Biological terms for evolution, please read details...?

There is no term for a species that's reached its maximum evolutionary potential, because there is no such thing in an absolute sense. Sharks and crocodiles can and have evolved into new species. However, you may be thinking of a situation when a species is at a *fitness maximum* given its environmental and historical constraints. These are times when a species has evolved as well as it can to a given environment, and so will stay stable so long as the environment doesn't change. It's also true that some genetic changes are easier to accomplish than others, and because of that, basic developmental pathways can be *cized*, meaning the evolutionary options act like a marble rolling around in a c, at least until such time as major selection pressures push it out again. Finally, a species (such as the cheetah) may p through a *genetic bottleneck*, or time during which the population was very small, and thereby lose most of its genetic variation. Until mutations build up again in the population, such a species will have a very low potential for evolutionary change.