HIERARCHICAL SELECTION AND FITNESS IN MODULAR AND CLONAL ORGANISMS

Modular growth generates at least three kinds of hierarchies: morpholo gical, functional, and demographic. The morphological hierarchy corres ponds to the phenotypic characters of both the units (modules) that ar e repeated by developmental processes, and the units (organisms, colon ies, and clones) that develop by iteration, specialization and integra tion of modules. The functional hierarchy concerns the levels of inter action or, in evolutionary terms, the functional relationships between fitness and the phenotypic characters at different levels of modular organization. Finally, clonal growth and reproduction results in a nes ted hierarchy of demographic units that are replicated by asexual prop agation. Each level of the demographic hierarchy that is characterized by specific birth and death rate, is a potential candidate for evalua ting fitness. We propose a formal approach in order to analyze the hie rarchical structure of phenotypic selection in modular organisms, and to evaluate the selective importance of various levels of modular orga nization. We derive a measure of selective importance from the sensiti vity of fitness to a unit change in the characters of a given level: t he sum of squared sensitivities associated with that level. We propose that clonal-level characters of disintegrated clones will make small contributions to the variation in fitness, while such characters will be more important if the clone is physically and physiologically integ rated. Moreover, we present a decomposition of fitness variation in re lation to the levels of trait variation. This decomposition demonstrat es that the levels where variation in fitness is observed do not alway s correspond to the interaction-levels at which the causal agents of s election are acting on particular traits. Following the logic of pheno typic optimization models, we consider three examples of selection in order to examine whether the different demographic levels are equally suitable for evaluating fitness. In two examples of density-independen t selection we show that the Malthusian parameter is identical at all levels in the hierarchy. However, the third example shows that this re sult is not valid in density-dependent selection models. The way densi ty-dependent regulation is supposed to operate in the model system det ermines which of the demographic levels should be used to evaluate fit ness. Consequently, there is no fundamental demographic level that sho uld a priori be chosen when measuring fitness.