It absolutely is, but it's defined backwards compared to how most people intuit it. You can't look at a population today and identify which individuals are the

It absolutely is, but it's defined backwards compared to how most people intuit it. You can't look at a population today and identify which individuals are the fittest in evolutionary terms. It's not possible to state with precision that someone who has e.g. more sex or more children or is taller or richer or poorer or better spoken or more sociopathic is more likely to succeed. Rather, fitness is something that can only be defined in reverse, because today's population is full of individuals with ancestors who have survived and reproduced, and so on all the way back. As Neal Stephenson puts it: "Like every other creature on the face of the earth, Godfrey was, by birthright, a stupendous badass, albeit in the somewhat narrow technical sense that he could trace his ancestry back up a long line of slightly less highly evolved stupendous badasses to that first self-replicating gizmo---which, given the number and variety of its descendants, might justifiably be described as the most stupendous badass of all time. Everyone and everything that wasn't a stupendous badass was dead."