Researchers have demonstrated that a Darwinian process operating on an intermediate timescale of days governs the immune response following a vaccination. Through a series of cellular generations spanning several weeks, the immune system produces defensive antibody molecules that are better and better fits against invaders. By abstracting the essential features of a Darwinian process from what is known about species evolution and immune responses, we can see that any Darwin machine must have six properties.
First, it must operate on patterns of some type; in genetics, they are strings of DNA bases, but patterns of brain activity associated with a thought might qualify. Second, copies are made of these patterns. (Indeed, that which is semi-reliably copied defines a unit pattern.) Third, patterns must occasionally vary, whether through mutations, copying errors or a reshuffling of their parts.
Fourth, variant patterns must compete to occupy some limited space (as when bluegrass and crabgrass compete for my backyard). Fifth, the relative reproductive success of the variants is inuenced by their environment (how often the grass is cut, watered, fertilized and trampled); this result is what Darwin called natural selection. And, finally, the makeup of the next generation of patterns depends on which variants survive to be copied. The patterns of the next generation will be variations spread around the currently successful ones. Many of these new variants will be less successful than their parents, but some may be better.
Sex and climatic change are not essentials, but they add spice and speed to a Darwinian process, whether it operates in milliseconds or millennia. Note that an essential is not Darwinian by itself: for example, selective survival can be seen when owing water carries away sand and leaves pebbles behind.
The Darwinian Mind
LET US CONSIDER how these principles might apply to the evolution of an intelligent guess inside the brain. Thoughts are combinations of sensations and memories--in a way, they are movements that have not happened yet (and maybe never will). They exist as patterns of spatiotemporal activity in the brain, each representing an object, action or abstraction. I estimate that a single cerebral code minimally involves a few hundred cortical neurons within a millimeter of one another either firing or keeping quiet.
Evoking a memory is simply a matter of reconstituting such an activity pattern, according to psychologist Donald O. Hebbs cell-assembly hypothesis. Long-term memories are frozen patterns waiting for signals of near resonance to reawaken them, like ruts in a washboarded road waiting for a passing car to re-create a bouncing spatiotemporal pattern.
Some cerebral ruts are permanent, whereas others are short-lived. Short-term memories are just temporary alterations in the strengths of synaptic connections between neurons, left behind by the last spatiotemporal pattern to occupy a patch of cortex; this long-term potentiation may fade in a matter of minutes. The transition from short- to long-term patterning is not well understood, but structural alterations may sometimes follow potentiation, such that the synaptic connections between neurons are made strong and permanent, hardwiring the pattern of neural activity into some regions of the brain but not into others.
A Darwinian model of mind suggests that an activated memory can compete with others for workspace in the cortex. Perceptions of the thinkers current environment and memories of past environments may bias that competition and shape an emerging thought.
An active cerebral code could move from one part of the brain to another by making a copy of itself, much as a facsimile machine re-creates a copy of a pattern on a distant sheet of paper. The cerebral cortex also has circuitry for cloning a spatiotemporal pattern in an immediately adjacent region less than a millimeter away, although our present imaging techniques lack enough resolution to see the copying in progress. Repeated copying of the minimal pattern could colonize a region, rather the way a crystal grows or wallpaper repeats an elementary pattern.