[empthought]

You probably know this, but the Darwin/FreeBSD relationship is much, much more complicated than a fork.

Darwin's the successor to NeXTSTEP, which included a Mach microkernel and some BSD APIs and userland. When they developed OS X, Apple updated much of the userland using more recent FreeBSD versions as a base.

Even more confusingly, Apple has contributed some subsystems to open source that have made their way into FreeBSD (notably the compiler and libc++).

So Darwin's not a fork in the same way that OpenBSD is a fork of NetBSD.