生物谷報道：野生索艾羊(Soay Sheep)已在蘇格蘭的圣基達島(St Kilda Is.)上與世隔絕,、毫無拘束地生活了上千年的時間,。處于這種環(huán)境下的索艾羊是研究自然進化的絕佳材料--圣基達島就像一個巨大的培養(yǎng)皿，而其中的索艾羊就像果蠅一樣,。
有兩種顏色的索艾羊：淺棕色和深棕色,。深棕色的索艾羊個體更大，從進化的角度來說,，形體越大越好--大型動物的適應性更好,，更有可能生存下去,。因此隨著時間的推移，索艾羊種群中深棕色的羊比例應該越來越大,。但過去20年記錄顯示的結(jié)果卻完全相反,。
英國布里斯托爾大學 （University of Bristol）大學Jon Slate領(lǐng)導的的研究小組發(fā)現(xiàn)了其中的原因。 Slate博士說,，這是由于羊毛顏色基因位點附近的基因決定的,，這個基因叫做適應性基因（fitness gene）。
較早前研究人員已鑒定出羊毛的顏色基因,，并發(fā)現(xiàn)深棕色的為顯性,。只要從父母的任何一方遺傳得到一個深棕色的等位基因，它就是深棕色的,；只有同時從父母身上都遺傳得到淺棕色的等位基因,，才會出現(xiàn)淺棕色的癥狀。
在這項新研究中,，他們發(fā)現(xiàn)在基因組相鄰的基因位點,，分別會影響形體的大小，小羊的存活率和生殖的成功與否,。他們不知道為什么這些位點會有這種作用,。當深棕色的等位基因遺傳給子代時，同時也遺傳正向影響形體大小的基因,，但也遺傳著負向影響小羊存活率和生殖能力的基因,。當淺棕色的特征遺傳給索艾羊時，索艾羊同時也獲得更小的形體的基因,，以及更好存活和生殖的基因,。
這意味著，淺棕色的索艾羊和僅有一份深棕色基因的深棕色索艾羊,，適應性要更強,。這就是淺棕色索艾羊數(shù)量增多的原因。更大的形體不足以抵消其它的遺傳因素,。Slate博士表示,，“如果不是形體大小的基因，那它們就更糟糕,。”
生物谷推薦英文原文：
Secret Of Scottish Sheep Evolution Discovered
ScienceDaily (Jan. 20, 2008) — Researchers from the University of Sheffield, as part of an international team, have discovered the secret of why dark sheep on a remote Scottish Island are mysteriously declining, seemingly contradicting Darwin's evolutionary theory.
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.
The researchers looked at coat colour in a feral population of Soay sheep on Hirta in the St Kilda Archipelago. On Hirta about three quarters of sheep have dark brown coats, while the remaining quarter have light sandy coats. 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.
Darwin'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. However, this study shows that the process of evolution by natural selection in Soay sheep, although still evident, is actually more complex than this.
Dr 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. They found that the dark coat trait is usually co-inherited with a set of genes that increase size but decrease reproductive success. The light coat trait is usually co-inherited with a set of genes that decrease size but increase 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. Sheep with two copies of the dark gene are larger still, but have poor reproductive success. Sheep with two copies of the light gene are small, but still have quite high reproductive success. This means that the two types of dark sheep although indistinguishable visually, vary in Darwinian fitness.
The study was based on over 20 years of field data, and involved researchers from the University of Edinburgh and the Queensland Institute of Medical Research in Brisbane, as well as members of the University of Sheffield.
Dr Jacob Gratten said: "The aim of this study was to improve understanding of how evolution by natural selection operates. The study addresses an important problem in evolutionary biology. It shows that predicting the evolutionary response to selection is difficult without knowledge of which genes are in close proximity to each other. It also highlights that an understanding of the underlying genetic basis of a highly visible trait was necessary in order to understand its evolution."
This research - ' A Localised Negative Genetic Correlation Constrains Microevolution of Coat Colour in Wild Sheep' will be appear in the journal Science.
