This is kinda backwards. Biodiversity is a result of random mutation over millions of years. Evolution is the result of selective pressures "choosing" from that diversity. It doesn't create or provide biodiversity, it depends on it, so biodiversity should be the one providing crabs. Evolution just thinks they're neat.
Man, I just started Man on the Inside by Michael Schur with Ted Danson, and it's pretty good, but it's not The Good Place. Looks like it's time for another rewatch