If he brought up the monkeys to men thing you may want to explain to him that that isn't how evolution works. In fact, it doesn't always kill off the latter species. Natural selection favors the better suited and may eventually kill off or absorb the less favorable to survive. This is in certain cases though. In other cases though, the new trait may in fact separate the two populations or change their characteristics without being better than the other. When the early humans got better legs for walking, we could walk from tree to tree and navigate the open planes while apes were able to continue living in trees without much interaction or competition with early hominids and thus we evolved separately