Unequal sisters: phenotypic feedbacks in the evolution of Conchifera (Mollusca)

Abstract Sister groups (two clades sharing a common ancestor) often differ greatly in taxonomic, phenotypic, and ecologic diversity, but explanations for these differences remain elusive. One hypothesis is that mate choice involving display promotes genetic isolation and thus speciation. We test this and other hypotheses by analyzing individual-level phenotypes that evolved in one but not the other of the two great molluscan sister clades Aculifera (chitons and relatives) and the much more diverse Conchifera (snails, clams, squid, and their relatives). The key difference lies in the morphological relationship between body and shell, which in Aculifera are tightly coupled, greatly limiting locomotion, activity, and mate choice. In Conchifera (especially in the major subclade Megalopodifera), the body and shell are more decoupled, facilitating a wide range of activity, modular evolution, and ecological expansion. Contrary to the primacy of mate choice as an engine of diversification, display-related mate choice is part of a larger theme of evolutionary feedbacks among habitat (including mate) choice, locomotor and other activity, and speciation. Some conchiferan clades never diversified and remain phenotypically conservative, highlighting the fact that the only unifying feature of a clade is its retrospective monophyly.