Regeneration as an evolutionary variable.

Regeneration poses a distinctive set of problems for evolutionary biologists, but there has been little substantive progress since these issues were clearly outlined in the monograph of T. H. Morgan (1901). The champions at regeneration among vertebrates are the urodele amphibians such as the newt, and we tend to regard urodele regeneration as an exceptional attribute. The ability to regenerate large sections of the body plan is widespread in metazoan phylogeny, although it is not universal. It is striking that in phylogenetic contexts where regeneration occurs, closely related species are observed which do not possess this ability. It is a challenge to reconcile such variation between species with a conventional selective interpretation of regeneration. The critical hypothesis from phylogenetic analysis is that regeneration is a basic, primordial attribute of metazoans rather than a mechanism which has evolved independently in a variety of contexts. In order to explain its absence in closely related species, it is postulated to be lost secondarily for reasons which are not understood. Our approach to this question is to compare a differentiated newt cell with its mammalian counterpart in respect of the plasticity of differentiation.