Final answer:
Relative fitness refers to an individual's reproductive success in relation to the average reproductive success in the population, and it's critical for understanding evolutionary changes driven by natural selection.
Step-by-step explanation:
An individual's reproductive success divided by the average reproductive success of the entire population is known as relative fitness. This measurement is a way of quantifying an individual's ability to survive and reproduce in comparison to others in the population. In the context of evolution by natural selection, relative fitness is a key concept as it helps to determine which individuals are contributing more offspring to the next generation and how the population may evolve over time. Natural selection favors the reproduction of individuals with favorable heritable traits, leading to evolutionary changes in the population.