Phylogenetics and phylogeography of North Pacific bay gobies: adaptive convergence, relictual endemism, and climate-driven population structure
- Author(s): Ellingson, Ryan
- Advisor(s): Jacobs, David K
- et al.
North Pacific bay gobies inhabit bays, beaches and estuaries of temperate Asia and North America, but are absent from the northernmost latitudes of the central Pacific. Morphological characters have conventionally subdivided the clade into two groups - an elongate infaunal Astrabe group, and a deeper-bodied Chasmichthys group - each with a disjunct East-West (amphi-) Pacific distribution. In chapter 1, I use multi-locus DNA sequence data to examine phylogenetic relationships of bay gobies. Basal divergence of the tree coincides with a dramatic global cooling event at the Eocene/Oligocene transition, and there is no evidence of subsequent trans-Pacific migration. These results suggest that several morphological characters previously used to define the Astrabe and Chasmichthys groups have arisen independently on both sides of the Pacific, revealing convergence of ecologically adaptive characters within a geographically divided clade. Chapter 2 uses inferences of vicariance via biogeographic events to time-calibrate this phylogeny. Divergence time estimates allow me to compare and contrast potential mechanisms of bay goby diversification on either side of the Pacific. Speciation in the West Pacific has been driven largely by interstitial colonization of gravel beaches of varying grain size, and by invasion of freshwater streams around the Sea of Japan. In the East Pacific, diversification appears to be related to an intense upwelling regime combined with isolation in large Miocene-era embayments on the coast of California. These results also provide strong evidence for relictual endemism in the Gulf of California, as the divergence of three out of four Gulf-endemic gobies substantially predates tectonic formation of the Gulf itself. In chapter 3, I use one member of the bay goby clade, Gillichthys mirabilis, to investigate population structure within the Gulf and on the adjacent Pacific outer coast. Phylogeography suggests a complex history of extirpation and colonization driven by Pleistocene sea-level fluctuations. A degree of discordance between mitochondrial and nuclear DNA patterns, however, raises the possibility that differential selection may also contribute to genetic subdivision, precluding north-south migration of mitochondrial haplotypes in the face of more extensive nuclear gene flow.