PHYLOGENETIC SUPPORT FOR THE EVOLUTION OF APOSEMATISM BEFORE GREGARIOUSNESS IN CONIFER SAWFLIES

The evolutionary origin of aposematism remains a long-standing puzzle: how can a conspicuous mutant persist and spread within a cryptic prey population? Two competing hypotheses have been proposed. The group-first hypothesis suggests that aposematism evolved secondarily in already aggregating, chemically defended cryptic ancestors, whereas the aposematism-first hypothesis suggests that aposematism arose in solitary ancestors, with gregariousness evolving subsequently. Currently, these two evolutionary pathways have only been studied in caterpillars of Lepidopteran species, but we lack wider taxonimical evidence especially from systems with cryptic but chemically defended species. We evaluated these hypotheses using phylogenetic reconstructions in chemically defended conifer sawflies (Hymenoptera: Diprionidae), a group whose larvae vary in both colouration and aggregation behaviour. Almost half the species examined (20/45) had aposematic and gregarious larvae. Aposematism and gregariousness were positively evolutionarily correlated (r = 0.29). Aposematic and gregarious species tended to evolve from aposematic, solitary ancestors, while species with cryptic and gregarious larvae (14/45) tended to evolve from aposematic and gregarious ancestors, a pattern consistent with the aposematism-first hypothesis.