Evolution and biogeography of Floreana’s lizards
Concordant phylogeographic patterns across codistributed species with similar dispersal abilities suggest that those species share historical biogeographic factors (Avise, 2000). In this study we show that two species of phylogenetically distant lizards share the same geographic pattern of genetic differentiation. Haplotypes from Floreana are significantly more similar to those from Champion, the closest islet with shallow water between, than they are to the more distant Gardner with deeper water between (Fig. 3). This genetic pattern was already noticed for M. grayii by Benavides et al. (2009). Pleistocene contact between Floreana and Champion (Fig. 1) most likely allowed gene flow between what are now separate populations of both lava lizards and leaf-toed geckos. Thus, our results support the model that Pleistocene changes in sea level had a strong influence on the evolution of land-locked vertebrates by allowing contact among populations from different islands during glaciation periods (Ali & Aitchison, 2014; Geist et al., 2014; Poulakakis et al., 2012). In contrast, lava lizard and leaf-toed gecko populations in Gardner have been isolated longer, possibly since they colonized this islet, which explains their strong genetic differentiation (Fig. 3). Although we have no data on the time of colonization of Gardner by any lizard species, exposed lava dating suggests that Gardner is at least one million years old (Geist et al., 2014); both lava lizard and leaf-toed gecko colonizers were present in the archipelago at that time (Benavides et al., 2009; Torres-Carvajal et al., 2014). The different haplotypes found in this study, as well as maximum sea depth (200 m), and long distance (8 km) between Gardner and Floreana suggest that Gardner’s populations have been isolated long enough to allow strong genetic differentiation through genetic drift. This is not a novel phenomenon for Galápagos organisms, as drift in isolation was identified as the main cause of genetic differentiation in populations of M. albermalensis on 12 islets that were connected to Santa Cruz island during the late Pleistocene (Jordan & Snell, 2008). Similarly, pronounced drift was reported in populations of Floreana’s mockingbirds (Mimus trifasciatus ) from Champion and Gardner (Hoeck et al., 2010).
Genetic differentiation between islands or islets in the Galápagos for a suite of species with limited dispersal ability is observable due to the rarity of overwater dispersal, which reduces the chances of gene flow, and the time that has elapsed since colonization or separation of populations through sea level rise. Rafting dispersal in the Galápagos is less frequent because tree coverage is generally thin, with only sparse vegetation along coastal areas, and there are no rivers capable of transporting rafts to the sea (Ali & Aitchison, 2014). However, during El Niño or other severe flooding events, large logs and trees can be seen washed out to the ocean, and now floating debris like plastics are becoming more common. As in other archipelagoes, such as Cape Verde, the Canaries and the Mascarenes, reptile assemblages in the Galápagos have resulted mostly by intra-archipelago colonization and subsequent anagenesis (Ali & Meiri, 2019). Morphological (Van Denburgh, 1912) and molecular (this study) evidence suggest that populations of geckos in Gardner are the same species as in Floreana. However, our knowledge on the evolution of Galápagos leaf-toed geckos (Benavides et al., 2009; Kizirian et al., 2004; Torres-Carvajal, Rodríguez-Guerra, & Chaves, 2016) suggests that Gardner populations could become different species in the near ’evolutionary’ future (assuming no extinction and limited or no gene flow). If this were true, we would be documenting a case of speciation at the earliest stages of separation of descendant lineages (de Queiroz, 2011). By contrast, the population of lava lizards from Gardner shows a greater genetic differentiation. Genetic distances between Gardner’s and Floreana + Champion’s populations (0.068 on average) overlap the range of interspecific distances within the Western Radiation clade (0.038–0.149). No morphological differences between specimens of lava lizards from Floreana and surrounding islets have previously been recognized (van Denburgh & Slevin, 1913); however, preliminary analyses suggest otherwise (O. Torres-Carvajal unpublished data). Thus, a closer examination of specimens of M. grayii from Floreana and surrounding islets is underway to clarify their taxonomic status.