Colonization and diversification of Galápagos terrestrial fauna: a phylogenetic and biogeographical synthesis

Abstract

Remote oceanic islands have long been recognized as natural models for the study of evolutionary processes involved in diversification. Their remoteness provides opportunities for isolation and divergence of populations, which make islands remarkable settings for the study of diversification. Groups of islands may share a relatively similar geological history and comparable climate, but their inhabitants experience subtly different environments and have distinct evolutionary histories, offering the potential for comparative studies. A range of organisms have colonized the Galápagos Islands, and various lineages have radiated throughout the archipelago to form unique assemblages. This review pays particular attention to molecular phylogenetic studies of Galápagos terrestrial fauna. We find that most of the Galápagos terrestrial fauna have diversified in parallel to the geological formation of the islands. Lineages have occasionally diversified within islands, and the clearest cases occur in taxa with very low vagility and on large islands with diverse habitats. Ecology and habitat specialization appear to be critical in speciation both within and between islands. Although the number of phylogenetic studies is continuously increasing, studies of natural history, ecology, evolution and behaviour are essential to completely reveal how diversification proceeded on these islands.

Figure 1

(a) Map of the Galápagos Islands and current hypotheses for the phylogenetic relationships of three Galápagos lineages as follows: (b) giant Galápagos tortoises, (c) terrestrial snails and (d) Darwin's finches. (a) Estimated minimum and maximum geological age for each island are in parentheses (D. Geist 2005–2008, unpublished data). Filled triangles indicate the summits of the six Isabela volcanoes. The star indicates the location of the hot spot, currently between Fernandina and volcano Cerro Azul of Isabela (Hooft et al. 2003). (b) Bayesian tree based on mtDNA control region sequences of extant and extinct Galápagos tortoises. Extinct taxa are asterisked. Placement of the three lineages on Santa Cruz is indicated with an arrow, and the pictures exemplify their different carapace morphologies. Redrawn from Russello et al. (2005). (c) Best maximum likelihood phylogenetic tree based on combined mtDNA COI and nDNA ITS1 sequence data. Two long branches were shortened and their actual lengths are reported in brackets below them. The snail outlines are roughly proportional to actual size. Species on older islands connect at deeper nodes, with a significant negative relationship between the depth of the clade encompassing an islands' species and the island's age. For each tree, numbers above branches are Bayesian posterior probabilities. Maximum likelihood bootstrap proportions more than 50% are given below branches for b. (d) Phylogenetic tree of Darwin's finches estimated from cytochrome b sequences and neighbour joining (Petren et al. 2005). There is significant genetic structure and paraphyly among populations of sharp-beaked ground finches and warbler finches, but lack of resolution among the phenotypically distinct species of ground finches and tree finches. Photographs are proportional to actual size.