Final answer:
Fitness in flies can be influenced by both genetics and the environment. The color of an organism's eyes can affect its fitness in relation to population density. Additionally, natural selection can be constrained by the relationships between different traits, as seen in the example of mice coloration.
Step-by-step explanation:
Fitness in flies can be determined by the environment and genetics. In one study, the relative fitness of Drosophila melanogaster was measured in relation to black eye color and population density. The experiment showed that as population density increased, the fitness of the flies with the black eye color allele decreased. Furthermore, natural selection can also be constrained by the relationships between different traits, as seen in the example of mice coloration. Light-colored mice may be more fit than dark-colored mice due to the negative fitness consequences of an intermediate coat color that doesn't blend in with the environment.