生物谷报道：野生索艾羊(Soay Sheep)已在苏格兰的圣基达岛(St Kilda Is.)上与世隔绝、毫无拘束地生活了上千年的时间。处于这种环境下的索艾羊是研究自然进化的绝佳材料--圣基达岛就像一个巨大的培养皿，而其中的索艾羊就像果蝇一样。
有两种颜色的索艾羊：浅棕色和深棕色。深棕色的索艾羊个体更大，从进化的角度来说，形体越大越好--大型动物的适应性更好，更有可能生存下去。因此随着时间的推移，索艾羊种群中深棕色的羊比例应该越来越大。但过去20年记录显示的结果却完全相反。
英国布里斯托尔大学 （University of Bristol）大学Jon Slate领导的的研究小组发现了其中的原因。 Slate博士说，这是由于羊毛颜色基因位点附近的基因决定的，这个基因叫做适应性基因（fitness gene）。
较早前研究人员已鉴定出羊毛的颜色基因，并发现深棕色的为显性。只要从父母的任何一方遗传得到一个深棕色的等位基因，它就是深棕色的；只有同时从父母身上都遗传得到浅棕色的等位基因，才会出现浅棕色的症状。
在这项新研究中，他们发现在基因组相邻的基因位点，分别会影响形体的大小，小羊的存活率和生殖的成功与否。他们不知道为什么这些位点会有这种作用。当深棕色的等位基因遗传给子代时，同时也遗传正向影响形体大小的基因，但也遗传着负向影响小羊存活率和生殖能力的基因。当浅棕色的特征遗传给索艾羊时，索艾羊同时也获得更小的形体的基因，以及更好存活和生殖的基因。
这意味着，浅棕色的索艾羊和仅有一份深棕色基因的深棕色索艾羊，适应性要更强。这就是浅棕色索艾羊数量增多的原因。更大的形体不足以抵消其它的遗传因素。Slate博士表示，“如果不是形体大小的基因，那它们就更糟糕。”
生物谷推荐英文原文：
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.
