The basic idea of natural selection is very simple. If some individuals have a trait that enhances their survival and reprroduction and if that trait can be passed from parent to offspring, then it will become more common in future generations. Darwin and Wallace formulated that principle 150 years ago, and it has been repeatedly tested and verified.
In Friday's issue of Science Jacob Gratten and colleagues point out that in Soay sheep dark coat color is associated with large size and that large size enhances both survival and reproduction (subscription required). The coat color difference (dark brown versus light tawny) is determined by alternative alleles at a single locus, with the dark brown allele being dominant to tawny. So you'd expect dark brown Soay sheep to become more common – and you'd be wrong. Over 20 years the dark colored phenotype has decreased at a rate of a little less than 0.4% per year.
What gives? Is the theory of natural selection wrong? No. You need to look a little closer at all of the traits that affect fitness.1
A more precise formulation of the principle of natural selection is that alleles that increase the fitness2 of individuals carrying them increase in frequency over time, and those that decrease the fitness of individuals carrying them decrease in frequency. It turns out that even though sheep that are homozygous for the allele that causes dark color (GG) have the same coat color as those that are heterozygous for the allele (GT), the overall fitness of the homozygous genotype (GG) is less than that of the heterozygous genotype (GT), and the fitness of the light sheep (TT) is indistinguishable from the fitness of the heterozygotes. So individuals carrying the G allele have lower fitness than individuals carying the T allele, explaining the decrease in frequency of the G allele and of dark sheep.
But what about the observation that dark sheep tend to be larger and that larger sheep are more likely to survive and more successful in reproducing? Well, that's true on average, but it's not true for differences in body size that are specifically related to differences in coat color.
The fitness and body-size effects associated with differences at the coat-color locus aren't due to differences among the alleles at that locus. They're due to differences among alleles at loci that are closely linked to the coat-color locus. The dark coat color allele is linked to alleles promoting large body size at a body-size QTL3 and to alleles causing lower fitness at a fitness QTL So when a dark-colored sheep passes the G allele along to its offspring, it also passes along alleles at QTLs that cause large body size and lower fitness.
1And unravelling those complexities is why the paper appeared in Science
2Roughly the product of survival probability and reproductive success.
3QTL: Quantitative Trait Locus
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