Problems with the Interpretation of Developmental Sequences

-Some systematists, pattern cladists in particular, have recently argued that phylogenetic patterns are essentially a reflection of the "orderliness of ontogeny." In fact, these authors contend that information derived from the study of ontogenetic sequences is the most reliable source to determine character polarities. Heterochronic analysis, in spite of its emphasis on process over pattern, shares many of the same assumptions about ontogeny. This is because both systematic and heterochronic analyses belong to the comparative tradition of embryology. Essentially, ontogeny is viewed as a sequence of morphological stages that are assumed to be conserved. Furthermore, in spite of claims that the analyses are based on Von Baer's law, the approach is fundamentally Haeckelian, since its comparison of ontogenies (at least in the manner currently done in systematics and heterochrony) requires that embryonic stages be homologized with adult stages. Nelson's revised "biogenetic law" is used as an example to illustrate these points. The nature and informational value of ontogenetic sequences are examined and I conclude that, in contrast with strictly temporal sequences, the only "meaningful" sequences (in the sense that they will be conserved through phylogeny) are the causal ones, where the antecedent stage is required for the expression of the subsequent one. However, I present some examples of causal sequences where inductive relationships change through phylogeny. Therefore, there are no good arguments to assume a priori that ontogenetic sequences are conserved. A more fundamental problem is pointed out when I discuss that the methodology of comparison of ontogenetic sequences, used by systematists and heterochronists, is not compatible with the dynamic perspective of development endorsed by experimental and mechanistically-oriented embryologists. Examples are provided where, although the process can be arranged in a sequence of stages, these are effectively meaningless and useless in a systematic context. Two additional empirical cases are discussed where a heterochronic analysis is integrated within a dynamical framework of development. A paradox emerges, since the resultant morphology in the derived species is not represented in the primitive ("ancestral") ontogenetic sequence, in spite of the fact that it has been produced by a regulation in the timing and developmental rates of the ancestral ontogeny. I conclude that these problems result from the usage of a static and unrealistic view of ontogeny. A major challenge in systematics will be to incorporate the dynamic perspective of development into a methodological framework amenable to comparative analysis. I agree with Nelson and others in that development is ordered and that this internal order structures pattern. However, the information is not necessarily in the ontogenetic sequence. [Ontogeny; development; character polarities; embryology; heterochrony; biogenetic law.] Systematics and heterochrony-here defined not only as the process of morphological evolution due to alterations of timing and rates of development, but also as the field of research that studies such processes-were intimately related a century ago. Both Von Baer and Haeckel strove to construct a general scheme of ontogeny which would directly provide insights into the nature of evolutionary process and pattern. For example, Haeckel's dictum of "ontogeny recapitulates phylogeny" was not only a statement about the nature of developmental processes, but it also implied that by observing and comparing ontogenies one could gain information about the relationships among species. The classical comparative approach of Von Baer, Haeckel and others lost popularity at the beginning of this century due to a shift in emphasis towards the study of the experimental aspects of ontogeny. It became clear that there was no good mechanistic justification for Haeckel's assumptions of conservatism of developmental sequences and the principle of terminal addition (see historical review by Gould, 1977). Von Baer's postulate that de-