An organism's "fitness" is measured by its ability to live long enough to reproduce and pass down traits to their offspring. In sexually reproducing organisms, those who are most fit are most likely to survive often have a specialized trait that gave them the advantage in doing so. The organism with the traits that allowed them to survive and reproduce passes the trait to it's offspring. Assuming that the environment doesn't overly change, those offspring have a greater chance to survive and reproduce themselves. Thus, this trait becomes a specialized adaption for the environment and allows the species as a whole to survive longer.
