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.