The origin, evolution, and diversification of rockfishes of the genus Sebastes (Cuvier)

Abstract

The evolutionary relationships of the livebearing rockfishes of the genus Sebastes have been a point of interest since their original description. With over 65 species found in the northeast Pacific (NEP), 27 in the northwest Pacific (NWP), seven in the Gulf of California (GC), four in the north Atlantic (NA) and at least two in the southern hemisphere (SH), they represent a fascinating lineage for studies of spatial and temporal patterns of dispersal, vicariance and speciation in the marine environment. Previous studies of Sebastes species have attempted to reconstruct their phylogeny using allozyme patterns or portions of a single mitochondrial gene while incompletely sampling the genus, resulting in a partial picture with low statistical support. In this study, genetic analyses using sequence data (5581 characters) from seven mitochondrial genes (cytochrome b, cytochrome c oxidase subunit 1, 12S rRNA, 16S rRNA, tRNA proline, tRNA threonine and the control region) and two nuclear genes (recombination activating gene 2 and internal transcribed spacer 1), along with a near complete sampling of species, have produced a well supported phylogenetic hypothesis of the relationships between Sebastes species as well as clarifying their position within the scorpaenid subfamily, Sebastinae. Though studies of similar magnitude have been conducted at the family and subfamily level, this represents the most detailed and extensive examination of biogeography and marine speciation within a single, widely distributed marine fish genus. Both Bayesian posterior and maximum parsimony analyses produced highly similar phylogenies suggesting an origin for Sebastes at high-latitudes in the NWP. The majority of previously proposed sub-generic groupings based upon morphology are found to be either para- or polyphletic. Using Bayesian-derived genetic distance measures together with rate smoothing techniques, a molecular clock was applied to the phylogeny. The clock-calibrated data suggest that Sebastes originated in the middle Miocene, concordant with fossil data, and began substantial diversification and dispersal in synchrony with high-latitude cooling and establishment of productive upwelling systems across the north Pacific (NP) in the late Miocene. Contrary to contemporary taxonomic criteria that often group Asian and North American species based on common morphology, the molecular phylogenies tend to indicate geographically circumscribed lineages with no evidence for repeated long distance dispersal between disjunct biogeographic provinces (e.g., Asian species nested within a North American lineage). No examples of large-scale glacial vicariance as would be suggested by Asian and North American sibling species were observed. To the contrary, sibling species tended to be in geographic proximity. While occasional long distance dispersal may occur, such as the single colonization of the SH, and thermal barriers presently exist between the NP, NA, GC, and SH taxa, the observable patterns in Sebastes suggest colonization occurs by stepwise invasion of newly available habitat when temperature conditions permit. Colonization events are spread throughout the sub-generic lineages. Vicariant isolation processes may occur on smaller geographic scales perhaps due to local isolating mechanisms such as glacial advance and retreat, sea level change, and ocean currents. Allopatric differences may be enhanced by a tendency for female mate choice and assortative mating in these livebearing species. The ongoing process of thermal advance and retreat is reflected in contemporary patterns of phylogeographic population genetic structure within species and may be enhanced under climate warming.