Modularity as a universal emergent property of biological traits.

An emergent property of an entity is one that results from the organization or interaction of its components. Here I argue that modularity (discreteness) is a universal emergent property of organismic traits that results from the universal bounded responsiveness, to inputs (e.g., genomic or environmental), of the phenotype at all levels of organization. While degree of modularity is variable and may be subject to change under selection, I argue that the consequences of modularity, such as the evolvability of modular traits (e.g., due to their somewhat independent expression and selection; and their potential for expression in new combinations) are not responsible for the existence of modularity itself. I discuss some other views of modularity in biology; and the related idea that increased complexity (e.g., increased numbers of kinds of modular traits) is a product of selection. I argue that emergent properties characterize innovative phenotypes at their origin. When emergent properties occur at more than one level of phenotypic organization, they sometimes lead to confusion due to confounding levels of organization with levels of selection, as has occurred in discussions of group selection in the evolution of sociality. Phenotypes, whose environmental sensitivity and (emergent) properties are emphasized here, seem likely to assume increasing importance as biology moves toward a better understanding of the genome as agent of both the transmission and the expression of traits.