Well?
I get the general idea that certain "mutations" have to occur with one organism to change their species, but what about mating?
Mutations occur all of the time. Most are benign and do nothing. Every now and then the mutation kills the critter, and even more rarely the mutation does nothing until some outside event comes along that makes that mutation important. The other critters vying for that particular niche die out because they lack that particular mutation, while the mutated critter takes over the niche. That is how evolution works.
Why would a viable critter die out just because a new model developed?
Because the new critter is able to compete better for the particular niche they are both fighting for. Remember, Mother Nature doesn't "like" or "dislike" her critters, she merely gives them a chance and those that are better and more efficient survive. Those that aren't....don't. It's simply numbers.