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Size homoplasy refers to the repeated evolution of similar body size in unrelated evolutionary lineages. It describes a situation where species that are not each other’s closest relatives converge on comparable scales of mass, length, or other size metrics, driven by similar ecological pressures or developmental constraints rather than shared ancestry. As a form of convergent or parallel evolution, size homoplasy can complicate interpretations that rely on size alone to infer relatedness or ecological similarity.

Causes and mechanisms include analogous selective optima for resource use, predator–prey dynamics, life-history trade-offs, climate and habitat constraints, and general allometric relationships. Because size can be achieved through different genetic and developmental pathways, independent lineages may reach similar sizes via distinct routes, a phenomenon that underscores the complexity of genotype–phenotype mapping.

Detection and study rely on phylogenetic comparative methods. Researchers map body-size data onto a well-supported phylogeny and assess phylogenetic signal, using techniques such as independent contrasts, phylogenetic generalized least squares, or Bayesian models to infer the history of size evolution. If multiple independent shifts toward similar sizes occur and do not align with phylogeny, this supports size homoplasy.

Common contexts include repeated shifts toward large size in island lineages (island gigantism) and toward small size in productive environments, as well as broad instances of convergent body-size evolution across distant clades facing similar ecological constraints. Size homoplasy highlights that quantitative traits can evolve in parallel or convergently without reflecting shared ancestry.