Colonization history, ecological shifts and diversification in the evolution of endemic Galápagos weevils

Abstract

Mitochondrial DNA sequence data were obtained for eight species of flightless Galapaganus endemic weevils and one winged close relative in order to study their colonization history and modes of diversification in the Galápagos Archipelago. Contrary to most other insular radiations, the phylogeny estimates we recovered for Galapaganus do not follow the progression rule of island biogeography. The penalized likelihood age estimates of colonization of the archipelago exceed the age of the emerged islands and underscore the potential role of now sunken seamounts for the early evolution of Galapaganus. The phylogeny proposes one intra-island origin for Galapaganus endemics, but monophyly tests suggest a larger contribution of in-situ speciation on older islands. Generalist habitat preferences were reconstructed as ancestral while shifts to highland habitats were reconstructed as having evolved independently on different islands. Magnitudes and patterns of diversification rate were found to differ between older and younger islands. Our analyses reveal that the colonization sequence of islands and timing of colonization of Galapaganus could be linked with the geological and volcanic history of the islands in a rather complex scenario. Even though most islands appear to have been colonized soon after their emergence, there are notable deviations from the pattern of sequential colonization expected under the progression rule when considering only the extant emerged islands. Patterns of diversification rate variation on older and younger islands correspond to the volcanic activity or remnants of such activity, while the pattern of independent evolution of restricted habitat preferences in different islands suggests that habitat shifts could also have contributed to species diversity in Galapaganus.

Nuclear and mitochondrial sequences confirm complex colonization patterns and clear species boundaries for flightless weevils in the Galápagos archipelago

“…The prevailing pattern of mitochondrial DNA (mtDNA)-derived time estimations supported colonization and within-island differentiation soon after island emergence for most of the islands (Sequeira et al 2008). This pattern favours dispersal scenarios over vicariant events to explain the bulk of inter-island diversification.…”

“…DNA was isolated following Normark (1996) and Sequeira et al (2008). The polymerase chain reaction (PCR) was used to amplify six nuclear gene regions: elongation factor 1 alpha (EF1-a); arginine kinase (ArgK); internal transcribed spacer 1 (ITS-1); and three anonymous loci (4b, 5, 6).…”

“…The mtDNA-derived phylogeny of Galapaganus showed high correspondence with morphological species definitions, with high levels of resolution and support for monophyletic species, and lead us to believe that inter-species hybridization is not prevalent in this group (Sequeira et al 2008 Figure 1. The Galápagos archipelago with island age estimates and distribution of Galapaganus species.…”

“…The possibility of hybridization of G. vandykei with Galapaganus collaris and Galapaganus galapagoensis on San Cristobal is investigated here by assessing concordance with a nuclearbased phylogeny. Nuclear sequence data and additional mtDNA sequences were added to those of Sequeira et al (2008), continuing investigation into the colonization history and modes of diversification in Galapaganus. Well-resolved mitochondrial and nuclear phylogenies allow testing of the following explicit predictions: (i) if the colonization history of the archipelago does not follow the chronological ages of the extant islands owing to the use of sunken seamounts as colonization platforms, then the phylogeny/ies should not strictly follow the progression rule; (ii) if larger and topologically complex, though young islands, such as Isabela, are not promoting intra-island speciation, then weevil species/populations from Isabela will not be each other's closest relatives in the estimated phylogeny; and (iii) if inter-species hybridization and mitochondrial introgression are not a widespread phenomenon in this radiation, then topologies of mitochondrial and nuclear-based phylogenies will be congruent.…”

Speciation Has a Spatial Scale That Depends on Levels of Gene Flow

“…Even if a speciation-area relationship exists, it need not be the direct result of area; for instance, larger islands tend to have higher habitat diversity, which could foster higher rates of ecological speciation (Losos and Parent 2009). Island age might also affect diversification, either by increasing the time over which speciation can occur or through other effects related to the dynamics of island aging (Gillespie 2004;Emerson and Oromi 2005;Sequeira et al 2008; discussed in Whittaker et al 2009). In addition, the degree of isolation from other landmasses might affect speciation rates if lower colonization rates to more isolated islands leave more niches open to be filled by in situ speciation (Gillespie and Baldwin 2009).…”