What Are the Units of Selection in Modular Organisms?

Abstract

In modular plants and animals, the conceptual distinction between idividual and group selection is not self-evident. In modular organisms, physically coherent structural individuals consist of semi-autonomous interactive and reproductive modules. In higher plans, reproductive structures develop on shoot modules repeated during the growth of an entire structural individual, which corresponds to a physically independent plants as well as to a physically coherent plant clone, The fitness of such a modular structural individual can be expressed in terms of module births and deaths, and of propagule production by reproductive modules. As these fitness components may be influenced by module-level interactions and those at the level of the structural individual, phenotypic selection has a hierarchical causal structure in modular organisms; several of interaction modify survival and reproduction at the level of the module. Since physically coherent clones are, in fact, groups of physically interrelated organisms (ramets), this implies a special king of group-level selection, called group modulated selection in the present study, in which group function as interactive units that modify the fitness components at a lower level, consisting of the reproductive units. It is these reproductive units that actually progagate genetic units or replicators to the succeeding generations.