The overarching aim of my research is to discover the origins and maintenance of marine biodiversity, with a particular focus on the biodiversity of coral reef fishes. I concentrate my research efforts on the interrelationships between species, the most common biological unit used to describe the diversity of life. I am broadly interested in the divergence of species through time and space, and the mechanisms that maintain species boundaries. Rooted in systematic biology, my research applies molecular phylogenetics and biogeography to address specific questions pertaining to the timing of extant species divergence in relation to biogeographical patterns. Using these temporal and spatial patterns as frameworks, I am now applying my skills to discover the drivers of ecological and phenotypic trait diversification in coral reef fishes, and how these traits might function to maintain species boundaries.
Why focus on species?
Biodiversity is a measure of the variety of life. It can refer to variation on a number of levels, although species are the most common unit used to describe biodiversity. Therefore, to understand the evolutionary processes that generate biodiversity and the mechanisms that help to sustain it, I study species. Species-level phylogenetics adjoins more common family-level phylogenetic approaches that include representative taxa to evaluate relationships among clades of species, and phylogeographic studies of relationships among populations within species. Complete species-level sampling provides the opportunity to robustly identify sister-species relationships, more accurately estimate the timing of speciation, and establish geographical relationships of closely related species. These key pieces of information form a framework that can be used to investigate the evolutionary processes that promote biodiversity.
Figure from Hodge et al., 2013 showing the biodiversity of Pomacanthus species.
Uncovering evolutionary processes
Figure modified from Hodge & Bellwood, 2015 illustrating the evolutionary process of peripheral budding.
One such evolutionary processes that I have identified through my research is the concept of peripheral budding. This process came to light when examining the geographical relationships of closely related species in the genus Anampses. The process involves successive divergence events from a single species and results in a phylogenetic tree topology where the divergence time is underestimated for that species. In the figure above, the successive divergences are illustrated as founder events (although they could occur via any mode of speciation) at times t1, t2 and t3 and the age of the species from which they diverged is estimated from the phylogeny as t3. This is an underestimation of the true species age, which is more accurately estimated as t1, or as t-zero, if the phylogeny contained more taxa. Peripheral budding is potentially prevalent among fishes and other organisms with comparable dispersal capabilities, like birds and insects.
My research has also described methods for further analysis of species-age estimates that overcome their misidentification in the case of peripheral budding, and tackle issues of pseudoreplication. The application of these methods revealed an increase in the geographical range area of coral reef fish species with age.
Figure from Hodge & Bellwood, 2015 showing an increase in species geographical range area with age.
Looking for patterns through time and space
Figure modified from Hodge et al., 2014 showing recent divergence estimates for sister-species from different ocean basins (grey bar) and biogeographical regions (coloured circles correspond to coloured regions on map).
My research has identified temporal and biogeographical patterns of species-level divergence that are consistent across four of the major coral reef fish families, the Chaetodontidae, Epinephelidae (formerly Serranidae), Labridae, and Pomacanthidae. For example, despite differing geological histories, all major ocean basins have supported recent species divergence. In contrast, species endemic to the Red Sea and Hawaiian Islands have vastly different age structures, suggesting different underlying evolutionary processes.
Figure from Hodge et al., 2014 showing steady divergence of species endemic to the Red Sea over the past ~16 million years (top) in contrast to two waves of divergence of species endemic to the Hawaiian Islands (bottom).
In a forthcoming article, my co-author and I outline the relative importance of biogeographical barriers in the divergence of reef fish species and map areas of geographical range overlap among sister-species. We describe the likely mechanism of a previously understudied biogeographical barrier in the Indian Ocean and show that it accounts for a substantial portion of recent reef fish divergences. Additionally, this work showed that the area of highest coral reef fish biodiversity also contains the largest number of sympatric sister-species. This supports the area as a centre of overlap and a evolutionarily dynamic region with a complex environment capable of sustaining closely related species in sympatry.
Figure from a forthcoming paper showing biogeographical barriers with corresponding splits between allopatric sister-species.
Figure excerpt from a forthcoming paper showing the concentration of sympatry among sister-species in the area concordant with the centre of coral reef fish biodiversity.
What traits function to maintain species boundaries?
My ongoing and future research focuses on the ecological drivers of diversification and reinforcement. Still within the context of phylogenetics and biogeography, I am investigating the role of sexual dichromatism and the interplay between natural and sexual selection in the diversification of wrasses and parrotfishes. Results from this work indicate that sexual dichromatism is more pronounced in fishes that occur only in brightly lit coral reef habitat. In addition, by focusing on sister-species this work has revealed interesting dynamics of sexual selection versus natural selection through time. Check back again soon for more on this. In the meantime here’s a picture of a terminal phase wrasse.
How have phenotypic traits like color pattern evolved and how do they function?
In 2016 I will begin an NSF Postdoctoral Research Fellowship in Biology, for research using biological collections. For this project, I seek to discover the function and ecological drivers of butterflyfish color patterns using mathematical modeling to simulate color pattern and phylogenetic comparative methods to test ecological predictions. I look forward to travelling to the Smithsonian National Museum of Natural History and the California Academy of Sciences to work with their fish collections.