What does "fitness" refer to in a biological context?

In a biological context, "fitness" primarily refers to the success of a genotype across generations. This concept encompasses not just the ability to survive but also the overall contribution of an individual’s genotype to the gene pool of the next generation. Fitness is most often measured in terms of reproductive success – specifically, the average number of offspring produced by an individual that survive and reproduce themselves.

This means that fitness extends beyond mere survival, as it emphasizes the importance of reproduction. An individual that might live long yet produces few or no offspring would not be considered as 'fit' as another individual that may have a shorter lifespan but produces many offspring that live to reproduce.

While the number of offspring is a critical component of fitness, it is essential to note that this is just one aspect of a broader definition that includes genetic contribution and success across generations. Understanding fitness in this way allows us to appreciate the mechanisms of natural selection and evolutionary theory more fully.