Old and new concepts in EvoDevo

The concept of the modular organization of the organism is central to both classical experimental embryology and modern evolutionary developmental biology (EvoDevo). The latter discipline often ascribes homology, or the diversity of forms arising from the same module or combination of modules, to gene mutations or changes of gene regulation. This gene-centred atomistic view does not yield reliable criteria for homology, nor can it account convincingly for phenomena such as convergence and cooption. To understand the logic behind this plasticity, attention should be shifted from variation of existing characters to the preconditions for homology. This could be done by identifying the basic modules and their molecular regulation within the context of the most general, shared embryonic morphology. Mapping divergent embryonic trajectories onto different fates of these modules may allow us to throw light on the constrained sets of different, but related morphologies which can arise from different combinations of modules.