Northern Melanesia

During the 1970s, E. Mayr and J. Diamond investigated jointly the biogeography, ecology and evolution of the island birds of northern Melanesia (Bismarck and Solomon Islands) regarding the origin of the montane avifauna (Mayr and Diamond 1976f), the species-area relation (Diamond and Mayr 1976a), and the species-distance relation (Diamond, Gilpin and Mayr 1976d). In these studies more than half of the work and results are J. Diamond's. Most montane species have lowland populations on some islands. For example, the Island Thrush (Turdus poliocephalus) and the "flycatcher" Rhipidura [spilodera] are montane on some islands but inhabit the lowlands of other islands in the same archipelago. These niche shifts can often be correlated with major differences in the biological environment of competing species in the lowlands and mountains. Most montane birds either dispersed by direct "jumping" from mountain to mountain or they evolved from a relative in the lowlands. Almost all of the "great speciators," with at least five distinct subspecies or allospecies in the Solomons, are rather common short-distance colonists. Correlated with their high inter-island immigration rates, long-distance colonists show no geographical variation throughout the Solomons.

In their large book, The Birds of Northern Melanesia. Speciation, Ecology, and Biogeography Mayr and Diamond (2001g)6 investigated the processes of geographic speciation, as documented in the northern Melanesian avifauna by island populations that have reached various stages of taxonomic differentiation. Overwater dispersal ability of each species and the weak to more conspicuous geographical variation of conspecific populations on different islands are discussed in detail. Speciation is complete when the new taxon developed not only reproductive (genetic) but also ecological isolation and was able to invade the range of its parent population (19 species pairs). These processes can be followed by a quantitative analysis of this rich, yet still "manageable" tropical bird fauna (191 native zoogeo-graphical species, i.e., taxonomically isolated species and superspecies). The authors also analyzed the varying degrees of endemism in the different island groups, the varying ecological strategies of species, their geographical origin (mostly from New Guinea and Australia), dispersal barriers and extinction of species as well as numerous other ecological and biogeographical aspects, including the effect of Pleistocene sea-level changes on the distribution and current differentiation of these island birds (Fig. 4.12).

Their study led Mayr and Diamond (2001g) to conclude: "We cannot recognize any case that could plausibly be interpreted as a stage in sympatric spe-ciation" (p. XXII). All evidence favors the model of allopatric speciation, more specifically peripatric speciation from founder populations: "Peripheral isolates attract attention because the geographically most peripheral taxon in a species or superspecies is usually the most strikingly divergent one. However, in Northern Melanesia it is rare for peripheral isolates to expand into [species rich] central locations and become important evolutionary novelties, because of the problems of upstream colonization and faunal dominance" (p. 287). Therefore, at least in

6 The basic manuscript was Mayr's, but in the end J. Diamond (*1937) had at least done half. Diamond is a professor of physiology at the University of California at Los Angeles and the author of several award-winning books. He has been elected to the National Academy of Sciences, the American Academy of Arts and Sciences, and the American Philosophical Society. He is also the recipient of a MacArthur Foundation fellowship and was awarded the 1999 U.S. National Medal of Science and the 2000 Lewis Thomas Prize of the Rockefeller University, New York. His father, L. K. Diamond, had also collaborated with Ernst Mayr in a study on human blood groups in 1956 (p. 281).

Fig. 4.12. Distribution of the monarch flycatcher superspecies Monarcha [manadensis]. A M. infelix, B M. mencke, C M. verticalis, D M. barbatus, E M. browni, F M. viduus, G M. manadensis in northern Melanesia. Plumage color is basically black and white. This superspecies is one of the extremes among "great speciators," with six allospecies, 11 megasubspecies, and two weak subspecies in Northern Melanesia, plus four allospecies outside Northern Melanesia. The superspecies thus comprises forms of a wide range of distinctness, from weak subspecies to very distinct allospecies, with many borderline forms about which it is difficult to decide whether to classify them as allospecies or subspecies. All Northern Melanesian taxa are allopatric, but speciation has produced sympatric related species on New Guinea and several other Papuan and Lesser Sundan islands. Adapted from Mayr and Diamond (2001, map 41 and plate 3) which see for identification of subspecies

Fig. 4.12. Distribution of the monarch flycatcher superspecies Monarcha [manadensis]. A M. infelix, B M. mencke, C M. verticalis, D M. barbatus, E M. browni, F M. viduus, G M. manadensis in northern Melanesia. Plumage color is basically black and white. This superspecies is one of the extremes among "great speciators," with six allospecies, 11 megasubspecies, and two weak subspecies in Northern Melanesia, plus four allospecies outside Northern Melanesia. The superspecies thus comprises forms of a wide range of distinctness, from weak subspecies to very distinct allospecies, with many borderline forms about which it is difficult to decide whether to classify them as allospecies or subspecies. All Northern Melanesian taxa are allopatric, but speciation has produced sympatric related species on New Guinea and several other Papuan and Lesser Sundan islands. Adapted from Mayr and Diamond (2001, map 41 and plate 3) which see for identification of subspecies this region and bird fauna, the general evolutionary significance of peripheral isolates is relatively low.

Although molecular sequence data are needed to confirm the genealogical relationships of the bird taxa studied, such information is unlikely to change the major conclusions reached. This massive handbook again demonstrated the overriding importance of time and barriers to dispersal for the speciation of island birds. It "is an impressive synthesis, unique in scope, and inspiring by the challenges it sets forth. It represents the fulfillment of a research program" (Grant 2002a: 1881). Hopefully, similarly comprehensive analyses of geographical differentiation and speciation will be published for other groups like insects, land snails or plants. The model system of birds should be compared with other groups of organisms to see whether the generalizations made are true also for other organisms or whether they have their own regularities and laws. The book on the bird fauna of northern Melanesia summarized Mayr's latest thinking in the field of biogeography.

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