However the findings aren't quite as Darwin's theory of evolution would lead scientists to naturally assume.

A 20-year study into the genetic evolution of Soay sheep, on the Scottish Island of Hirta, has found it is the smaller - not larger - members of the flock who have much more success reproducing.

Dark-coated rams and ewes are bigger in size than their little and lighter-coated counterparts - and are significantly less likely to reproduce successfully.

Darwin's theory would have predicted that because dark-coated sheep appear fitter they would do better than light-coated sheep, until only dark sheep remained in the population.

But the study proves natural evolution is more complicated than just identifying bigger and fitter animals and predicting that they will squeeze out competitors.

One of the lead researchers, Dr Jacob Gratten, said: "The aim was to improve understanding of how evolution by natural selection occurs. The study shows that predicting the evolutionary response to selection is difficult without knowledge of which genes are in close proximity to each other."

Soay sheep are among the most primitive breeds in the world and are thought to have arrived on the island with the first human settlers, about 4,000 years ago.

In 1932, 107 sheep were moved from Soay to the larger island of Hirta. Every lamb born in the study area is tagged, weighed, and has a blood sample taken, and is followed throughout its lifetime.

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