Final answer:
Stalk-eyed flies have evolved traits such as eye stalk length due to selective predation by birds, affecting their visibility and survival.
Step-by-step explanation:
The stalk-eyed flies are subject to selective predation by birds, as hypothesized by J.W. Tutt in 1896. This selection pressure influences the length of their stalks and their mating choices, because these physical traits affect their visibility to predators. A similar process of selection can be seen in butterflies, where traits such as eyespots can make them more visible to predators, depending on the environment and the behavior of the predators. The studies suggest that these kinds of physical traits are selected against in natural populations when they increase visibility and predation risks. However, this selection can also favor traits if they result in a protective mimicry or other advantages that increase the survival of the species.