The molecular basis for phenotypic changes during pig domestication

Pig domestication was initiated some 10,000 years ago. Thus, within a fairly short period of time, from an evolutionary perspective, a remarkable change in phenotype has taken place. Until recently (the last few hundred years), the selection intensity was weak but selection on traits such as behaviour and disease resistance must have occurred early. Docile animals resistant to stress were likely to be kept by the early farmers. Less obviously, coat colour is a trait that also was altered early during domestication. New coat colour variants occur by spontaneous mutations, but in nature there is a strong purifying selection eliminating such mutations because they provide less efficient camouflage or fail to attract mates. In contrast, such mutations have accumulated in domestic animals—why? One reason is of course relaxed purifying selection, but this is not the only reason. A less efficient camouflage of the domestic stock could be advantageous for the farmer and maybe it was used to distinguish improved domestic forms from their wild counterparts. Today, coat colour is often used as a breed-specific marker. For instance, a Large White pig should be white and a Piétrain pig should be spotted. Furthermore, there is strong selection for white colour in some breeds because of consumer demand for pork meat without any pigmented spots in the remaining skin. Charles Darwin was the first to realize that the phenotypic change in domestic animals resulting from selective breeding is an excellent model for phenotypic evolution due to natural selection (Darwin 1859). In fact, he became a pigeon breeder himself and used domestic animals as a proof-of-principle for his revolutionary theory on natural selection as a driving force for evolution. The first chapter of The Origin of Species (Darwin 1859) concerns observations on domestic animals, and nine years later he published two volumes on The Variation of Animals and Plants under Domestication (Darwin 1868). In the latter book he describes the phenotypic changes that have occurred in the pig and other domestic animals as a consequence of domestication. As a result of the development of molecular tools in the form of well-developed genetic maps and large number of genetic markers we are now in position to start unravelling the molecular basis for phenotypic changes in the pig and other domestic animals.