Field homology as a way to reconcile genetic and developmental variability with adult homology.

The theoretical and developmental fundament of field homology is here examined, particularly as applied by the authors to comparative neurobiology. Preliminary considerations explore conceptual differences between sameness (homology) and similarity. The source of sameness (the biological evolutionary relationship properly sought in homology analysis) is thought to lie in morphostatic evolutionary and morphogenetic processes, which constrain organismal variation at the level of its fundamental structural organization (Bauplan). This occurs via regulation of the branching mode of the morphogenetic sequence or epigenetic landscape. Of fundamental importance in this context is the role of developmental (morphogenetic) fields. The latter concept is analyzed in its general properties and is postulated to underpin the stability of the developing Bauplan down to the ultimate conserved details. Developmental fields subdivide during ontogenesis into ever smaller fields in a complex hierarchy, defining at each stage the developmental entities which are subjected to regulatory, morphostatic effects via the genome and indirect phenotypic selection. These fields thus represent the natural characters for considerations of embryonic homology, and underlie adult homology, rather than arbitrarily selected embryonic parts. Field subdivision proceeds into the constitution of individually specified cell populations. Field regulatory properties, however, do not extend to all differentiation phenomena observed in embryos. This means there is a limit to the applicability of field homology analysis, leaving space for biological variation and convergence outside of proper homology relationships. Genetic and developmental variability are compensated by the regulatory functions of the developmental fields insofar as they relate to correct Bauplan construction. These ideas suggest the convenience of a more systematic use of field homology methods, which start with appropriately identified developmental fields to expand knowledge on adult homology (sameness) and eventually also on accessory structural and functional similarities or differences.