What is the implication of "fitness" in evolutionary biology?

The concept of "fitness" in evolutionary biology primarily refers to an individual's ability to pass on its genes to the next generation, which is captured well by the notion of greater genetic contribution. This encompasses not only the number of offspring that an organism produces, but also how well those offspring can survive and reproduce in their own right. Fitness is often measured in terms of reproductive success, which includes not just quantity but quality of offspring.

Understanding fitness in this broader context helps clarify why other interpretations of the term are less accurate. For instance, merely having physical strength does not ensure reproductive success or genetic contribution, as an organism may survive well but fail to reproduce effectively. Similarly, potential for survival is an essential aspect of life but does not automatically equate to fitness unless it correlates with reproductive outcomes. Lastly, the concept of fitness is not limited to certain species; it applies universally across the spectrum of life, indicating that all organisms have their unique adaptations that contribute to their fitness in various environments. Thus, the essence of fitness revolves around successful gene propagation rather than singular traits or species limitations.