Evolutionary patterns in ontogenetic transformation: from laws to regularities

The concept of heterochrony derives from classical approaches to the study of ontogeny and phylogeny. Under the influence of landmark books by deBeer (1930) and Gould (1977), the traditional theories have been revised to fit into the conceptual framework of modern genetics and evolutionary theory. The current scheme, however, suffers from a problem of lack of precise definitions. The term heterochrony is now used to refer to a developmental process as well as to an evolutionary pattern. That is, it refers to a microevolutionary process of adaptation, operating in local populations under selection and to a macroevolutionary pattern based on undefined internal laws of form. Such conceptually contradictory frameworks are a source of confusion and of empirical misuse of concepts. We propose to reduce the dependence of current thinking about heterochrony on the concept of "timing" and instead focus on the organization of sequences of developmental events in ontogeny. Although Haeckelian views have been rejected, most experts would agree that some subtle parallelism between ontogeny and phylogeny does occur. This relationship deserves renewed attention and urodeles are particularly suited to study it due to their variable patterns of ontogeny and complex life cycles. Current reductionist attempts to apply the morphological terminology and postulates of classical heterochrony concepts to cellular and molecular (genetic) aspects of morphogenesis are problematic. Molecular heterochrony requires a linear or strictly hierarchical structure of gene regulation of development. In addition, isomorphism between genetic mutations and morphological changes would be required for the existing terminology to apply. Finally, we caution against a broad interpretation of heterochronic processes at the molecular level, since the approach may end up permitting the meaningless interpretation of any developmental change as heterochrony.