“Dmitri hypothesized that the key factor selected when our ancestors domesticated wild animals was tameness. Thus, the unit of selection was not related to an animal’s physical attributes but to its behavior. This was an audacious guess since most scientists then assumed that physical morphology was the unit of selection. How could Dmitri test his bold theory? Only by going back to the time when animals first started becoming domesticated. For dogs, this would have meant experimenting on wild wolves. But getting a supply of wolves would have been very hard in Siberia, so he chose the silver fox. He designed a selective breeding experiment that focused only on tameness as the selection factor.”