Morphological Evolution
Body Form Evolution
The transition from quadrapedal to limbless body plans has occurred repeatedly in numerous independent lineages of squamates. However, few genera of lizards possess both fully limbed and limbless species. The known species-level diversity of skinks of the genus Brachymeles is concentrated in the Philippines, with species exhibiting a full range of limb development, including fully-limbed, intermediate, and limbless forms. The focus of my dissertation study was to investigate the process of limb reduction and loss of Brachymeles using a multi-locus dataset and particularly dense geographic sampling. I was interested in exploring species-level diversity within the group, as well as conducting comprehensive phylogeographic analyses of the group. Given an historical phylogenetic framework, I tested the polarity of evolutionary change in body form, estimated the number of times limbs have been lost in Brachymeles, and surveyed morphological changes associated with limb reduction. The genus Brachymeles has quickly become an excellent model system to address a variety of hypotheses related to body form evolution, miniaturization, limb reduction and loss, and correlated character evolution. Exploring patterns of diversification and body form change within this amazing radiation of lizards resulted in the discovery of many interesting patterns which are summarized in the following two studies:
Siler, C. D. and R. M. Brown. 2011. Evidence for repeated acquisition and loss of complex body form characters in an insular clade of southeast Asian semi-fossorial skinks. Evolution 65:2641-2663.PDF
Siler, C. D., A. C. Diesmos, A. C. Alcala, and R. M. Brown. 2011. Phylogeny of Philippine slender skinks (Scincidae: Brachymeles) reveals underestimated species diversity, complex biogeographical relationships, and cryptic patterns of lineage diversi cation. Molecular Phylogenetics and Evolution 59:53-65. PDF
Functional Morphology and Evolutionary Genomics
In addition to the studies on the polarity of character change, evolutionary patterns of limb-reduction and loss have provided biologists with a rich suite of hypotheses for tests in a phylogenetic framework. Recent phylogenetic studies have taken comprehensive approaches to understanding the correlated patterns of morphological change, the number of times these changes have taken place, and the directionality of these transitions. For exemplar taxa, a few studies have examined this process from a developmental or functional framework; however, no approaches have been able to take an integrative, multi-taxon approach to studying the process. Recently, I developed a collaborative research proposal that is currently under review at the National Science Foundation. The project will address questions concerning the irreversibility of complex characters via an integrative approach combining the fields of functional morphology and developmental genomics. This project is a collaborative undertaking, involving researchers at Clark University, the University of Massechusetts – Amherst, and Yale University.
Morphological Disparity and Species-level Diversification
I am interested in examining morphological disparity through time and exploring specific components of morphology that have contributed to species-level diversification. Several groups of Philippine vertebrates represent ideal systems for studying these patterns.
The genus Kaloula consists of species of frogs that are commonly referred to as Narrow-Mouth Frogs because of their small, pointed mouths. The diversity of Philippine species is often divided up into three distinct ecomorph groups. The first being the shrub frogs with species posessing expanded toe disks used for an arboreal lifestyle. The second group consists of burrowing frogs with narrow toe disks who burrow during the year except for the rainy season. Finally, the third group has have evolved a completely arboreal lifestyle. These species spend their lives within tree-holes and have wide toe disks. I am collaborating on a project investigating the evolution of ecomorph classes among Philippine Kaloula frogs using a phylogenetic framework and dense geographic sampling. We are interested in the amount of morphological change through time and the number of times each ecomorph class evolved.
Agamid lizards in the Philippines represent another system I am currently exploring as part of my research program. Three species are recognized to occur in the Philippines, all with little information available on precise type localities. Variation in crest morphology exists across the Philippines, and I am interested in exploring disparity in crest morphology as it relates to diversification patterns observed across the archipelago.