The Orangeband Surgeonfish, Acanthurus olivaceus. Photo from reefguide.org
Marine systems challenge the view that speciation is the result of geographic isolation. Many marine taxa have large effective population sizes, which slows lineage sorting, larval dispersal phases that may extend for days, weeks, and sometimes months, potentially connecting far flung populations, and they exist in an environment that lacks obvious physical isolating barriers. All of these characteristics should theoretically lead to panmictic populations with little chance of divergence and yet marine ecosystems, particularly coral reefs, are among the most species-rich on the planet.
In their recent Molecular Ecology paper, Gaither et al. (2015) used mitochondrial and nuclear intron sequences and 3,700+ genome-wide SNPs to test the mechanism of speciation in two members of the Orangeband Surgeonfish species complex, Acanthurus olivaceus and A. reversus. The authors focused on peripheral populations that split from the main central-west Pacific lineage at approximately the same time (~0.5 million years ago; see figure below). The Hawaiian population is morphologically indistinguishable from the main central-west Pacific lineage but the Marquesas population differs morphologically and in its ecological niche.
Distribution and phylogeny of recently diverged species Acanthurus olivaceus and A. reversus. Figure taken from Gaither et al 2015.
Using genome scans the authors showed that while both populations had signatures of genetic drift, there was strong disruptive selection in 59 loci sampled in fish from the Marquesas, including an opsin gene, perhaps under selection due to ecological differences in Marquesas reef habitats:
Coral reefs are generally characterized by clear oligotrophic waters, however, coral reefs in the Marquesas are atypical. Episodic increases in nutrients around the islands, attributed to a combination of ocean currents, El Niño events, and terrestrial run off, yields algal blooms and lower visibility… [T]he low light conditions that characterize the Marquesas could be driving the selection in visual pigments detected here in A. reversus. Future studies targeting opsin genes across taxonomic groups will help inform the role of these pigments in adaptation of fishes at the Marquesas.
The results of Gaither et al. point to geographic isolation as the main driver of divergence in Hawaii while selection, perhaps for ecological conditions, has driven divergence in the Marquesas. This work suggests a role for neutral and non-neutral mechanisms of speciation in marine organisms.
Michelle R. Gaither, Moisés A. Bernal, Richard R. Coleman, Brian W. Bowen, Shelley A. Jones, W. Brian Simison and Luiz A. Rocha (2015) Genomic signatures of geographic isolation and natural selection in coral reef fishes. Molecular Ecology. DOI: 10.1111/mec.13129