Ontogeny, Phylogeny and Evolution of Form: An Algorithmic Approach

-A computer model that simulates general aspects of ontogeny is presented as a heuristic device for studying the relationship between ontogenetic changes and the evolution of morphologies. The model consists of a set of developmental rules limited to known cellular properties and activities of geneproducts; it assumes development to be an algorithmic and hierarchical process. Morphological patterns are generated through the interactions of developmental rules. Thus, by its very nature the model establishes pleiotropies and complex characters. To represent mutations, changes were introduced in the developmental rules. Phylogenetic analysis of mutant forms showed that morphology retained significant information about ancestry. Genetic parallelisms, as a rule, did not result in morphological parallelisms, because the effect of a given "genetic rule" is context-dependent. Morphological parallelisms that did arise were a consequence of different mutations that affected certain populations of cells in related ways. [Ontogeny; phylogeny; computer model; pleiotropy; heterochrony.] The occurrence of major morphological transformations through evolutionary time must result from changes in the genetic rules that guide development. Yet, the majority of neo-Darwinian evolutionary theory deals with changes in allele frequencies in populations rather than with the origin of new phenotypes. This gap in the study of evolution results from the lack of a theory of development that is able to predict morphologies from patterns of gene activity. To date, theories useful for understanding particular aspects of development include reaction-diffusion systems (Turing, 1952; Meinhardt, 1982), models of mechanical interactions (Oster et al., 1983; Harris et al., 1984), and formal one-dimensional algorithms (Lindenmayer, 1982; Wolfram, 1984). None of these theories, however, address the relationships between gene activities and morphological features. Development is an inherently hierarchical and algorithmic process. It is our contention that a set of "genetic rules" provides the algorithm for development. The genetic rules operative at any given point in development are the biological activities of the gene products present at that time. These gene products can cause the appearance of new morphological or biochemical properties in the cells of the developing organism and may cause activation and inactivation of other genes. Developmental events range from the molecular to the macroscopic; genetic rules operating on the product of the preceding processes produce a sequence of forms, or an ontogeny. The characteristics of development and its morphological products depend on the genome of the individual in question and on the way in which cells respond to conditions in their internal and external environment (Lindenmayer, 1982; Maynard Smith, 1983; Wolpert, 1983). Since developmental events are not instantaneous, ontogeny is ideally viewed as a continuous series of biological forms whose adjacent members may differ quite subtly, as in the presence or absence of a single molecular species. For practical purposes, however,, ontogeny is usually divided into stages; the number of stages and their characteristics depend on the level of resolution at which analysis is performed. A fundamental requirement for understanding both macroevolution and development is an appreciation of the relationships between genes and morphologies. The idea that genes can directly code for structures has been one of the most re-