Evolutionary novelties: the making and breaking of pleiotropic constraints.

Body plans are remarkably well conserved, but on (very) rare occasions important novelties evolve. Such novelties involve changes at the genotypic and phenotypic level affecting both developmental and adult traits. At all levels, duplications play an important role in the evolution of novelties. Mutations for duplications, including mutations for duplications of body parts, as well as mutations for other changes in the body plan, in particular homeotic ones, occur surprisingly frequently. Hence the limitation of mutations appears to be relatively unimportant for the conservation of body plans. However, mutations for duplications of body parts and homeotic changes rarely persist in populations. We argue that the root cause of the conservation of body plans is the strong interactivity during the patterning of the embryonic axes, including the interactivity between patterning and proliferation processes. Due to this interactivity, mutations cause many negative pleiotropic effects (malformations and cancers) that dramatically lower fitness. As an example, we have shown that in humans there is extreme selection against negative pleiotropic effects of the, surprisingly frequent, mutations affecting the number of cervical vertebrae. Moreover, we argue for the relevance of relaxed selection, which temporarily allows just-arisen novelties to persist, for the effective breaking of pleiotropic constraints. We illustrate this with two empirical examples.