British biologists have been stunned by results from a study into the wild Soay sheep of the remote northern Scottish island, which concluded climate change had caused the flock to shrink.

The Soay sheep, considered one of the world's oldest breeds, live on Hirta, in the St Kilda archipelago in the storm-battered Outer Hebrides, and have been closely studied for nearly a quarter of a century.

The Hirta Island flock has about 2000 Soay.

The law of evolutionary theory says the brown, thick-coated ungulates should have grown progressively bigger.

Tough winters mean that bigger sheep have a better chance of survival than smaller ones, and should eventually they would dominate in the flock's numbers.

But in 2007, shocked researchers realised that the average size of the Hirta sheep, instead of rising, had been progressively falling.

The answer British biologists said last week lay in climate change.

A team led by Tim Coulson, a professor at Imperial College London, pored over data for the animals' body size and life history over 24 years.

The study found that the average weight of the Soay sheep fell by 81 grams per year and smaller sheep were likelier to survive into adulthood instead of perishing as lambs.

Similar effects have been observed in fish, marine iguanas, large-horned Canadian sheep and North American squirrels.

"Climate change is overriding what we would expect through natural selection," Dr Coulson said.

Dr Coulson believes that shorter, milder winters mean that lambs do not need to put on as much weight in the first months of life in order to survive their first birthday, as they did when winters were colder.

"In the past, only the big, healthy sheep and large lambs that had piled on weight in their first summer could survive the harsh winters in Hirta," he said.

"But now, due to climate change, grass for food is available for more months of the year and survival conditions are not so challenging – even the slower-growing sheep have a chance of making it, and this means smaller individuals are becoming increasingly prevalent in the population."

Man-made climate change is already having an impact on species in terms of habitat and migratory patterns.

But scientists say it is hard to predict which will be winners and losers form the change, partly because of the complexity of separating our evolutionary pressures from environmental factors.

"Unfortunately, it is too early to tell whether a warming world will lead to pocket-sized sheep."