Evolution of Morphological Variation

Understanding evolutionary patterns of morphological variation help us to determine the way in which biodiversity evolves. My research uses a robust phylogeny of skinks to approach a variety of questions about morphological variation.

Osteological variation

The systematic organization of skinks has been based primarily on the structure of different osteological features of the skull and axial skeleton.  The characters that define the different subfamilies and many of the groups within subfamilies are based on osteology. Recent molecular examinations of higher level skink relationships have shown that the current understanding of skink systematics is incorrect, warranting a re-examination of these osteological features.

In the Sphenomorphus group, many characters associated with the hard palate were used to define the relationships of the 30 genera. I am re-examining the palate characters in light of my molecular phylogeny and looking for other osteological features that correspond with the molecular data.

Body-size evolution

Skinks show rapid, continuous and reversible changes in adult body size throughout their diversity.  Species range in size from less than 20mm to over 102 cm (over a 500 fold difference in size). In the Philippines, species have reduced in size multiple times and there is clear evidence of a reversal to increased size within one clade.  As part of my dissertation, I became interested in the patterns of body-size change and what morphological features were correlated with extremely small size, or miniaturization.  I am currently examining cleared and stained specimens of miniaturized Philippine skinks to see if the multiple independent reductions in size have resulted in similar changes to osteological features.

Hemipene morphology

The structure of the hemipenis is a character commonly examined in morphologic studies of snakes. Examination of this features has seen limited use in understanding lizard relationships.  In an attempt to find new morphological characters to help diagnose the relationships I find using DNA data, I have started exploring the morphology of lizard hemipenises.  I have already found numerous useful characters that correspond with clades recovered in my molecular phylogeny. The image of a hemipenis at left is from a clade of Philippine skinks which my colleagues and I have described as a new genus.  The large lobed structure on the lower shaft of the hemipenis is unique among the skinks we have examined and is used as a diagnostic character for the genus (manuscript in press).

I think that detailed exploration of morphology outside of traditional scale counts and osteology will allow taxonomists to reconcile the incongruence between current morphological taxonomy and molecular phylogenies.

Skink systematics

Scincidae is the most species rich family of lizards. Skinks inhabit a variety of different niches, have a global distribution, and have a range in body size from 20 mm to over 102 cm. This large and diverse group provides a great system for examining many different evolutionary questions.

Resolving skink taxonomy

With over 1,450 species, Scincidae is the largest family of lizards. This globally distributed group shows a high level of morphological variation, making it an ideal group to study morphological evolution. I work on the Sphenomorphus group of skinks, which are mostly distributed in Southeast Asia and Australia, and consist of about 500 species in 30 genera.  There are many taxonomic issues in this group that I am working on resolving.  Additionally, I am using this large diverse group to understand rates of speciation and morphological diversification.

Alpha taxonomy

Discovery and description of new species is crucial to proper characterization of the world’s biodiversity. As part of my work on skinks I continually find and describe new species. My work also resolves taxonomy in geographically widespread groups that actually represent multiple species.

Current Projects

Taxonomic revision of the Sphenomorphus decipiens species group

Taxonomic revision of the Sphenomorphus abdictus-coxi-jagori complex

Island Biogeography

Many species in Southeast Asia have geographically widespread distributions that span multiple islands.  Using a comparative approach, I am investigating whether these species share common evolutionary histories due to the geologic history of the island archipelagos. The Philippines are a ideal region for these studies.  The history of the region is fairly well understood and there are multiple co-distributed species.

Additionally, I am interested in pan Southeast Asian biogeography, spanning some of the most famous barriers in the world such as Wallace’s line and Lydekker’s line. I am using a variety of skink groups to study these barriers, comparing the history of the islands to the relationships of the organisms that inhabit them.

Current Projects

Comparative biogeography of narrow-mouth toads (Microhyla) across the Sunda Shelf.

Collaborators: Jeet Sukumaran, Jesse Grismer

Diversification Between Two Continents: Dispersal Driven Biogeography of the Cryptically Diverse Emerald Skink (Lamprolepis smaragdina). in prep

Collaborators: JA McGuire, CC Austin, D Iskandar, AC Diesmos, RM Brown, J Supriatna.

Phylogenetic Theory

I am working on: (1) how to appropriately accommodate among site rate variation through partitioned phylogenetic analyses; and (2) extending species tree methods to large taxa datasets (hundreds of species).

Among Site Rate Variation

Accommodating rate variation in estimations of phylogeny has been considered critical for accurate phylogenetics for two decades.  This is commonly accomplished by incorporating rate variation through a gamma parameter and a proportion of invariant site and, more recently, by partitioning data into rate classes.  Most often, data is partitioned based on interpretations of secondary structure of the molecule being analyzed.  Recently, methods have been developed to categorize nucleotide data into rate classes based on their individual substitution rates.  Fellow KU graduate student Jamie Oaks and I have found that allowing these new models to categorize the nucleotide data results in rapid convergence on a preferred topology with a higher likelihood than structural partitioning schemes.  We are still working on testing this method through simulation to see if it is consistently accurate in topology estimation.

Species Trees with Several Dozen Taxa

    Species Tree methods (BEST, *BEAST, MDC, and STEM) allow for the incorporation of variation in coalescent time in multigene geneologies. In other words, if the genes you have sampled for the same species show different relationships, these methods accommodate that difference by allowing for differences in genes coalescence.

    These methods help to overcome some of the assumptions that are violated by the data concatenation approach.  The problem is that Species Tree methods are computationally complex, and therefore take a long time to converge even with a small (50 taxa) dataset.

    I am working on a hierarchical approach to Species Trees that can be used for large taxa phylogenies to allow me to estimate the species tree for a 250 species dataset with six independently evolving loci.

Understanding Biological Diversity

Quantifying and discussing biodiversity is important for determining conservation priorities. In other words, conservation efforts can only be as effective as the way in which biodiversity has been assessed. It is therefore important for biodiversity investigators to be mindful of the assumptions behind their assessments of regional diversity.

In the Philippines, I am examining the pattern of species richness on different islands.  Two islands, Luzon and Mindanao have similar species richness, with 11 endemic species. It could therefore be argued that these islands are equally important for conserving this group of lizards. However, the phylogenetic relationships are very different between the species on the different islands, with much higher species diversity on Mindanao compared to Luzon. This example demonstrates the challenge in deciding whether conservation decisions should be based on species number or genetic diversity. I believe that my research will help to answer this question.