DARWINIAN SELECTION IN THE BRAIN

Abstract

In this book, Gerald Edelman explains how process akin to natural selection, but one which operates in somatic time on the interactions between neurons, is responsible for order in the brain. The theory of neuronal group selection was proposed by Edelman over ten years ago and has since been developed by Edelman and his colleagues at Rockefeller University, Leif Finkel and George Reeke, into well-posed and synthetic theory of nervous-system development. In this review, I will focus on the issues of most concern to evolutionary biologists: in what sense is the brain selective system, and what are the similarities and differences between neuronal group selection and the genetical theory of evolution. There are two basic aspects to any selective system: variation in and heritability. Consider first variation in fitness. In genetic evolution, the word fitness is used in least two different senses: the first in an operational sense and the second in design or engineering sense. There are several measures of operational fitness, including the per capita rate of increase (first used by Fisher [1930]) and expected reproductive success. Fitness in the sense of better design (often referred to as adaptedness) is far more difficult to define in genetic evolution, and there is no universal definition, since it depends on the specifics of the species and its environment. In neuronal group selection, fitness is also used in the operational and engineering senses as discussed shortly. What sort of entities possess fitness? Again, this depends upon the problem of interest, but candidates in genetic evolution include bits of DNA, genes, chromosomes, organisms, populations, and possibly whole species. A final point to realize about in evolution is that it is always dependent on the environment. What is fit in one environment may be unfit in another. In neurobiology, candidates for units of selection are the synapse, neuron, neuronal group, nuclei, and whole cortical columns. It is basic to the theory of neuronal group selection that groups of neurons function as units of selection. A neuronal group is defined as a set of more or less tightly connected cells that fire predominantly together (p. 198). With terms appropriately translated, this definition of group corresponds nicely to the definitions of group structure used in population genetics (see, for example, Uyenoyama and Feldman [1980]). In neuronal group selection, also has an operational and design sense, referring either to