They bred foxes based on low fear response to humans and in just a few generations physiological changes occurred they started getting doglike physical features... floppy ears and the like.
If you are talking about what I expect you are, that's not quite what happened. It was a fairly famous Russian experiment to breed domesticated foxes - not specifically low fear response (that's too specific a trait to breed for), but tameness, which isn't exactly the same thing.
What they got wasn't actually doglike physical features. What they got was a fox that retained many traits of puppyhood later into life. One upshot of this was that their coats changed, mostly ruining the breed for farming purposes.