Did they ever figure out a way to explain Homos in Public School Biology cl?

In terms of evolution, how can 2 species that cannot reproduce, multiply in numbers? Some say mules, but that is not logical as it takes a donkey and a horse to create a mule. What other non aual life forms exist where they cannot further their species yet grow in population? The only thing I can think of is a some type of gene mutation, but I think people have a problem with saying 's are the result of mutated genes