A recent paper in Genome Research titled Relaxation of selective constraint on dog mitochondrial DNA following domestication concludes that domestication of wolves and their transformation into dogs were facilitated by relaxed selection and increased latent variation, from which subsequent selection could operate upon. Here is a good popular press summary. The basic finding is that domestic dogs in their sample (13 dogs, 6 wolves and 3 coyotes had their full mtDNA sequenced) exhibited greater accumulation of mildly deleterious mutations, in a nearly neutral fashion. Relaxation of selection tends to result in increased diversity, polymorphism, as the genome is not "purified" and stabilized toward one dominant allele. This does not imply that dogs are not subject to selection, but the selective pressures upon them are very different than those for their wild cousins.
The interesting thing for me is that I'm rereading The Genetical Theory of Natural Selection, originally published in 1930. On page 11 R.A. Fisher, the author, states:
The property of the particulate theory of conserving the variance for an indefinite period explains at once the delayed or cumulative effect of domestication in increasing the variance of domesticated species, to which Darwin calls attention. Many of our domesticated varieties are evidently ill-fitted to survive in the wild condition. The mutations by which they arose may have been occurring for an indefinite period prior to domestication without establishing themselves, or appreciably affecting the variance of the wild species. In domestication, however, not only is the rigour of Natural Selection relaxed so that mutant types can survive, and each such survival add something to the store of heritable variance, but novelties of form or colour, even if semi-monstrous, do undoubtedly attract human attention and interest, and are valued by man for their peculiarity. The rapidity with which new variance is accumulated will thus be enhanced....
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