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Renée Duckworth
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The unifying theme of my work is the recognition of the active role of organisms in generating ecological and evolutionary change. The two central goals of my research are to understand how behavior evolves and how evolutionary changes in behavior influence population dynamics that ultimately shape macroevolutionary processes. To achieve these goals, I explicitly connect the mechanisms underlying individual variation in behavior to evolutionary processes by integrating approaches from physiological and evolutionary ecology, quantitative genetics, and phylogenetic comparative methods.



1. Origin and evolution of distinct dispersal strategies

The mechanisms underlying the colonization of new environments – from the initial arrival of new individuals to population establishment – remain poorly understood. Because the success of colonizers depends on their ability to survive and reproduce in novel ecological conditions, they must either be pre-adapted to the new environment or be flexible enough to respond rapidly and adaptively to novel conditions. To address this problem, I investigate the dynamics of dispersal evolution in the context of range expansion as well as use large-scale ecological experiments to manipulate the process of colonization.


I discovered the ongoing range expansion of western bluebirds (Sialia mexicana) in the northwestern United States which has resulted in the displacement of its close congener, mountain bluebirds (S. currucoides) and found that western bluebirds display distinct dispersal phenotypes where aggressive males are more dispersive than nonaggressive males.





Using large-scale field experiments, empirical measures of lifetime fitness and molecular pedigree reconstruction I found that integration of aggression and dispersal led to rapid changes in aggression across the range that corresponded to western bluebird’s history of colonization and the competitive displacement of mountain bluebirds.




The adaptive polymorphism in dispersal strategies was maintained prior to range expansion, potentially to facilitate recolonization of ephemeral fire-produced habitat on which this species historically depended. In the future, I plan to compare colonization of natural post-fire habitats with experimentally created nest box populations to determine how human conservation efforts to save this species may be influencing its behavioral and morphological evolution.




2. From micro to macroevolution: linking behavioral change to evolutionary diversification

A major question in evolutionary biology is what determines the rate of evolutionary change? Do changes in organismal form reflect only external environmental change or are there intrinsic characteristics that differ among taxa that can act as drivers or inhibitors of evolution? Because behavior is at the forefront of the organism-environment interaction, it has long been considered an important pacemaker of evolutionary change. To investigate how behavior influences the rate of evolution, I developed a conceptual framework that makes specific predictions about how different developmental and evolutionary mechanisms of behavioral change can impact selection pressures and ultimately affect the evolutionary trajectory of populations. In my empirical work, I use both field experiments on natural selection and comparative studies of species diversification to test the predictions of this framework.


In a field study, I showed experimentally that behavioral interactions sorted western bluebird males into distinct breeding habitats where they experienced differential selection on morphology. These results suggest a mechanism by which behavior and morphology can become integrated and morphological polymorphisms associated with dispersal can evolve.



In a comparative study in collaboration with Jonathan Losos, I am currently investigating how evolutionary shifts in habitat use affect rates of diversification in two speciose clades of birds, the starlings and thrushes. To carry out this project, I have used Bayesian analysis to construct a single tree of more than 250 species and am currently analyzing this tree to compare diversification rates among cavity nesting and non-cavity nesting clades.




One interesting preliminary result is that, after controlling for phylogeny, cavity nesting species have a higher risk of extinction compared to non-cavity nesting species. The implication of this study is that the evolution of traits which are adaptive in the short term may predispose clades to extinction in the long term potentially influencing diversification rates among clades.




3. Physiological mechanisms underlying adaptive integration of behavioral phenotypes

Correlated expression of multiple distinct behaviors is thought to result from the pleiotropic effects of hormones that simultaneously affect their expression. Such pleiotropic effects have the potential to constrain the flexibility of behavior. I combine experimental manipulation with observations of natural variation to gain insight into how hormones mediate the development and expression of both flexible and inflexible behavioral phenotypes.





We are currently teasing apart the causal links between prolactin elevation, parental behavior and male plumage color by experimentally switching male behavior using hormonal implants. How do females respond when the phenotypes are reversed and bright males suddenly become better parents than dull males?





Ultimately, we’d like to understand, not only the proximate physiological mechanisms that influence male parental behavior but also, what cues in the environment stimulate males to pursue one parental tactic over another. So far, we’ve found that males’ plumage coloration and hence their reproductive tactic are partly age-dependent and partly influenced by environmental context. Male plumage color during their first molt depends largely on their physiological condition, however, in older males, plumage coloration is mostly a function of their pairing status in the previous year.