Aspects of two species of organisms that are equivalent because these aspects were acquired by both species from a common ancestor.
This is equivalent to the definition of a homology, that is, an aspect of an organism, or of two organisms, that exists in the manner that it does because the same aspect also existed in ancestral species. Particularly given sufficient time that genetic drift will have been expected to have eliminated such identity, then existence of homologies is assumed to be a consequence of stabilizing selection.
See also homologous structure and vestigial structure. Contrast with analogy and convergent evolution.
Note that base sequence can also be identical by descent, and indeed a key aspect in comparing the sequences of two organisms is the search for such homology, that is, genotype seen among two or more organisms that is identical by descent. Given otherwise sufficient evolutionary divergence between two lineages, then such identity again is assumed to be a consequence of stabilizing selection, that is, that it plays some role in the maintaining the fitness of both organisms.
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