Which of the following best describes vestigial structures?

Vestigial structures are features or organs that have lost most or all of their original function through the course of evolution. The concept stems from the understanding that certain anatomical features may have been useful in ancestral species but are no longer necessary for survival or reproduction in modern descendants. A classic example of vestigial structures includes the human appendix, which is thought to have played a role in the digestion of cellulose in the diets of herbivorous ancestors.

These structures can provide significant insights into the evolutionary history of a species, suggesting how organisms can change over time due to shifts in environment or behavior that render certain features obsolete. This evolutionary perspective underscores the importance of understanding vestigial structures as remnants of ancestral adaptations, rather than focusing on their current state of functionality.

While functional structures that serve vital purposes and structures with high levels of variation refer to different biological concepts, these are not relevant to the definition of vestigial structures. Similarly, absent structures do not accurately describe vestigial features, as vestigial structures are indeed present in the organism, albeit with diminished functionality.