Developmental logic: establishing a relationship between developmental process and phylogenetic pattern in primitive vascular plants

Abstract

Cladistic methods serve as a powerful example of the current shift in epistemology from a largely inductivist stance toward more `hypothetico-deductive' or `strong inference' approaches. However, in the area of character analysis, especially in addressing the roles of developmental and evolutionary processes influencing our understanding of homology, synapomorphy, and evolutionary pattern, it appears that little has changed. This omission has the very real potential of substantially limiting resolution and reliability of cladistic hypotheses, and points to the need for an expanded methodology that simultaneously addresses evolutionary pattern and underlying causal factors. Due to unique aspects of both plant development and evolutionary history, early vascular plants offer attractive features that make them highly suitable as a test case along these lines. Expanding on the Zimmermann's `telome theory' of a strong inference approach to developmental process is suggested by employing an analogy with logic gates and logic circuits in programming theory. Hypothesized units of developmental dynamic, termed `evolutionary developmental gates' (EDG) define a relationship between external and internal `environmental' factors, and developmental outcome, using a logical conditional. EDGs are then assembled into networks, specifying a relationship between developmental processes and resultant morphological structure, that may be tested. Testing utilizes several lines of evidence including comparing EDGs with specific developmental data, computer modelling, and evolutionary comparisons based on fossil and living taxa. To illustrate this approach, and to initiate analysis of what constitutes a set of developmental processes necessary for describing primitive vascular plant form, four developmental modules comprised of networks of EDGs are proposed. These include: (1) establishment of self-recognition at the shoot apex along with initiation of a hormone (auxin) gradient, (2) normal cell division modeled as an iterative and recursive process, (3) developmental cascades leading to tissue differentiation of epidermis, cortex, and vascular tissues, and (4) establishment of new apices either by bifurcation or de novo formation. Although clearly more detail is required to enable important evolutionary comparisons, it is anticipated that networks of EDGs can comprise a more realistic perspective with which to estimate homology in evolutionary studies.