Abstract
The Galápagos Archipelago is a place where terrestrial adaptive radiations of finches, mockingbirds, and tortoises have been studied extensively. In contrast, little is known about the potential for marine species to diverge among islands. The overall degradation of coral reefs in the Galápagos makes understanding the mechanisms and factors of speciation, the engine of biodiversity, important, and timely. While speciation in marine archipelagos has been described in the past, such as for cone snails in Cabo Verde Archipelago and limpets in Hawaii, adaptive radiations in the marine environment are still rare and poorly understood. In this study, we focused on the Galápagos blue-banded goby, Lythrypnus gilberti, a small endemic fish that is found in shallow subtidal rocky habitats. Using RAD sequencing, we analyzed 19,504 loci that were either neutral, or potentially under directional selection. As expected, considering the small geographic range, population structure based on neutral markers was weak. For loci under directional selection, however, marked differences between islands suggested potential for local adaptation. Our data suggest that for marine species, where dispersal barriers are less apparent, mechanisms of local adaptation may also be at play in the Galápagos Archipelago.
Highlights
The role of isolation has been a central concept to the field of speciation (Coyne and Orr 2004; Price 2007; Rocha and Bowen 2008; Bernardi 2013)
A total of 67 samples were collected along a longitudinal gradient from seven localities in the southern islands of the Galapagos Archipelago, including four main islands: San Cristobal (CRI), Espanola (ESP), Floreana (FLO), and Santa Cruz (Academy Bay, ACA), and three minor islands: Corona del Diablo (CDD), South Plaza (PLA), and Los Hermanos (HER) (Fig. 1, Table 1)
Speciation in marine environments has traditionally been seen as very different than its terrestrial counterparts for two main reasons: on one hand, physical barriers are less obvious in the ocean, and on the other hand, most marine organisms have long pelagic larval durations that translate in vast dispersal capabilities, which in turn result in panmixia (Palumbi 1992; Miglietta et al 2011)
Summary
The role of isolation has been a central concept to the field of speciation (Coyne and Orr 2004; Price 2007; Rocha and Bowen 2008; Bernardi 2013). Marine systems, and by extension marine island systems, have traditionally not been seen as conducive to local adaptation and speciation (Pinheiro et al 2017). While some spectacular examples do exist, such as for Cabo Verde Archipelago Conus marine snails (Cunha et al 2005, 2008; Duda and Rolan 2005; Peters et al 2016), Cellana limpets in Hawaii (Bird et al 2007, 2011; Bird 2011), Northeastern Pacific Sebastes rockfish (Johns and Avise 1998; Alesandrini and Bernardi 1999; Burford and Bernardi 2008), and Antarctic notothens (Janko et al 2011), in general, long pelagic larval durations, a common characteristic of marine fishes, have led scientists to assume that island speciation is of limited importance. Even ichthyologist David Starr Jordan suggested to the leader of one of the first scientific expeditions to the Galapagos (1898–1899) not to waste much time on the topic
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