Vestigial limb

Vestigiality refers to genetically determined structures or attributes that have lost some or all of their ancestral function in a given species, but have been retained during the process of evolution. Assessment of the vestigiality must generally rely on comparison with homologous features in related species. The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful, however if the lack of said feature provides no advantage and/or its presence provides no disadvantage the feature may not be phased out by natural selection and persist across species. Typical examples of both types occur in the loss of flying capability in island-dwelling species.

Vestigial features may take various forms; for example they may be patterns of behavior, anatomical structures, or biochemical processes. Like most other physical features, however functional, vestigial features in a given species may successively appear, develop, and persist or disappear at various stages within the life cycle of the organism, ranging from early embryonic development to late adulthood.

Vestigiality, biologically speaking, refers to organisms retaining organs, which have seemingly lost the entirety of the original function. The issue is controversial and not without dispute; nonetheless, vestigial organs are common, evolutionary knowledge. In addition, the term vestigiality is useful in referring to many genetically determined features, either morphological, behavioral, or physiological; in any such context however, it need not follow that a vestigial feature must be completely useless. A classic example at the level of gross anatomy is the human vermiform appendix — though vestigial in the sense of retaining no significant digestive function, the appendix still has immunological roles and is useful in maintaining gut flora.

Similar concepts apply at the molecular level — some nucleic acid sequences in eukaryotic genomes have no known biological function; some of them may be "junk DNA", but it is a difficult matter to demonstrate that a particular sequence in a particular region of a given genome is truly nonfunctional. The simple fact that it is noncoding DNA does not establish that it is functionless. Furthermore, even if an extant DNA sequence is functionless, it does not follow that it has descended from an ancestral sequence of functional DNA. Logically such DNA would not be vestigial in the sense of being the vestige of a functional structure. In contrast pseudogenes have lost their protein-coding ability or are otherwise no longer expressed in the cell. Whether they have any extant function or not, they have lost their former function and in that sense they do fit the definition of vestigiality.

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