Introduction. Evolution on Pacific islands: Darwin's legacy

Steven A Trewick, Robert H Cowie

Charles Darwin stepped ashore on the Pacific Island of San Cristobal in the Galápagos archipelago on 17 September 1835. No doubt grateful to be on the steady, dry land, the young Darwin was at first unimpressed by the landscape ‘Nothing could be less inviting than the first appearance’ (Darwin 1839, p. 454). Nevertheless, Darwin was to make important inferences from the observations of island biotas, and later described his experiences in The Voyage of the Beagle as ‘by far the most important event in my life and has determined my whole career’ (Darwin 1958, p. 12). Since the key insights of Darwin and also of Alfred Russel Wallace, island systems have been foci for evolutionary research. Much of this research has been directed at islands in the Pacific region (notably the Galápagos and Hawaiian Islands and parts of Melanesia), with many fundamental ecological and biogeographical concepts emerging from studies in these locations, not least those of Charles Darwin himself.

This special issue therefore commemorates the bicentennial of Darwin's birth and the 150th anniversary of the publication of The Origin by bringing together modern molecular research on the biogeography and evolution of Pacific Island terrestrial faunas. While intended as a celebration, it is also part of the great man's legacy, a legacy that we hope if he were alive today he would find as stimulating as he did his own encounters with the marvellous faunas of these islands.

The application of molecular (mostly DNA sequencing) approaches has enabled questions about biodiversity and biogeography to be critically and stringently addressed. In particular, studies have explored phylogeography and species radiations of insular biotas as well as the dispersal and geographical origins of these biotas. Importantly, molecular phylogenetics has enabled testing of evolutionary hypotheses that were first stimulated by observation of the island faunas. For instance:it is the circumstance that several of the islands possess their own species of tortoise, mocking-thrush, finches, and numerous plants, these species having the same general habits, occupying analogous situations, and obviously filling the same place in the natural economy of this archipelago, that strikes me with wonder.Darwin (1845, p. 97)

Molecular research in this arena has tended to have a local perspective but a broader, comparative approach is now feasible and necessary. Improved interdisciplinary exchange (in particular, the contribution of geologists on the age and formation of islands), increasing ease and decreasing cost of acquisition of molecular data, availability of multiple mitochondrial and nuclear markers and the renaissance of dispersal as an acceptable hypothesis of evolutionary and biogeographical processes are providing the environment in which to achieve this. The future of island biogeography lies in the integration of methods and evidence, and the comparison of systems (i.e. a shift in geographical and taxonomic scale). Recent commentaries (e.g. Cowie & Holland 2006; Trewick et al. 2007) have highlighted the need for a shift towards more integrated studies if hypotheses about the evolution of island biotas are going to be tested properly. The present issue brings together contributions from a broad taxonomic and geographical range, presenting data, analyses and interpretations that aim to take a comparative and holistic view of evolution on islands.

In order to provide at least some cohesiveness among the various contributions, we constrained the contributions to a focus on terrestrial animals, for no other reason than that these are our own focal organisms. For similar reasons, but also because we wanted to highlight the most modern advances, we also constrained the focus to molecular studies. And by involving relatively younger workers in many of the contributions, we also hoped to offer fresh and vibrant new insight as well as to provide these up-and-coming researchers with increased visibility. It is up to the reader to assess the extent to which we have succeeded in these regards.

The geology of the Pacific islands underlies biological evolution on them. Therefore, the first paper in this issue reviews what is known of the formation of these islands in space and time, and their geographical trajectories as the major tectonic plates that make up the Pacific and Pacific Rim move with respect to each other. Many of these islands are ‘hot spot’ islands that have never been in contact with continental regions and the biotas of which are necessarily derived from individuals that dispersed over the ocean. Others are continental islands, but the influence of dispersal on the development of their biotas is also substantial, and it is evident that the processes of biotic assembly on oceanic versus continental islands do not constitute a simple dichotomy. The geological review (Neall & Trewick 2008) is intended to provide an accessible summary of the history of the Pacific Ocean and geology of its islands as a foundation for the biological papers that make up the remainder of the issue, and for future research.

The review of Pacific geology is followed by five regional biological reviews. Each of these collates and attempts to synthesize molecular studies of phylogenetics, biogeography and phylogeography of terrestrial animals in New Caledonia (Grandcolas et al. 2008), New Zealand (Goldberg et al. 2008), French Polynesia (Gillespie et al. 2008), the Galápagos Islands (Parent et al. 2008) and the Hawaiian Islands (Cowie & Holland 2008). Each deals with the origins of the various components of the faunas as well as the intra-archipelago biogeography and phylogeography, but are limited to the taxa and scope of the research papers they review. Hence, one important outcome of these reviews is to highlight the taxa and topics that remain understudied.

Following these review papers is a series of new research articles. These deal with beetles in Japan (Sota & Nagata 2008), land snails in Belau (Rundell 2008) and the Ogasawara Islands (Davison & Chiba 2008), iguanas in Fiji (Keogh et al. 2008), crickets in New Zealand, Australia and New Caledonia (Pratt et al. 2008), weevils in the Galápagos (Sequeira et al. 2008) and moths in the Galápagos (Schmitz et al. 2008) and Hawaii (Rubinoff 2008).

No other published compilation has focused explicitly on molecular approaches to Pacific-wide biogeography nor taken an explicitly phylogenetic perspective. For instance, the landmark contribution edited by Keast & Miller (1996) focused more on patterns of diversity, enumeration of biodiversity and interpretation of those numbers in terms of the determinants of diversity, and lacked a molecular perspective. The book edited by Wagner & Funk (1995), a watershed in Pacific biogeography, was taxonomically more scattered, based on morphology to a much greater extent, and dealt only with Hawaii. We hope, therefore, that the present compilation of review papers and research articles provides stimulating, productive and interesting insights into evolution at work, and an additional stimulus for further research on the fabulous terrestrial faunal diversity of the islands of the Pacific.

Twenty-four years after setting foot on the Galápagos and visiting other islands in the Pacific (New Zealand, Tahiti), Darwin's seminal work on evolution was published, and observations made on islands were an essential component of the evidence he presented in his inimitable style: ‘Thus the several islands of the Galápagos Archipelago are tenanted, as I have elsewhere shown, in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world’ (Darwin 1859, p. 400). The significance of that observation does not diminish with repetition.

It is that great work (On the Origin of Species) that set us all upon the odyssey we now pursue. And in the 150 years since its first publication, the evidence for and understanding of evolution has advanced profoundly, not least due to developments in genetic theory and methodology. At the current rate of increase in data acquisition and analysis, we can expect our understanding of the details of evolution to advance in the future even more rapidly. However, as we interpret these data we should seek to emulate Darwin's character: his patience and caution, his skills of observation and his application of a rigorous scientific approach of hypothesis testing. Perhaps Darwin's greatest legacy was to advance hypotheses that turned evolutionary studies into testable science.

Acknowledgments

Selection and spelling of island names in all papers in this special issue follows Motteler (2006), with older or alternative names sometimes given in parentheses. We thank James Joseph and Victoria Brown of the Royal Society for discussion of our initial idea for a special issue celebrating Darwin's legacy in the Pacific and for their guidance in editing this issue. Thanks also to Brenden Holland for his editorial assistance, David Penny for his comments and the many reviewers who were instrumental in improving the quality of the entire issue.

Footnotes

  • One contribution of 15 to a Theme Issue ‘Evolution on Pacific islands: Darwin's legacy’.

References