Homology

Definition

Similarity in structure, gene sequence, or developmental pattern between organisms due to shared ancestry rather than convergent evolution. In comparative anatomy and systematics, homology indicates that a trait originated in a common ancestor and was inherited by descendant lineages, even if modified in form or function.

Etymology

From Greek homologia, agreement or correspondence

Example

The forewings of beetles (elytra) and the forewings of grasshoppers (tegmina) are homologous as insect wings derived from the ancestral pterygote wing, despite their different hardened vs. membranous forms; in contrast, elytra are not homologous to the hardened forewings of earwigs, which evolved independently.

Related Terms

• analogy
• Convergent evolution
• serial homology
• paralogy
• orthology
• synapomorphy
• Homoplasy
• Plesiomorphy

Usage Notes

Distinguished from analogy (similar function without common ancestry) and homoplasy (similarity from independent evolutionary origins). Homology is a hypothesis based on evidence from comparative anatomy, embryology, genetics, or the fossil record, not an observation. Serial homology refers to repeated structures within one organism (e.g., legs of a centipede). In molecular systematics, orthologous genes (diverged by speciation) are distinguished from paralogous genes (diverged by gene duplication). Specialists may use 'homologous' loosely for any matching parts, but precise usage requires evolutionary inference.