EVOLUTIONARY PATTERNS IN ONTOGENIC TRANSFORMATION - FROM LAWS TO REGULARITIES

The concept of heterochrony derives from classical approaches to the s tudy 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 ev olutionary 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, ope rating in local populations under selection and to a macroevolutionary pattern based on undefined internal laws of form. Such conceptually c ontradictory frameworks are a source of confusion and of empirical mis use of concepts. We propose to reduce the dependence of current thinki ng about heterochrony on the concept of ''timing'' and instead focus o n the organization of sequences of developmental events in ontogeny. A lthough 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 partic ularly suited to study it due to their variable patterns of ontogeny a nd complex life cycles. Current reductionist attempts to apply the mor phological terminology and postulates of classical heterochrony concep ts to cellular and molecular (genetic) aspects of morphogenesis are pr oblematic. Molecular heterochrony requires a linear or strictly hierar chical structure of gene regulation of development. In addition, isomo rphism between genetic mutations and morphological changes would be re quired for the existing terminology to apply. Finally, we caution agai nst a broad interpretation of heterochronic processes at the molecular level, since the approach may end up permitting the meaningless inter pretation of any developmental change as heterochrony.