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 finchesiconically 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.