More than 140 years ago, Charles Darwin noticed something peculiar about domesticated mammals. Compared to their wild ancestors, domestic species are more tame, and they also tend to display a suite of other characteristic features, including floppier ears, patches of white fur, and more juvenile faces with smaller jaws. [This is called the domestication syndrome.]
Why [does] breeding for tameness causes changes in such diverse traits[?]
In the hypothesis proposed by Wilkins and co-authors Richard Wrangham of Harvard University and Tecumseh Fitch of the University of Vienna, domesticated mammals may show impaired development or migration of neural crest cells compared to their wild ancestors.
Tests of the neural crest hypothesis may not be far off, as other scientists are rapidly mapping the genes that have been altered by domestication in the rat, fox, and dog. The hypothesis predicts that some of these genes will influence neural crest cell biology.This is a lovely example of the scientific method in operation.
Darwin's observations are in the first paragraph. The problem is in the second. The hypothesis to explain the problem is in the third. A possible test of the hypothesis is in the fourth.
Wikipedia: Domesticated silver fox
Scientists in Novosibirsk, Russia began an experiment over 50 years which resulted in the first true, domesticated foxes.
Domesticated red fox, red variety
Domesticated red fox, silver variety.
The fox is an especially good test animal for this as the scientists in Novisibirsk, Russia, have three varieties of foxes: wild fox, their domesticated variety, and a variety selected for "wildness." The latter amplifies the wild traits of the fox.