Phylogeny and biogeography of Galapagos lava lizards (Microlophus) : inferred from nucleotide sequence variation in mitochondrial DNA

The genus Microlophus has representatives in South America and in the Galápagos Islands. Although the phylogenetics of the seven endemic Galápagos species have been previously investigated, no consensus exists as to the relationships among the island species or of the relationships of the island species to the mainland taxa. In this study, variation in the nucleotide sequence in portions of the mitochondrial cytochrome b and 16S ribosomal RNA genes is analyzed and used to propose hypotheses regarding the phylogenetic relationships and biogeographic history of the Galápagos species of Microlophus. The major findings of this study are:

1. the present-day species of Galápagos lava lizards are not a monophyletic assemblage; there are two main lineages, with the first including M. bivittatus and M. habeli while the second is comprised of all other island species;
2. M. bivittatus and M. habeli are more closely related to M. occipitalis from mainland South America than they are to the other Galápagos species;
3. the ancestor of the remaining island species could not be unequivocally resolved but, of the mainland South American taxa examined, M. theresiae, M. thoracicus, and M. tigris were identified as possible sister taxa;
4. of those populations previously recognized as M. albemarlensis, four groups consistently emerge; M. indefatigabilis is resurrected for the populations from Santa Cruz, Baltra, and Daphne islands and M. jacobi is resurrected for the populations from Santiago and Bartolomé islands;
5. M. delanonis is the most basal species within the main lineage, followed by M. grayi; M. pacificus is the sister taxon to M albemarlensis; M. indefatigabilis is the sister species to M. jacobi; M. duncanensis is the closest relative to the two latter species;
6. the Galápagos lava lizards are almost certainly derived from South American species and dispersal to the islands most likely occurred via rafting on mats of vegetation swept out to sea;
7. at least two colonization events are necessary to account for the present-day distribution of the Galápagos lava lizards;
8. the lineages of Microlophus inhabiting the Galápagos Islands diverged from their closest mainland relatives approximately 7.1 -8.1 million years ago (mya) and the Galápagos species separated from each other from 2.1 - 6.6 mya;
9. the estimates of divergence times for the Galápagos species pre-date the age of the current Galápagos Islands;
10. the combination of morphological data, cytochrome b data, and 16S rRNA data support the proposition that Plesiomicrolophus be synonymized with Microlophus.