I'm recalling evolution here. GURUKID hit the nail on the head. A species cannot be considered fit if it's offspring are sterile. It could be considered fit if it produced many eggs in its lifetime, many offspring, or number of gametes produced during time of reproducing. The number of offspring surviving to breed is, therefore, the most important.
You can check out the link provided, but I recall it being a huge tenet of Darwinism that a species needs to survive AND reproduce to be fit for evolution.