Vestigiality

Vestigiality is the condition of having an inherited trait reduced in size or function compared with its ancestral form. A vestigial trait may retain a function, acquire a new one, or have little function.

Evolution: Evolution is the change in heritable traits of populations across generations. Changes in inherited traits across generations can reduce structures when their ancestral functions matter less.
Whale pelvis: The whale pelvis is a reduced set of pelvic bones embedded in the body wall of modern whales. Its reduced form reflects the hind limbs of land-dwelling ancestors.
Homology: Homology is similarity between traits due to shared ancestry. Shared ancestry helps identify a reduced structure as an inherited remnant.
Common descent: Common descent is the principle that organisms inherit traits from shared ancestors. Similar reduced structures can preserve evidence of shared ancestry.
Natural selection: Natural selection is the process by which heritable differences cause organisms to leave different numbers of descendants. Reduced structures can persist when selection against them is weak or their costs are small.
Human coccyx: The human coccyx is the small terminal bone formed by fused vertebrae at the base of the spine. It is reduced relative to the tail-bearing structures of human ancestors, though it retains functions.
Comparative anatomy: Comparative anatomy studies similarities and differences in the structures of different organisms. Comparisons reveal how a trait's form differs from related organisms' forms.
Evolutionary biology: Evolutionary biology studies the processes and patterns of biological evolution. Vestigiality links present-day anatomy with changes across evolutionary history.
Genetic drift: Genetic drift is random change in allele frequencies, especially pronounced in small populations. Drift can alter traits that provide little advantage or disadvantage.
Human appendix: The human appendix is a narrow pouch attached to the large intestine, with immune and microbial roles. Its size and role differ from larger cecal structures in some herbivorous ancestors.

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