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Adaptive Shift and Dispersal in a Tropical Ant Fauna

Edward O. Wilson
Evolution
Vol. 13, No. 1 (Mar., 1959), pp. 122-144
DOI: 10.2307/2405948
Stable URL: http://www.jstor.org/stable/2405948
Page Count: 23
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Adaptive Shift and Dispersal in a Tropical Ant Fauna
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Abstract

1. The zoogeography of the Melanesian ponerine fauna is preliminarily analyzed. Most of the fauna has apparently been derived ultimately from Oriental stocks entering by way of New Guinea; some invading species are able to spread beyond this island to Queensland and outer Melanesia. A smaller part of the fauna has been derived from old Australian stocks that have entered by way of New Guinea or New Caledonia. Faunal flow from New Guinea through outer Melanesia has been unidirectional, with an ever diminishing number of species groups found outward from the Bismarcks to the Fiji Islands. New Caledonia draws almost all of its fauna from eastern Australia and has engaged in very little direct faunal exchange with the remainder of Melanesia. 2. A cyclical pattern of expansion, diversification, and contraction is hypothesized to account for later evolutionary events following initial dispersal. Following invasion of Melanesia (Stage I, primary), the pioneer populations may then diverge to species level (Stage II) and further diversify. Eventually the source populations outside Melanesia tend to contract, leaving the species group as a whole peripheral and Melanesian-centered (Stage III). Endemic Melanesian species occasionally enter upon a secondary phase of expansion (Stage I, secondary) but are rarely if ever able to push beyond Australia or the Philippines. 3. Stage-I species are characterized on New Guinea by their greater concentration in "marginal" habitats, including open lowland forest, savanna, and littoral. The central habitats, including denser lowland forest and montane forest, contain significantly larger faunas as well as a higher percentage of Stage-II and Stage-III species. Stage-I species are also characterized by their individual occurrence in a greater range of major habitats. Finally, these species make up a significantly higher proportion of the faunas of the archipelagos of central Melanesia, including the Bismarck Archipelago, the Solomon Islands, and the New Hebrides. 4. On the basis of these data it is suggested that ponerine species normally invade New Guinea by way of the marginal habitats. Evolutionary opportunity is nevertheless limited in the marginal habitats, and there is a strong selective pressure favoring re-entry into the inner rain forest habitats. In general, Stages II and III, leading to the origin of the great bulk of the Melanesian fauna and its most distinctive endemic elements, are played out only in the inner rain forest.

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