A phenotype-centered view of evolution needs to start with a solid idea about the nature of the phenotype. This chapter and the next are devoted to two universal properties of phenotypes, plasticity, or responsiveness to environmental inputs; and modularity, or subdivision into semi-independent and dissociable parts (chapter 4). Of these two properties, plasticity is probably the more fundamental, for the ability to replicate, which distinguishes organic from inorganic nature, requires molecules which are interactive and precisely responsive— adaptively plastic. So plasticity must have been an early universal property of living things. The universality of modularity is a secondary, or “emergent” result of the universality of plasticity (see Wilczek, 2002, on emergent universality in physics). Any organism whose size, whether due to accretion or growth, is large enough to create internal environmental differences, such as those between the inner and the outer regions of a clump of material, has the potential for regional internal differentiation. As differentiation evolves to produce specialized parts and an internal division of labor, internal heterogeneity gives rise to conditional switches between developmental pathways. The result is a stucture characterized by somewhat discrete parts—modularity. Thus, given plasticity as a universal property of living matter, modularity follows. The present chapter describes some of the remarkable mechanisms of phenotypic plasticity. One reason to focus on mechanisms is to indicate the material basis for the evolution of plasticity, which is a product of concrete devices that are subject to genetic variation and selection. A cursory look at these mechanisms, however incomplete, by itself suggests the importance of plasticity in development and evolution, for the mechanisms of plasticity include some of the most ingenious and widely conserved creations of nature. Mechanisms of plasticity are further discussed in chapter 23, which describes how organisms assess environmental conditions when they adaptively switch between alternative developmental pathways. Phenotypic plasticity has already been defined as the ability of an organism to react to an environmental input with a change in form, state, movement, or rate of activity.
Hormone Signaling and Phenotypic Plasticity in Nematode Development and Evolution