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More the merrier

作者:屋庐篚    发布时间:2019-03-07 01:10:05    

By Matt Walker THE rich diversity of species in the Amazon basin is mainly down to buried ridges that criss-cross the region, a new study suggests. The finding debunks a long-held theory that it’s rivers that isolated groups of Amazonian animals and cranked up their evolution. We know there are up to three times as many vertebrate species in any given area of the lowland Amazon forests than is average for the rest of the world. Why the diversity should be so high is a long-standing puzzle. According to one popular theory the network of Amazonian rivers and tributaries left land animals isolated in pockets where they rapidly diversified from each other. If this is true, animals on opposite sides of the river should be genetically distinct, and these differences should become exaggerated as the river mouth widens. Evidence to support this idea has been sketchy. Some studies suggested that rivers are a barrier to birds and monkeys, but others show no such pattern for rodents or amphibians, says Claude Gascon of Conservation International’s Center for Applied Biodiversity Science in Washington DC. Gascon and his colleague Stephen Lougheed of Queen’s University in Kingston, Canada, thought that another sort of barrier might have been at work—huge geological ridges, hundreds of kilometres long, that dominated the western Amazonian landscape five to ten million years ago. Several of these ridges, now buried under sediment, criss-cross the region. “We decided to rigorously test the idea,” says Gascon. To do this, their team looked at the diversity of poison-dart frogs ( Epipedobates femoralis) in the region. The frogs live in trees and avoid rivers. The researchers collected DNA from frogs along both sides of the Rio Juruá river which runs west to east, and on both sides of a ridge 200 kilometres wide that bisects the river north to south. Tests on the frogs’ DNA showed that groups on opposite banks of the river were more genetically similar than those from different sites on the same bank, and widening rivers had no effect. But two genetically distinct groups were present on either side of the ridge (Proceedings of the Royal Society B, vol 266, p 1829). “The patterns of variation don’t match up with the presence of the river,” says Gascon. “But they match up very nicely with these ridges.” The results suggest the ridges have been a major evolutionary force in the region over the past few million years. Gascon thinks that patterns of bird and primate diversity that seem to correspond to rivers are simply an artefact. “Taxonomists that work with these groups tend to be fine splitters,

 

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