Modularity in Development and Why It Matters to Evo-Devo

The concept of modularity is fundamental to research in bothevolutionary and developmental biology, though workers in eachfield use the idea in different ways. Although readily and intuitivelyrecognized, modularity is difficult to define precisely. Mostdefinitions of modularity are operational and implicit, particularlyin developmental biology. Examination of several proposed definitionspoints to some general characteristics of developmental modules,for example their internal integration, and suggests the importanceof devising a definition applicable at different levels of thebiological hierarchy. Modules, like homologs, must be definedwith respect to a specified level of the hierarchy, and a generaldefinition should support both analyses of the evolving causalrelationships between levels, and studies of the interconnectionsbetween modules of the same type. The designation of a developmentalstructure, process, or function as a "module" is a testablehypothesis; this hypothesis is confirmed in the case of thedorsal marginal zone of the amphibian gastrula, which acts asa morphogenetic module. Discussions of developmental modularitycan provide a meeting place for developmental and evolutionarybiologists by helping us articulate key questions at the intersectionof the two fields, and design experiments to begin answeringthem.