine J.M., Schmidt P.R. and M.D. Bertness. 1998. Flow-driven variation in intertidal community structure in a Maine estuary. Ecology 79 (4): 1395-1411.
BIOL 230 - Evolutionary Biology
BIOL 615 - Graduate Seminar in Ecological Genetics