The fitness of a trait can change over time when something called frequency dependent selection occurs. Frequency dependent selection occurs when the fitness of a trait depends on how common individuals with that trait are in a population. Positive Frequency Dependent Selection occurs when a trait has higher fitness when it is common than when it is rare. Negative Frequency Dependent Selection occurs when a trait has higher fitness when it is rare than when it is common. Negative frequency dependent selection will turn out to have some important implications for evolution of other traits, so we will consider a couple of examples.
Female fruit flies in a population prefer to mate with a male with an unusual phenotype.
Suppose for example that most individuals in the population have red eyes, but a few have white eyes. White eyed males will attract more females. If most males in the population have white eyes but a few have red eyes, females will mate preferentially with the red-eyed males. Consider what will happen to eye color over time in such a fruit fly population. If white eyes start out rare, females will mate preferentially with males with white eyes, so white-eyed males reproduce most and white eyes will become common.