Nat Rev Genet. Author manuscript; available in PMC 2010 Feb 1.
Published in final edited form as:
Although the operation of natural selection requires that genotypes differ in fitness, for some geneticists it seems easier to understand natural selection than fitness. Partly this reflects the fact that the word “fitness” has been used to mean subtly different things. Here I distinguish among these meanings (e.g., individual versus absolute versus relative fitness) and explain how evolutionary geneticists use fitness to predict changes in the genetic composition of populations through time. I also review the empirical study of fitness, emphasizing approaches that take advantage