Final answer:
Vestigial structures, such as the human tailbone, whale pelvis, and the eyeballs of cavefish, are remnants of organs that once served a function in ancestors but have since become unnecessary or used for different purposes.
Step-by-step explanation:
Vestigial structures are parts of an organism that are no longer used for their original function due to evolution. These structures serve as important evidence for evolution, indicating changes from using the structure to not using it, or using it for a different purpose.
For instance, the tailbone in humans (A) is a vestigial structure; it is the remnant of an ancestral tail and no longer serves its original purpose. Eyeballs in cavefish (B) have become unnecessary due to the dark cave environments in which the fish live, rendering vision non-essential. The pelvis bone in whales (C) is a residual part from when their ancestors walked on land. Although whales no longer have hind legs, they still possess remnants of pelvis bones.
The concept of vestigial structures also extends to physiological functions, such as sight in whales (D), where certain species have reduced vision because it is less necessary in their dark, deep-water environments. For balance in monkeys (E), their tails no longer serve as a primary balance aid as they have adapted other means of balance. Lastly, in frogs, certain bone structures (F) may no longer serve as attachment points for limbs if those limbs have been lost or transformed through evolutionary processes.