|As an Integrative Biologist, my research seeks to connect four fundamental aspects of the evolution of organismal diversity: (1) lineage diversification, (2) historical biogeography, and the influence of (3) morphological and (4) molecular evolution on organismal performance. Although there are a number of approaches that one may utilize in order to gain insight into these processes, my research program is firmly rooted in phylogenetic systematics with a more recent foray into population genetics. Given my emphasis on phylogenetics, I have a fifth area of interest, the theory and practice of phylogenetic systematics. Thus, I would describe myself as a phylogenetic systematist with expertise in molecular genetics, phylogenetic comparative methods, and theoretical aspects of phylogenetic analysis, with a strong and growing interest in molecular population genetics.|
The Evolution of Gliding Performance in Flying Lizards: The flying lizards (genus Draco) of Southeast Asia, a radiation of approximately 45 species, represent an excellent model system with which to address questions pertaining to body size evolution and its role in the evolution of gliding locomotor performance. Draco exhibit substantial size variation, with species differing by more than an order of magnitude in maximum body mass. My phylogenetically-controlled analysis of allometric scaling in Draco found that they do not deviate significantly from isometry small species and large species are essentially scale-replicas of one another, without significant shape variation. Thus, larger species do not account for their increasing body mass with equivalent increases in the area of the lift-generating surfaces. Therefore, large Draco species are predicted to be relatively poor gliders unless they can compensate behaviorally or physiologically. With empirical gliding performance data obtained via field trials, I tested locomotor performance predictions derived from the allometry study in a comparative framework. These analyses illustrated that larger species of Draco are indeed relatively poor gliders, as predicted by the allometric scaling study. The fact that Draco are unable to compensate physiologicallor behaviorally indicates that the evolution toward larger size (which has occurred 4 times during Draco evolution) does not come without a cost for flying lizards.
|Historical Biogeography of Sulawesi Frogs, Lizards, and Monkeys: My biogeographical studies are focused primarily on the island of Sulawesi and adjacent islands in eastern Indonesia. This region has experienced an incredibly complex tectonic history and consequently is particularly challenging for biogeographers. Sulawesi itself is a composite island formed from five discrete paleo-islands sutured together during the past 15 million years following extensive micro-plate movements from various corners of the South Pacific region. Furthermore, subsections of these geologically distinct units have been temporarily fragmented via fluctuations in sea level over the past several million years. Consequently, many Sulawesi taxa exhibit regional endemism within the confines of Sulawesi, as well as on its satellite islands. The Draco lineatus complex (ilustrated in the accompanying figure) provides an excellent example. Together with Ben Evans and Rafe Brown, I am investigating Sulawesi biogeography using a comparative approach that involves phylogenetic and coalescent population genetic analyses of nine focal taxa (including frogs, lizards, and monkeys). Our approach is predicated on multilocus data and to that end we have developed genomic libraries for our focal taxa and are screening 20 independent anonymous nuclear loci for ~250 individuals per taxon. These data will first be analyzed using phylogenetic methods to identify points of secondary contact or primary divergence, but will be followed up with coalescent-based population genetic analyses to estimate divergence timing and migration (gene flow).|
|Phylogenetics and the Evolution of Flight Performance in Hummingbirds: Hummingbirds utilize the most energetically demanding form of locomotion (sustained hovering) of any vertebrate taxon. Nevertheless, hummingbirds are most diverse at high elevations in the Andes where oxygen availability is limited, air density is low, and where it therefore should be most difficult to utilize this form of locomotion. Despite that I am ostensibly a herpetologist, much of my recent research effort has been directed toward a multilocus species-level phylogenetic analysis of hummingbird relationships. A major objective of this research (undertaken in collaboration with Doug Altshuler at UC Riverside and Robert Dudley of UC Berkeley) is an investigation of the relationship between body size, morphological predictors of flight performance such as wing loading and aspect ratio, flight kinematic parameters such as wingbeat frequency and stroke-amplitude, and burst-flight performance across elevational gradients. All of this comparative work is undertaken in the context of our robust multilocus phylogenetic estimate. My interest in hummingbirds extends beyond the allometry and evolution of flight performance, and includes historical biogeography and the timing of hummingbird diversification and its relationship with Andean uplift and the diversification of obligate hummingbird-pollinated plants.|
|Molecular Evolution of Hummingbird Globin Genes and Respiratory Physiological Performance: Together with Dr. Chris Witt of the University of New Mexico and Dr. Robert Dudley of UC Berkeley, I am investigating the evolution of hummingbird globin genes and its consequences for respiratory physiological performance.Globin proteins such as (hemoglobin) are responsible for oxygen transport and are therefore likely targets of selection in high-elevation hummingbirds. In order to detect signatures of positive selection on these respiratory proteins, we are sequencing eight hemoglobin and myoglobin respiratory genes for 275 hummingbird species in relation to lineage-scale colonization of, and radiation within, the high Andes. We are analyzing the DNA sequence data using recently developed maximum likelihood and Bayesian methods that allow for the study of selection at each codon, and on each branch of the phylogeny. Our preliminary data for the hemoglobin a-A subunit identified 11 independent instances in which hummingbird lineages invaded high elevation habitats, and experienced subsequent positive selection on amino acid positions near the oxygen binding sites. The reconstruction of the hemoglobin a-A subunit shown here indicates several amino acid positions under positive selection and their positional relationship with the O2 binding site and an amino acid substitution for bar-headed goose documented to provide enhanced O2 binding.|
|Phylogenetics, Mitochondrial Introgression, and Incomplete Lineage Sorting in Crotaphytid Lizards: I have a long-standing interest in the phylogenetics and historical biogeography of collared and leopard lizards. My recent work with this group has been focused on documenting a unique pattern of repeated mitochondrial introgression that has occurred between Crotaphytus collaris and its neighboring non-sister species, C. bicinctores and C. reticulatus. I am particularly interested in the possibility that cytonuclear disequilibrium could explain the biased transmission of mitochondrial genomes across the geographical landscape.|