Research

My work explores the integration of the phenotype—how groups of morphological, physiological, and behavioral traits interact and evolve. Natural selection is expected to favor not only traits that fit well with an organism’s environment, but also integrated groups of traits that work well together. As populations adapt to their environment, natural selection may shape the genetic and physiological underpinnings of these integrated groups. The evolutionary, genetic, and physiological mechanisms responsible for phenotypic integration are the primary focus of my research.

G-matrix evolution in anoles

Caribbean Anolis lizards have been a classic study system for evolutionary biology. On the Greater Antilles, anoles have undergone adaptive radiations allowing them to colonize multiple environmental niches. Ecological types such as trunk-crown, trunk-ground, and twig forms have evolved multiple times on different islands. Each type has its own characteristic morphology.

 In collaboration with Butch Brodie and Jonathan Losos, I am exploring the underlying quantitative genetic structure of the morphology of these ecological types. By measuring the G matrix, which describes how traits vary and covary genetically, in multiple species, we are asking whether the pattern of genetic integration has been shaped more by adaptation or phylogeny.

Physiology of integration in juncos

Because of their ability to regulate the expression of many traits simultaneously, hormones may be a major physiological factor underlying the integration of the phenotype. In songbirds, testosterone is particularly important for the expression of several traits related to obtaining mates. In collaboration with Ellen Ketterson, I have been exploring individual variation in testosterone-mediated phenotypes in dark-eyed juncos.

Surprisingly, we found that many important traits are associated with short-term fluctuations in testosterone levels, rather than steadily elevated levels. These short-term elevations are mediated by social interactions and by the release of gonadotropin-releasing hormone (GnRH) in the brain. Using injections of GnRH, we found that short-term testosterone elevations were associated with variation in territorial behavior, parental behavior, and plumage. These short-term elevations seem to underlie the fundamental trade-off between mating effort and parental effort experienced by male juncos.

Evolutionary mechanisms of phenotypic integration

One of the major evolutionary mechanisms leading to integration is correlational selection, which occurs when groups of traits interact to affect fitness. Over time, correlational selection is expected to shape the G matrix, generating genetic correlations between functionally related traits. In juncos, larger males are more successful in male-male interactions, while males with whiter tails are more attractive to females. We found that these two traits combined to determine male mating success, causing correlational sexual selection. In addition, these two traits were genetically correlated, suggesting that correlational selection may be maintaining the integration of these traits.

We are also currently exploring how selection acts on short-term testosterone elevations. Because these elevations are so closely related to mating effort and parental effort, they are expected to be closely associated with fitness or components of fitness.

Theoretical work

Interactions are important not only at the level of the individual, but also between conspecifics. The theory of interacting phenotypes examines how interactions affect the evolutionary process. In collaboration with Butch Brodie, I have been working on several extensions of this theory, including the development of statistical methods to measure model parameters.

Collaborators