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Speciation in Brachyrhaphis fishes

In the mid-1990's I first found the livebearing fishes in the genus Brachyrhaphis while collecting in northwestern Costa Rica. I was struck by the remarkable similarity between populations in the species B. rhabdophora and the well-known guppies of Trinidad. Both systems housed populations that experienced strong divergent selective pressures in response to the presence or absence of predators. My dissertation work focused broadly on evaluating the extent to which parallel evolution had occurred within B. rhabdophora. I showed that populations had repeatedly, and independently, diverged for a variety of life history traits in response to differences in predator-mediated mortality. Remarkably, my results mirrored those previously found in the guppy system, suggesting that evolution is predictable and consistent--similar selective pressures lead to similar evolutionary outcomes. More recently, work in my lab has focused on other traits that have diverged in response to predation environment, including morphology, swimming performance, and behavior. All of this led to the hypothesis that predator-mediated evolution is driving early stages of reproductive isolation within B. rhabdophora. A broader look at the genus revealed a pair of sister species that occur on the Pacific slope of western Panama that are largely segregated into predator environments (B. roseni) and predator-free environments (B. terrabensis). This provided us with an important contrast to evaluate the extent to which trait evolution in the early stages of reproductive isolation (within B. rhabdophora) is reflected after speciation is complete (between B. roseni and B. terrabensis). Work in my lab, conducted initially by PhD student Spencer Ingley and a handful of undergraduate students, shows that a variety of traits have diverged between B. roseni and B. terrabensis in ways similar to those documented in B. rhabdophora. Our current research is now focused on understanding the specific traits and mechanisms that contribute to reproductive isolation. We are also beginning work using next generation sequencing data to evaluate whether or not the same genetic architecture is responsible for parallel evolution in Brachyrhaphis, which we hope will ultimately reveal the genetic underpinnings that lead to speciation.

Demographic effects of predator-driven life history evolution

Ecology and evolution have long been viewed as sister disciplines. Ecological interactions are known to be powerful drivers of evolutionary change through the mechanism of natural selection. Yet surprisingly little work has focused on the effects of evolution on ecological processes. My early work on the evolution of life history traits led me to question how life history diversification in fish populations would affect population growth and long-term demographic trends. Using serial mark-recapture approaches to estimate mortality rates in the wild, coupled with estimates of fecundity from field collections, we have employed a population matrix modeling approach to calculate demographic trends in natural populations. Our results in Brachyrhaphis fishes show when life history traits diverge, it shifts the relative importance of different vital rates to overall population growth. Yet populations still maintain stable and sustainable growth even under different selective regimes.

Behavioral ecology

Of the many phenotypic traits that natural selection can act upon, behavior is one of the most interesting. Some behaviors are fixed, or canalzied, but many behaviors are quite flexible. These allows organisms to show a variety of responses to different stimuli. We have started to explore different facets of the evolution of behavior. What started as simple questions asking how living in an environment with constant predation risk shaped prey behavior, has now expanded to several lines of inquiry focused on what we consider to be interesting current problems in behavioral research. For example, there is a great deal of interest in the evolution of animal personality. In fact, we have published several papers focused on links between personality and speciation. However, we are now finding that some individuals are remarkably consistent in their personality tendencies, while others are much less so. This variation has captured our attention and may have important implications for how we understand the evolution of behavior. We are also interested in the evolution of copying behavior, and its consequences for mate choice and decision making. The fish we work with, it turns out, are quite interesting in the ways that they gather and use information from other individuals to make decisions. And finally, one of the most exiciting projects that we are currently working on explores the evolution of lateralized behavior. We have found that fish show a side bias in the way that they orient their bodies (left or right) to view different stimuli. By comparing fish that are morphologically bilaterally symmetrical to fish that are asymmetrical with respect to the male mating structure (the gondopodium, which corkscrews either to the left or right), we are exploring how handedness has evolved and how it might ultimately be linked to brain lateralization in vertebrates.