Hierarchical Selection and Fitness in Modular and Clonal Organisms

Abstract

Modular growth generates at least three kinds of hierarchies: morphological, functional, and demographic. The morphological hierarchy corresponds to the phenotypic characters of both the units (modules) that are repeated by developmental processes, and the units (organisms, colonies, and clones) that develop by iteration, specialization and integration of modules. The functional hierarchy concerns the levels of interaction 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 nested hierarchy of demographic units that are replicated by asexual propagation. Each level of the demographic hierarchy that is characterized by specific birth and death rate, is a potential candidate for evaluating fitness. We propose a formal approach in order to analyze the hierarchical structure of phenotypic selection in modular organisms, and to evaluate the selective importance of various levels of modular organization. We derive a measure of selective importance from the sensitivity of fitness to a unit change in the characters of a given level: the 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 integrated. Moreover, we present a decomposition of fitness variation in relation to the levels of trait variation. This decomposition demonstrates that the levels where variation in fitness is observed do not always correspond to the interaction-levels at which the causal agents of selection are acting on particular traits. Following the logic of phenotypic 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-independent selection we show that the Malthusian parameter is identical at all levels in the hierarchy. However, the third example shows that this result is not valid in density-dependent selection models. The way density-dependent regulation is supposed to operate in the model system determines which of the demographic levels should be used to evaluate fitness. Consequently, there is no fundamental demographic level that should a priori be chosen when measuring fitness