I'm using Darwin to mean Evolution by Natural Selection. Perhaps Lamark. I honestly know very little about Lamark, but my understanding is that his theories didn't turn out to be very strong. Interestingly, even if they don't describe biology important ideas metaphysics.
I don't think so. Darwinian evolution is not just a description of a natural process. It is a powerful metaphysical principle. IE, Darwinism could happily exist in theory, without any physical form. It just happens to have a physical form, biology. But this fact is the reason that it has had such an impact on our vocabulary.
People that have used Darwinian-like thought before can apply it elsewhere. This kind of analysis draws a lot from that. I think there are few philosophical principles that have this power. I cannot think of a many pieces of philosophical work with this kind of an impact.
