It's not a "fork" per se, but it is reasonable to describe Darwin as a member of the BSD family tree, descended largely from FreeBSD. My understanding, based on my memory and the [Wikipedia page](https://en.wikipedia.org/wiki/XNU) is that the XNU kernel was built out of Mach, but then had 4.3BSD grafted on to provide a normal-looking UNIX API, which was later upgraded using FreeBSD. But it isn't a normal BSD, certainly; Darwin still shows its Mach heritage in the way it does process management and message passing.