There is quite a bit of sharing between Apple and FreeBSD. For example libdispatch has been ported to FreeBSD. Also when Apple imported pf into Darwin; FreeBSD

There is quite a bit of sharing between Apple and FreeBSD. For example libdispatch has been ported to FreeBSD. Also when Apple imported pf into Darwin; FreeBSD used that same port to update their port of pf from OpenBSD. As others have mentioned, stuff like the TrustedBSD and Audit framework came from Darwin, and features such as Seatbelt and sandboxes are slowly being considered or improved upon by Capsicum for example.