Tracing early stages of species differentiation: Ecological, morphological and genetic divergence of Galápagos sea lion populations

Abstract

BackgroundOceans are high gene flow environments that are traditionally believed to hamper the build-up of genetic divergence. Despite this, divergence appears to occur occasionally at surprisingly small scales. The Galápagos archipelago provides an ideal opportunity to examine the evolutionary processes of local divergence in an isolated marine environment. Galápagos sea lions (Zalophus wollebaeki) are top predators in this unique setting and have an essentially unlimited dispersal capacity across the entire species range. In theory, this should oppose any genetic differentiation.ResultsWe find significant ecological, morphological and genetic divergence between the western colonies and colonies from the central region of the archipelago that are exposed to different ecological conditions. Stable isotope analyses indicate that western animals use different food sources than those from the central area. This is likely due to niche partitioning with the second Galápagos eared seal species, the Galápagos fur seal (Arctocephalus galapagoensis) that exclusively dwells in the west. Stable isotope patterns correlate with significant differences in foraging-related skull morphology. Analyses of mitochondrial sequences as well as microsatellites reveal signs of initial genetic differentiation.ConclusionOur results suggest a key role of intra- as well as inter-specific niche segregation in the evolution of genetic structure among populations of a highly mobile species under conditions of free movement. Given the monophyletic arrival of the sea lions on the archipelago, our study challenges the view that geographical barriers are strictly needed for the build-up of genetic divergence. The study further raises the interesting prospect that in social, colonially breeding mammals additional forces, such as social structure or feeding traditions, might bear on the genetic partitioning of populations.

Speciation genetics: current status and evolving approaches

“…It is intuitively clear that the match between body coloration and substrate is relevant to predator-mediated selection, as has been recently shown for Peromyscus mice (Mullen et al 2009). Another convincing non-genic mechanism that generates an immediate association between ecological adaptation and mate choice is habitat learning (Beltman & Haccou 2005), which has been suggested to be relevant in several vertebrate systems (Musiani et al 2007;Wolf et al 2008). One has to be careful, however, to a priori attribute the link between ecological adaptation and assortative mating to learning, as simple non-genic mechanism of assortative mating, in which the mating trait arises as a pleiotropic effect of genes responsible for ecological adaptation, is also credible in viral evolution (Duffy et al 2007).…”

Ecological opportunities and specializations shaped genetic divergence in a highly mobile marine top predator

“…For example, top predators inhabiting neighbouring areas, such as grey wolves in boreal coniferous forest and tundra/taiga, that are known to specialize on different prey, can be genetically and phenotypically differentiated [4]. Other examples include Galapagos sea lions from two distinct rookeries foraging in benthic and pelagic habitats, sympatric populations of killer whales specializing on fish or marine mammals in the North-east Pacific and sympatric generalist and specialist killer whales in the North-east Atlantic (NEA) [5][6][7]. Similarly, sympatric individuals of several birds and post-glacial temperate lake fish species, showing contrasting morphs adapted to distinct feeding ecology, are at different stages of genetic isolation [8,9].…”

Hybridization masks speciation in the evolutionary history of the Galápagos marine iguana

“…P P -S R B S P P -S R G P P -S R S P P -S R P B P P -S R I P P P -S R P C L O -S R L ( 1 9 9 3 ) P P -S R P C ( 1 9 9 3 ) P P -S R E C ( E /D /C /B ) San Cristóbal scenario, whereby regular dispersal facilitates gene flow between islands, is a more probable explanation for this weak phylogeographic structure; a similar situation is found for another semi-aquatic Galápagos organism, the Galápagos sea lion [49].…”