Darwin was an amateur. So was Einstein. And both were subject matter experts. It might help to remember that the root of "amateur" is "amator", lover. An amateur is one who does what they do out of love.

Amateur astronomers still make meaningful contributions, especially in the realm of variable stars.

Amateur archaeologists and paleontologists are often welcome at many digs. Not only are they free labor: they provide watchful eyes and minds.

An amateur isn't really someone who knows nothing about a particular subject. On the contrary, every amateur by definition knows at least something about their subject matter. An amateur is simply someone who engages that subject in a non-paid capacity, for personal enjoyment or some other reason. Indeed, amateurs can be, and many are, subject matter experts.