Sheep with two copies of the light gene are small, but still have quite high reproductive success.
Sheep with two copies of the dark gene are larger still, but have poor reproductive success. This discovery means that sheep with one copy of the dark gene and one copy of the light gene are quite large and also have quite high reproductive success. The light coat trait is usually co-inherited with a set of genes that decrease size but increase reproductive success.
They found that the dark coat trait is usually co-inherited with a set of genes that increase size but decrease reproductive success. However, this study shows that the process of evolution by natural selection in Soay sheep, although still evident, is actually more complex than this.ĭr Gratten and colleagues used a statistical genetics approach similar to that used by medical geneticists, when trying to map genes for human disease such as heart disease or diabetes. However, despite the fact that the dark-coated Soay sheep are larger, which is usually linked to survival and reproductive success, the frequency of light-coated sheep has increased over the last 20 years.ĭarwin's theory would have predicted that because dark-coated sheep appear fitter that they would do better than light-coated sheep, until only dark-coated sheep remained in the population. On Hirta about three quarters of sheep have dark brown coats, while the remaining quarter have light sandy coats.
The researchers looked at coat colour in a feral population of Soay sheep on Hirta in the St Kilda Archipelago. Dr Jacob Gratten and Dr Jon Slate, from the University's Department of Animal and Plant Sciences, led the team, which found that the gene responsible for dark coat colour is linked to other genes that reduce an animal's fitness.
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