homoplasia

Homoplasia, also known as homoplasy, is a term in evolutionary biology describing similarity between traits in different species that is not due to inheritance from a common ancestor. The word derives from Greek roots meaning “same” and “formation.” In practice, homoplasy refers to features that arise independently in separate lineages.

Homoplasy arises through several processes. Convergence occurs when unrelated lineages evolve similar traits in response to comparable selective pressures. Parallelism involves related lineages developing similar traits independently, often from a similar ancestral condition. Reversal, or evolutionary reversal, happens when a derived trait reverts to its ancestral form. Collectively, these processes produce analogous similarities rather than homologous ones.

Examples commonly cited include the wings of birds and bats, which evolved independently to enable flight, and the camera-like eyes of cephalopods and vertebrates, which converge on a similar solution despite distant relatedness. Another classic example is the streamlined body shape of ichthyosaurs and dolphins, which reflects convergent adaptation to aquatic life. Such cases illustrate how similar ecological challenges can shape comparable structures even in distantly related organisms.

In phylogenetics, homoplasy can complicate the reconstruction of evolutionary trees because similar characters may be misinterpreted as evidence of shared ancestry. Researchers mitigate this by using multiple genes or genomes, applying probabilistic models of character change, and weighting characters to reduce the impact of highly homoplastic traits. Understanding homoplasy is essential for distinguishing true homology from convergence, parallelism, or reversals in evolutionary analysis.