How do organisms respond and adapt to complex, variable natural environments?

Our research integrates environmental physiology, ecology and evolution to address this question, using a combination of laboratory, field and modeling approaches. Much of our work is with temperate insects and their interactions with plants, but together with recent graduate students and colleagues we have also studied bacteriophage, echinoderm larvae, and tropical butterflies.

One major theme in recent years is plastic and evolutionary responses to human-induced environmental changes—climate change, invasive species, agroecosystems—and their ecological consequences. A few of our current research projects:

  • Mechanisms and evolution of phenotypic plasticity
  • Ecological and evolutionary responses to climate change

  • Ecology and evolution of invasive species
  • phenotypic selection on body size and life history in nature

Mechanisms and evolution of phenotypic plasticity

Phenotypic plasticity—the expression of different traits in different environments—is ubiquitous, and adaptive plasticity that enables organisms to express favorable traits in appropriate environments is a critical evolutionary means of adapting to variable environments. But understanding the underlying mechanisms of plasticity and how these affect the evolution of adaptive plasticity are major challenges. We are using thermal reaction norms- traits that vary continuously as a function of temperature- as a system for exploring these challenges. One project, in collaboration with Fred Nijhout (Duke) involves a widespread pattern of plasticity in ectotherms called the temperature-size rule: in most species, the final (adult) body size of an organism is smaller at warmer rearing temperatures. For insects, the temperature-size rule is determined by the different thermal sensitivities of the underlying processes of larval growth, enzyme degradation and hormone secretion. Our recent studies with Manduca sexta (Tobacco Hornworm) and Pieris rapae (Cabbage White Butterfly) indicate larval hostplant quality can alter thermal reaction reaction norms for size, and in some cases reverse the temperature-size rule. Population comparisons in these species show evolutionary differences in thermal reaction norms among geographic populations, and that these patterns can depend on adaptation to local hostplants. These interactions between resource quality and temperature in determining plasticity may have important implications for evolution of adaptive plasticity and the evolution of body size. A second project, in collaboration with Ray Huey (University of Washington), Dick Gomulkiewicz (Washington State) and Christina Burch (UNC), explores mechanisms and evolution of thermal performance curves: the plasticity to temperature of biological rates such as growth, development, feeding, locomotion and fitness. These studies suggest that the underlying biophysical mechanisms can constrain patterns of genetic variation in performance curves, altering evolutionary changes in performance curves in response to selection due to changing thermal environments.

Ecological and evolutionary responses to climate change

There is now abundant evidence that climatic conditions are changing across the globe, and that many species are responding with changes in seasonal timing, local population abundances, and geographic range. We are interested in evolutionary responses to this climate change: are morphological or physiological traits more likely to evolve? Will change change cause greater selection and evolution in traits expressed in juveniles or in adults? In collaboration with UNC colleague Lauren Buckley, we are currently exploring these questions in several species of Colias (Sulphur) butterflies along an elevational (and climatic) gradient in the Rocky Mountains in Colorado. Analyses suggest that both mean and extreme temperatures have been increasing this region during the past 30-40 years. We are using lab and field studies to document morphological and physiological traits in larval and adults that affect performance and local adaptation to climate; compare these results with historical data and museum records, to detect potential evolutionary changes in these traits; and develop biophysical and demographic models to predict fitness consequences of larval and adult traits in different climate conditions. Our larger goal is to test whether evolution can reduce the negative ecological consequences of climate change in high-altitude systems.

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Ecology and evolution of invasive species

Invasion and colonization by exotic species of plants and animals into new regions and ecological communities are increasingly important, largely as a result of human activities. We are interested in whether invading insects may evolve rapidly in their new geographical ranges, and how such evolution may affect—and be affected by—ecological interactions with plants and other insects. In addition, evolution in invasive species can provide excellent ‘natural experiments’ to study local adaption to climate and other factors in their new geographic ranges. One project explores the responses of M. sexta (Hornworm) populations to an invasive plant species, Devil’s Claw, in the southeastern US during the last century. Our studies show that these Hornworm populations oviposit and feed on Devil’s Claw, but have lower rates—especially at lower temperatures– of larval growth, development and survival on Devil’s Claw relative to other, more typical hostplants. However, Hornworm larvae on Devil’s Claw escape parasitism by a major larval parasitoid that causes high Hormworm mortality on other hostplants. As a result, warmer summer temperatures and escape from natural enemies has facilitated the use of this novel hostplant by Hornworm populations in this region. A second major project examines rapid evolution of phenotypic plasticity and life history traits—body size, development time, fecundity, and immune response—in Pieris rapae, the Cabbage White butterfly. P. rapae is native to Europe, but independently invaded and colonization North America in 1860, and Japan by about 1700. We are studying evolutionary divergence in these traits for populations in both North America and Japan, to evaluate local adaptation of populations in response to environmental gradients in temperature, seasonality, and natural enemies.

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Phenotypic selection on body size and life history in nature

Many studies have documented consistent directional selection for increasing body size in natural populations: in many species, larger size is associated with greater fecundity in females and greater mating success in males. It is widely assumed that selection for increased adult size is balanced by opposing, correlated selection for more rapid juvenile development to escape juvenile mortality from natural enemies and other factors, but there is surprisingly little evidence from natural populations to support this assumption. We are examining phenotypic selection on size and development rates in relation to natural enemies in several insect species in the southeastern US. For example, M. sexta (Hornworm) populations have several generations each year; and both climate and the abundance of a major larval parasitoid, Cotesia congregata, vary seasonally across the generations. Our field studies show selection for large adult size and for rapid larval development in both summer and fall generations, even when parasitoid abundance is quite low. But adult size and larval development time are not correlated, so that selection for rapid development does not constrain the evolution of larger adult size. In collaboration with Sarah Diamond (North Carolina State) and others, we are also conducting meta-analyses that consider the scores of studies of phenotypic selection in natural populations that are now available, to assess patterns of directional, stabilizing, and correlated selection. Our analyses to date provide little support for the hypothesis that opposing selection between body size and development time is common in natural populations.