It is both - it is a a separate subject (course unit), but one that is often hidden inside another subject (unit) to make it palatable to students. To do this you give it another name and mix in some other stuff - I guess a bit like the way nasty tasting medicine is handled :)

The problem with teaching population genetics is not that it is not important or interesting, but that it involves maths. Biology students and maths are a bit like oil and water.