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鈥檚 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鈥檚 Center
for Applied Biodiversity Science in Washington DC.
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Gascon and his colleague Stephen Lougheed of Queen鈥檚 University in Kingston,
Canada, thought that another sort of barrier might have been at work鈥攈uge
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. 鈥淲e decided to rigorously test
the idea,鈥� 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
Juru谩 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鈥� 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). 鈥淭he patterns of variation don鈥檛 match up with the presence
of the river,鈥� says Gascon. 鈥淏ut they match up very nicely with these
谤颈诲驳别蝉.鈥�
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.
鈥淭axonomists that work with these groups tend to be fine splitters,鈥� he says. He
suggests they may record different species on opposite banks based on very minor
details.
