Domesticating Foxes »

Dmitri Belyaev, a Russian scientist, was curious about the mechanism(s) that changed wild wolves into our domestic dogs. In a short 20 years of selectively breeding silver foxes for tameness, he was able to "evolve" his wild silver foxes into surprisingly dog-like creatures. They showed affection to their human caretakers, their ears lowered to resemble border collies, their coats became fluffy and grew in black and white spots, and their breeding cycles became year-round rather than seasonal.

Tests confirmed lower adrenaline and higher serotonin levels in the brains of the selectively-bred animals. In 20 years. The experiment has continued for nearly 50 years with the same results.

More information from this American Scientist article.