Humans are causing evolution to slip into reverse for one of the species of finches that is said to have inspired Charles Darwin after he returned from his famous visit to the Galapagos Islands.

Scientists have found that one of "Darwin's finches" living in the remote Pacific archipelago has begun to lose the distinguishing trait that could be causing it to split into two different species.

The medium ground finch is normally found in two distinct forms - individuals with a larger beak or those with a smaller beak - 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.

"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," said Professor Andrew Hendry of McGill University in Montreal, who led the study.

"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 of some sort is responsible," he said.

There are 14 different species of finch in the Galapagos and all are believed to have evolved from a single species that was blown by accident to the archipelago from the South American mainland.

Darwin collected many dead specimens of the finches on his visit to the islands in 1835 but it was not until 1845 that he began to realise that they could have all evolved from a common stock.

Darwin 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 is 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.

The medium ground finch, as its name implies, feeds on medium-sized seeds.

Its two most closely related species are the small ground finch, which feeds on small, soft seeds, and the large ground finch, which lives on larger, harder seeds.

Professor Hendry explained that the medium finch differs from the other two species in that it has a "bimodal" population - in other words members of this species can be classified as having either large or small beaks, with few in between.

Intermediate beak sizes for medium ground finches are rarely if ever seen in the wild, which indicates that normally it is a disadvantage to be born with a beak that is half way between the two size extremes.

However, 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 beaks of intermediate size.

The human population of the town of Puerto Ayora on Santa Cruz - where the scientists studied medium ground finches - has mushroomed tenfold over the past 30 years.

However, when the researchers analysed medium ground finches living away from densely-populated areas of the island they continued to find that the birds fell into one of two groups - those with big or those with small beaks.

"What we found is that the process that is causing a species to split into two species is now being reversed," Professor Hendry said.

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," Professor Hendry said.

"There's plenty of evidence of humans having caused species to go extinct but we're not in essence doing anything to reverse the loss of biodiversity.

"Although the most immediate goal of biological conservation should be to preserve species at self-sustaining population sizes, a long-term goal should be to preserve their ability to diversify.

"Only then can we reverse declines in biological diversity."