熱帯多雨林はなぜ種の多様性が高いか 分類学の立場から
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概要
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Species-richness of plants is a consequence of the continuance of existing species, speciation, immigration, and extinction. Remarkable species-richness of tropical rain forests is due to ecological and geological circumstances. One of the main geological factors is continental drift, in particular the drift in the Cretaceous and Tertiary during which angiosperms radiated into the dominants of the extant land flora. In the Upper Tertiary in tropical Asia, and around the end of the Tertiary in tropical America, Laurasian and Gondwanan continents were united and their different floras merged there. During such events speciation perhaps took place in new localities and habitats to increase species-richness. Another, more effective factor is climate change of the Quaternary, during which species composition and distributional range probably fluctuated to a considerable degree. Small, isolated populations may have speciated into neoendemic species. As suggested by the refugia hypothesis, particularly in tropical America, there are isolated, species-rich regions of rather small area, to which many endemic species are concentrated. The remarkable species-richness of tropical rain forests is due largely to the presence of a number of endemic species. For instance, a large number of species are confined to less than three sites and most of species-rich sites are u!tramafic (serpentine) in Mt. Kinabalu, northern Borneo. A high frequency of endemic species perhaps is a product of the dynamic historical change of the tropics. Some of tropical species may have originated by rapid (or explosive) speciation from small populations. One such example seems to be rheophytes, i.e. plant species that are confined to streambed subject to flooding after heavy rains. The narrow leaves characteristic of rheophytes are considered to have been derived by heterochronic evolution, in which rather simple mutations are postulated to have been involved.
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