http://news.independent.co.uk/environment/article361830.ece
Darwin's finches show how man harms evolution
By Steve Connor
Published: 04 May 2006
They were the birds that were said to have inspired Charles Darwin to
formulate his theory of evolution more than 10 years after his famous
visit to the Galapagos Islands.
Darwin's finches iconically depicted how biological diversity and
natural selection lead to the origin of new species - but now
scientists have detected signs of evolution "in reverse".
Biologists have found that one of Darwin's finches living in the remote
Pacific archipelago has begun to revert to an earlier form because of
interference caused by a growing human population. Humans are causing
evolution to slip into reverse for one of the finch species that lives
on the islands. Scientists have found that the finch is losing the
distinguishing trait that was causing it to split into two different
species - its beak.
The medium ground finch is normally found in two distinct forms - one
with a larger beak the other with a smaller one - but when humans come
to live alongside the finch, this "bimodal" beak size tends to
disappear.
The scientists believe that the arrival of people on the islands may be
causing evolution to run in reverse by causing the two extreme versions
of the ground finch to revert to individuals with intermediate beak
sizes.
In effect, people appear to be unwittingly eliminating the evolutionary
disadvantage of having a beak that is half-way in size between the two
extremes, according to a study published in the Proceedings of the
Royal Society B. As yet the researchers do not understand why people
are having this effect but the implications are that the influx of
tourists and migrants to the Galapagos is helping to eradicate a source
of biological diversity that has made the islands famous.
Professor Andrew Hendry of McGill University in Montreal, who led the
study, said: "We need to make more effort to enable those species that
are in the process of diversifying to continue to diversify and thereby
generate new species. It is appropriate to describe it as evolution in
reverse. It's an evolutionary split within a species that is being
reversed and we think human activity is responsible," he said.
Darwin collected many dead specimens of the finches on his visit to the
islands in 1835 and in 1845 he began to realise that they may have
evolved from common stock. He eventually recognised that it was the
size and shape of each finch's beak that determined what it could eat
and in which ecological niche it was best suited to survive.
"Seeing this gradation and diversity of structure in one small,
intimately related group of birds, one might really fancy that from an
original paucity of birds in this archipelago, one species had been
taken and modified for different ends," Darwin wrote in the 1845
edition of the Journal of Researches.
Intermediate beak sizes for medium ground finches are rarely if ever
seen in the wild, which indicates that it is usually a disadvantage to
be born with an intermediate beak size.
But when the scientists studied populations of medium ground finches
living near to human settlements on the island of Santa Cruz, they
found that most of them had reverted to having intermediate-sized beaks.
The researchers can only speculate about the causes of the reverse. It
could be the result of people introducing alien plants with
intermediate-size seeds, or it could be the deliberate feeding of wild
birds with imported rice.
"Humans are changing the nature of the adaptive landscape. They are
homogenising it. Being intermediate was once bad for the survival of
this finch, now it is no longer the case for birds living near people.
"There's plenty of evidence of humans having caused species to go
extinct. We're not, in essence, doing anything to reverse the loss of
biodiversity," Professor Hendry said.
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