Evolutionary biology

La eficacia biológica is fitness, and this is the one place where English borrowed trouble from its own gym. « Fitness » on a poster is being in shape; « fitness » in this field is a count of descendants and nothing else — a sickly animal that leaves twelve young is fitter than a magnificent one that leaves none. Spanish never risks the confusion, because the two are separate words: la forma física for the body, la eficacia biológica for the count. Reading « the fittest » as « the strongest » is the oldest mistake in the subject, and only English spelling invites it.