Homology describes traits inherited from a common ancestor that may have different functions in modern organisms. The human arm, bat wing, and whale flipper are classic homologous structures.
These structures often share the same underlying anatomical pattern even when natural selection has modified them for grasping, flying, or swimming. Their similarities are evidence of common ancestry rather than evidence that the structures perform identical jobs.
Homology is commonly confused with analogy. Analogous structures have similar functions but evolved independently, as wings in birds and insects did. In modern biology, homology also applies beyond visible anatomy: genes, developmental pathways, and DNA sequences can be homologous when they descend from a shared ancestral feature.