What does fitness measure in the context of evolutionary biology?

In the context of evolutionary biology, fitness refers to an organism's ability to survive and reproduce in its environment. It encompasses not only the survival aspect but also how effectively an organism can pass on its genes to the next generation through reproduction. Therefore, measuring fitness includes looking at both the survival rates of individuals and the total number of offspring they can successfully raise to reproductive age.

This comprehensive view of fitness highlights why it is important to consider both survival and reproductive success together. An organism might survive for a long time, but if it does not reproduce, its genetic contribution to the gene pool is minimal. Conversely, an organism that produces a high number of offspring, but does not survive long, may still be considered fit because of its reproductive success. This dual focus is essential in understanding how traits that enhance both survival and reproduction can spread through populations over time, influencing evolution.