Canalized pathways of change and constraints in the evolution of reproductive modes of microarthropods

Abstract

This study investigates how the course of evolutionary change of an organismal pattern is canalized by organismal properties. As an example we use the mechanisms of indirect sperm transfer of some microarthropod groups. Miniaturized droplet spermatophores, characterized by a rather similar pattern of structural and functional components, are shown to have evolved independently in the Acari-Actinotrichida and the Pseudoscorpiones within the Arachnida, and in the Entognatha (Collembola and Diplura-Campodeoidea), Symphyla, Pauropoda and Pselaphognatha within the Antennata. At least in the phylogenetic lineages leading to the various antennatan groups, evolution of miniaturized spermatophores took place in a similar sequence of transformation steps. It is likely that — originally — large sac-spermatophores, with a rigid sheath, were deposited on the ground. The subsequent sequence of evolution involved carrying structures, a viscous sheath of the spermatophore-droplets, a stable water balance of the spermatophore under habitat conditions, and miniaturized spermatophore-droplets with immobilized sperm cells. Finally, mate dissociation became a common mode of behaviour in all groups mentioned. Each of the transformation steps was not only an adaptation to a particular selective condition, but additionally a precondition for further adaptive innovation. In this way the sequence of evolutionary change was rigidly determined. Moreover, integration of subsequently evolved components of the spermatophores into a complex network of interacting components obviously caused constraints of interaction, which in turn have affected evolutionary stability of the character-patterns. Using water mites as an example, it is shown how in a changed environment few behavioural changes have initiated an evolutionary sequence which finally has led convergently in several lineages to semidirect or direct sperm transfer and to a massive repatterning of the original reproduction pattern.