The origin of species in Melanesia

Abstract

The Birds of Northern Melanesia: Speciation, Ecology, and Biogeographyby E. Mayr and J. Diamond. Oxford University Press, 2001. £45.00 hbk (492 pages) ISBN 0 19 514170 9View Large Image | Download PowerPoint SlideThis book is about the origin of species. Darwin, focusing mainly on how selection drives evolution over time, missed the point of speciation, which was first outlined in detail in the 1930s when Ernst Mayr studied the birds of the Whitney South Sea Expedition. Data from the Melanesian archipelago provided a variety of ‘snapshots’ of different stages in the process by which ancestral species give rise to two or more ‘daughter species’. This archipelago has some ‘great speciators’ – common forest birds with a moderate ability to disperse, but also poorly differentiated and widespread forms. Since the 1970s, Jared Diamond's studies of variation in dispersal strategies of Melanesian birds have added a new dimension to Mayr's work.The strength of this book is the partnership between these two authors, which allows the variation in biogeographical pattern to be coupled with variation in dispersal and habitat use. They present their accumulated wisdom, but leave it to others to scrutinize their ideas using newer tools and concepts. So the main value is that Mayr and Diamond, each of whom at one time provided some truly innovative thinking, summarize their deductions as a coherent story. Their book is nicely produced. Nine colour plates by Douglas Pratt illustrate interesting species and cases of intraspecific variation. The book is also well narrated, and is organized as 36 short chapters, each covering a specific question and providing a summary. This is followed by distribution maps and several appendices with data. Those who have been critical about some previous analyses now have access to the relevant data.What makes Melanesian birds so interesting is the great variation in their ability to disperse and speciate (something worth noting by those biogeographers who use computers to find the generalized pattern). Although few island birds are flightless, many are reluctant to leave forest cover, although uncommon birds of scattered and unstable habitat patches disperse well. Diamond provides catchy terms for different strategies, such as ‘supertramp’ for the typical colonists of small volcanic islands where competition is relaxed. Species richness per island is explained mainly from immigration–extinction equilibria. Although some suggestive statistics are provided, it is difficult to reject alternative explanations of species–area curves, and the role of humans in impoverishing small islands.Speciation events are mostly thought to be of Pleistocene age, many of them related to varying connectivity among islands as sea-levels change. The authors give little attention to alternatives and to the universality of their ideas. Similar diversification patterns are also found in some mainland situations with tropical montane habitat mosaics near coasts that have been constantly warm since the upper Tertiary. Studies of molecular evolution here (and elsewhere) suggest that biogeographical species are often much older than suggested in earlier Pleistocene speciation scenarios. Not much thought is given to the possible role of Tertiary events, such as vicariance or relictuation in mobile arc systems along the East Gondwanic margin. Although the authors identify New Guinea as the immediate source area for most Melanesian birds, they might not have noticed its geological complexity, most of it being part of the Australian plate, but the north coast, along with Melanesia, being part of an ancient south Caroline-east Philippines Arc [1xHall, R and Holloway, J.D. See all References][1]. It requires other approaches to disentangle the role of colonization of islands from processes operating along the island arc – possibly exemplified by barred rails Gallicrex torquatus and G. insignis, some Ducula pigeons, lories Lorius and Chalcopsitta, pygmy parrots Micropsitta and Actenoides kingfishers.The insights that Mayr and Diamond gained from Melanesian birds are fascinating, and this region still contains an irresistible challenge for further biogeographical research.