Final answer:
Vestigial structures, such as the human appendix, persist because they typically do not significantly affect reproductive success, and therefore are not strongly selected against. Their presence supports the concept of evolution by illustrating common ancestry and changes over time rather than an end goal to evolution.
Step-by-step explanation:
Vestigial structures are anatomical remnants that were important in the organism's ancestors but are no longer used in the same way. A commonly cited example of a vestigial structure in humans is the appendix, which was once larger and more functional in ancestral forms where it played a significant role in digestion. Similarly, the pelvic bones found in whales are vestigial, indicating their land-dwelling ancestors despite their current aquatic lifestyle.
Vestigial structures persist despite natural selection because their reduced size and function typically do not pose enough of a disadvantage to significantly impact an organism's reproductive success. In many cases, these structures may shrink or become less functional because their maintenance would otherwise use resources without offering benefits. A full-sized, unused structure could indeed reduce an organism's fitness if it impedes the organism or uses significant resources, so natural selection often leads to the reduction of these structures over time.
Moreover, although often misunderstood, vestigial structures do not indicate a purpose or end goal to evolution. They are more accurately seen as evidence of the process of evolution and the changes that occur over time due to varying selective pressures. They illustrate the concept of common ancestry, supporting the idea that different species have evolved from common ancestors, altering their features as needed or as a consequence of genetic changes over many generations.