Brian C. GOODWIN
The life of form. Emergent patterns of morphological transformation — (CRAS 323 (2000) 1:1-144)
The focus of the sciences of complexity is the emergence of form in complex dynamic systems.
There are biological phenomena that stubbornly resist this molecular, genetic and historical reductionism. The first of these is development itself, the emergence of complex organisms from eggs or buds. This cannot be explained in terms of a genetic program for the simple reason that the generation of three dimensional, macroscopic form (morphogenesis) cannot be explained in terms of the changing molecular composition of the developing organism, for which a genetic program provides, in principle, a complete description. The difficulty here is a general one, not particular to biology: composition does not determine form. Furthermore, adding historical constraints (in the form of initial conditions, say) and environmental influences does not alter this general conclusion. To explain how macroscopic form is generated, it is necessary to include a description of the spatial pattern of forces, or the relational order, that characterises the particular system under study, which is called a field. Solids, liquids, liquid crystals, gases are all described by theories that combine compositional properties with a description of how the state of any region is influenced by, and influences, neighboring regions within the system, defining its relational order as a field. Developing organisms are described by morphogenetic fields which are the organised spatio-temporal context within which changing molecular composition (controlled by a genetic program) exerts its influence. … To understand developing organisms, it is necessary to have an adequate theory of morphogenetic fields.
The second area of biology where the Neo-Darwinist approach has difficulties is the study of parallel and convergent evolution. e.g. segmentation has arisen independently in annelids and arthropods. Explanations of this homeoplasy then require an appeal to functional constraints, different groups evolving similar body structure as adaptations to their habitats. However, the diversity of life styles and habitats between annelids and arthropods makes functional convergence a rather implausible hypothesis, and one is drawn to alternatives such as robust developmental mechanisms as a source of explanation. This implies that there are basic generative mechanisms readily accessible to metazoan morphogenesis so that particular patterns, such as segmentation, can emerge independently in different lineages.
A third area is where similar developmental strategies are found in taxa as distant as Radiata on one hand, and Ascidians and other Deuterostomes on the other hand. How is one to account for such similarities, polyphyletic in origin, without invoking constraints that are intrinsic to morphogenesis, irrespective of lineage or selective pressure?
The causes, or the generative mechanisms, of biological form are not genes themselves but the morphogenetic fields to whose particular properties genes contribute. As stated by Gilbert et al. (1996): « Just as the cell (and not its genome) functions as the unit of organic structure and function, so the morphogenetic field (and not the genes or the cells) is seen as a major unit of ontogeny whose changes bring about changes in evolution ». E.g. the segmentation genes identified in Drosophila contribute to the formation of segments, but the production of the physical structures identified as segments arise from the coordinated activities of cell adhesion molecules, the properties of the extracellular matrix, the differential contractions and expansions that give rise to the extended band stage 11, and many other factors distributed in spatial patterns that perform the work involved in directed cell movements, shape changes and tissue deformation. These activities, organised coherently over macroscopic dimensions, are morphogenetic fields, the proximal causes of organismic form as it emerges during epigenesis. Regulatory genes act within, and modify, such a context, functioning as co-ordinated networks of interacting elements, primarily transcription factors, that modulate one another’s activities in a manner quite similar to neurones in neural nets.
Similar genes (homeotic) does not mean same form (e.g. fly and chick).
François KEPES 