Homology describes similarities in different species that result from inheritance from a common ancestor.
A homologous trait may serve different functions in different organisms while retaining an underlying structural relationship. The forelimbs of humans, bats, whales, and cats are a classic example: their bones correspond to one another even though they are used for grasping, flying, swimming, and walking.
Homology is evidence of common ancestry and is central to comparative anatomy and evolutionary classification. Similarities can occur at many levels, including anatomy, development, DNA sequences, and biochemical pathways. A shared genetic sequence may be homologous even when the visible structures or functions have changed.
Homology is often confused with analogy. Analogous traits have similar functions but evolved independently, such as the wings of birds and insects. Such similarity is usually associated with convergent evolution rather than inheritance from the same ancestral structure. Evolution can therefore produce either inherited similarity with changed function or independent similarity under similar environmental pressures.