Differentiation of developmental plasticity as a major cause of morphological evolution in stag beetles (Coleoptera: Lucanidae)

Abstract

Stag beetles show great developmental plasticity (DP) in their external morphology. In intraspecific evolution, DP functions as a stabilizer for morphological change, whereas interspecifically, the evolution of external morphology is primarily a process of differentiation in DP. Greater DP in external morphology has evolved as a species trait through selection for survival in conditions that vary stochastically, rather than a long-term unidirectional trend in conditions. All the interspecific transitions towards larger size, multimodal morphology and adaptive expansion are preceded by an increase in DP. Here, differentiation of DP is more a cause, rather than a result, of species-level evolution. This contrasts with the differentiation of other important traits, such as the morphology of the genitalia, which follow the conventional known process of small effects accumulating to larger effects from an intraspecific stage to an interspecific stage, in which DP plays no significant role. The DP of external morphology, as an emergent property of species, provides the means by which they can survive and subsequently evolve to be better adapted to novel conditions. This contrasting mode of evolution through differentiation of DP should play a crucial role in species survival and the maintenance of biodiversity in the Anthropocene.