Homology -A Continuing Challenge

The notions of essential similarity and/or common ancestry underlie the majority of homology concepts that have been proposed so far. Both notions have limitations and lead to difficulties. As an alternative, a flexible approach taking into consideration the existing variety of structural relationships is suggested. This approach recognizes different kinds and grades of struc- tural relationships, i.e., 1:1 correspondences and partial correspondences. The latter include, for example, relationships that occur between members of a morphocline. The 1:1 correspondences may be seen as borderline cases in which the partial correspondences approach 100%; thus the two are not fundamentally different from each other. Structural relationships (which may or may not be termed homologies) are relative to the level of organization, the background theory, and, in phylogenetic considerations, the level in the cladogram. Transformational analysis investigates structural relationships in terms of the processes that produce them. It underlines the dynamic aspect of plant form and facilitates comparison where the search for structural correspondence is inappropriate. In such cases it may supersede structural homologization, whereas in other cases, it may be complementary to the latter. Transformational analysis may reflect phylogeny. However, in cases where sufficient phylogenetic evidence is lacking, it may represent formal transformations.