Abstract
In the past decade, the study of dispersal of marine organisms has shifted from focusing predominantly on the larval stage to a recent interest in adult movement. Antitropical distributions provide a unique system to assess vagility and dispersal. In this study, we have focused on an antitropical wrasse genus, Semicossyphus, which includes the California sheephead, S. pulcher, and Darwin's sheephead, S. darwini. Using a phylogenetic approach based on mitochondrial and nuclear markers, and a population genetic approach based on mitochondrial control region sequences and 10 microsatellite loci, we compared the phylogenetic relationships of these two species, as well as the population genetic characteristics within S. pulcher. While S. pulcher and S. darwini are found in the temperate eastern Pacific regions of the northern and southern hemispheres, respectively, their genetic divergence was very small (estimated to have occurred between 200 and 600 kya). Within S. pulcher, genetic structuring was generally weak, especially along mainland California, but showed weak differentiation between Sea of Cortez and California, and between mainland California and Channel Islands. We highlight the congruence of weak genetic differentiation both within and between species and discuss possible causes for maintenance of high gene flow. In particular, we argue that deep and cooler water refugia are used as stepping stones to connect distant populations, resulting in low levels of genetic differentiation.
Highlights
In marine fishes, population structuring at large scales is generally weak due to high effective population sizes and/or high migration rates
Ecology and Evolution published by John Wiley & Sons Ltd
All phylogenetic reconstructions showed S. pulcher as a very closely related sister species to S. darwini, with S. reticulatus being distantly related to these two species (Fig. 1), regardless of marker or reconstruction method used
Summary
Population structuring at large scales is generally weak due to high effective population sizes and/or high migration rates. Speciation in the sea is thought to be counteracted by high gene flow enabled by dispersive larval forms and a rarity of strong physical barriers to dispersal and intermixing Attempts at predicting population structure and gene flow among populations of marine fishes based on a number of variables, in particular the pelagic larval duration (PLD) of a given species, mostly resulted in contradictory findings (Waples 1987; Doherty et al 1995; Shulman and Bermingham 1995; Riginos and Victor 2001; Selkoe and Toonen 2011). While correlations have been tenuous, the methods used to test these predictors have been compromised by the inherent constraints of the metrics and methods used rather than a necessarily weak relationship in nature (Weersing and Toonen 2009; Faurby and Barber 2012)
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