Tool Module: Neural Darwinism

The process of selection that is the basis of evolution operates not only on the individual and species levels, but also on the cellular level, in the various major systems of an organism.

One of the people who has done the most work on the hypotheses of Darwinian selection at the cellular level is Gerald Edelman, winner of the 1972 Nobel Prize for Medicine. Edelman’s research showed that the immune system is not programmed in advance to face all potential invaders and microbes. It is rather the pressure of the antigens (from the invaders) that selects antibodies from among the infinite variety that the immune system produces at random.

Edelman subsequently became known for his theory of neuronal group selection (or Neural Darwinism), of which we will provide only a very general overview here (see the linked modules for details). This theory too is based on a selective process, but in this case one that accounts for the development and functioning of the human nervous system. This nervous system comprises nearly neurons, each of which makes thousands of connections. But the human genome comprises only 30 000 to 100 000 genes. Hence it is mathematically impossible that our genome specifies the wiring of our entire brain in detail. To explain how our genes can nevertheless construct our nervous system, Edelman calls on the mechanism of competition and selection of neuronal groups.

Edelman’s central hypothesis is that the highly complex mapping of the brain occurs through a selective process. First, an individual’s genome generates varied neural networks. Then, from this primary neuronal repertoire that is defined by the genome (and distinct for each species), certain networks of neurons are selected that respond especially well to external stimuli that are important for the organism.

We thus find the same two key concepts here as we do in the process of natural selection that is the origin of species: the production of a variety of forms (in this case, neural structures) and a mechanism that selects those forms that are best adapted (here, the selective stabilization of neural circuits). The connections that get used the most become stronger, while the others disappear, thus creating neural networks that are unique to each individual. The forces that lead to this selective stabilization of synapses are primarily fundamental biological ones, such as the need to eat and reproduce. But forces in the physical and social environment come into play as well. Edelman calls these forces “values”. They have nothing to do with values in the moral sense, but rather with the fundamental needs of living organisms.

The selected circuits form what Edelman calls neural maps. These maps, which are massively interconnected, in turn form associations by entering into “temporal resonance”. This process might be the basis for our ability to categorize our perceptions, which might operate by combining the activities of various maps in the cortex, some of which were sensitive to shapes, others to colours, others to touch, and so on.

Note that in this model, there is no central supervisor that imposes coherence on our perceptions. Instead, various maps are simply excited at the same time, activating millions of neurons in parallel, which in turn activate other maps that comprise millions of neurons as well. And it is through this process of “re-entry” that perceptions, motor behaviours, conceptual thought, and even consciousness itself come into being.