My dissertation looked at the scales (both spatial and temporal) over which we observe genetic variation in several coral reef fish. The goal of these observations is to improve our understanding of the evolutionary processes responsible for the profligate diversity of the coral reefs of Melanesia. Concurrent with the evolutionary aim there is a strong emphasis on providing information that generates real and actionable conservation measures, through the understanding of population connectivity, endemism and biodiversity identification throughout the region.
I have developed a hierarchical approach towards studying these processes. The first project, which represents the finest scale approach, uses the mitochondrial control region to look at gene flow within several common coral reef species (both fish and invertebrate) within the Fijian archipelago. Here I have found that despite the presence of a dynamic oceanographic environment, we see little geographic structuring of genetic diversity, suggesting the potential for larval connectivity of populations within the country. Upon completion of this project the Fijian government incorporated these findings in establishing a network of marine protected areas.
The second chapter looked at the genetic structure in five putatively monospecific fishes from the Indo-West Pacific (Amphiprion melanopus, Chrysiptera talboti, and Pomacentrus moluccensis [Pomacentridae] and Cirrhilabrus punctatus and Labroides dimidiatus [Labridae]) that express regional color variation unique to Fiji. Mitochondrial control-region sequence analysis showed shallow to deep genetic divergence in all five species (sequence divergence 2-17%) with clades concordant with regional color variation. These results were partially supported by nuclear RAG2 data. An analysis of molecular variation (AMOVA) mirrored the phylogenetic results with Fst values range from .91 to .7 indicating high levels of geographic partitioning of genetic variation. Concordance of genetics and phenotype demonstrate the genetic uniqueness of southwestern Pacific color variants, indicating that these populations are at a minimum distinct evolutionarily significant units and perhaps distinct regionally endemic species. This has been published as an article in Conservation Biology (Drew et al. 2008).
Having identified Fijian populations as being genetically unique, I explored the genetic diversity of two species (A. melanopus and P. moluccensis) across the entirety of their range. Both species have evolved a southwest Pacific endemic form (chapter 2), representing an initial speciation event. Additionally, these species also show evidence for speciation along the western boundary of their ranges, in Sumatra and Bali Indonesia, suggesting that peripatric speciation can occur multiple times within a single lineage. Taken together, this evidence suggests that these species represent marine superspecies. These findings therefore suggest that the pelagic nature of larval dispersal does not necessarily preclude genetic subdivision; and that widespread species may harbor recondite patterns of biodiversity across their ranges. Part of this chapter has been published in Molecular Phylogenetics and Evolution (Drew and Barber, 2009).
I continue to expand the geographic range of my thesis, by examining the phylogeography of the widely distributed fish, Halichoeres hortulanus. Using information from the mitochondrial and nuclear genomes we demonstrate that there are clear differences between Pacific Ocean and Indian Ocean fish (mtDNA Fst =. 38). We show that this biogeographic boundary appears to act as a near complete one-way filter with haplotypes moving from the Pacific to the Indian, but not in the other direction. Similarly we demonstrate a signal of rapid population expansion in the Indian Ocean samples suggesting that there has been a recent invasion from East to West. Our results indicate that while the fish has the potential for long distance dispersal, several factors, including paleohistory and modern day ocean currents serve to restrict this dispersal capability. We presented this chapter this summer at the 8th Indo-Pacific Fish Conference.
