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Development and Evolution of Fern Floras of Oceanic Islands

Rolla Tryon
Biotropica
Vol. 2, No. 2 (Dec., 1970), pp. 76-84
DOI: 10.2307/2989765
Stable URL: http://www.jstor.org/stable/2989765
Page Count: 9
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Development and Evolution of Fern Floras of Oceanic Islands
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Abstract

Ferns are especially useful for the study of certain biogeographic problems of migration and speciation because the species have a high and nearly equivalent capacity for spore dispersal by air transport. Dispersal can bring spores of many species to an island from a source area; the ones that become established are not a random assortment of the source species. Island floras have a major element of widely distributed source area species, although these are a minor element in the source area. The endemics on islands are mostly related to narrowly distributed source species, although these are a minor element in the island flora. In an archipelago the endemics are nearly always related to source species rather than to other species of the insular flora. Establishment of a species on an island is an individual, rather than a population, phenomenon because dispersal cannot bring the variability of a source population to the island. Adaptability of the genotype-phenotype of the single spore to a new environment and sensitivity to selection are characteristics that dominate success in migration and evolutionary potential under geographic isolation. Widely and narrowly distributed source species differ in these characteristics, and these differences account for the relative proportions of the two source groups in the insular flora and their relations to insular endemics. Re-immigration can maintain gene-flow between islands and is a deterrent to the evolution of species-flocks in an archipelago.

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