ABSTRACT
The principles underlying the development of an organism are among the most actively studied and discussed issues in biology. Various and sometimes contradictory experimental data have inspired different theories. One such problem has been the focus of research attention during the last two years. In their article entitled “Is neural development Darwinian?” Purves et al. (1996) critically assessed the concept of ontogenetic development of the nervous system, first formulated in 1888 by Wilhelm Roux (Roux, 1974) and later developed by Ramon-y-Cajal (1929). According to this concept, during neurogenesis there is a competitive struggle among outgrowths, and perhaps even among nerve cells for space and nutrition. The key point is that the nervous system contains more neural elements (neurones, neuronal branches, synaptic connections, and groups of interconnected neurones) early in development than in maturity. Purves et al. supply ample data on the developmental increase in the number of neural elements, synaptic contacts, and neurites’ branching. They conclude that an unwarranted enthusiasm for the idea that neural development proceeds by winnowing an initial excess can only obscure the essentially constructionist nature of the development of the mammalian brain, and may impede an effort to understand it. The idea offered by Purves et al. was questioned by Sporns and Tononi (1994) who claimed that it ignored other brain theories based on variation and selection (Edelman, 1987). These selectionist theories have three major components: “The first is the generation of variability within populations of neurones, which is manifest structurally through cell replication and cell death, and ongoing neurite extension, retraction and synaptic remodeling, and dynamically
through continuous modifications of firing patterns. The second is the interaction of the variable circuitry and firing patterns with the organism’s environment. The third is differential amplification or attenuation of the contribution of neuronal or synaptic populations to neuronal function, either by local rules of plasticity based on correlated firing, or by global changes mediated by diffuse ascending (value) systems” (Sporns, 1997, p. 291).
