Suppose, for example, you are handed an unfamiliar piece of fruit. You automatically classify it by its physical appearance, smell, taste, and the circumstances under which it is given. A large amount of in- formation is activated within seconds, not just the comparison of the fruit in hand with other kinds but also the emotional feelings, recollec- tions of previous discoveries of similar nature, and memories of dietary customs that seem appropriate. The fruit—all its characteristics com- pounded—is given a name. Consider the durian of Southeast Asia, re- garded by aficionados as the greatest of all tropical fruits. It looks like a spiny grapefruit, tastes sweet with a transient custardlike nuance, and when held away from the mouth smells like a sewer. The experi- ence of a single piece establishes, I assure you, the concept "durian" for a lifetime.
The natural elements of culture can be reasonably supposed to be the hierarchically arranged components of semantic memory, en- coded by discrete neural circuits awaiting identification. The notion of a culture unit, the most basic element of all, has been around for over thirty years, and has been dubbed by different authors variously as mnemotype, idea, idene, meme, sociogene, concept, culturgen, and culture type. The one label that has caught on the most, and for which I now vote to be winner, is meme, introduced by Richard Dawkins in his influential work The Selfish Gene in 1976.
The definition of meme I suggest is nevertheless more focused and somewhat different from that of Dawkins. It is the one posed by the theoretical biologist Charles J. Lumsden and myself in 1981, when we outlined the first full theory of gene-culture coevolution. We recom- mended that the unit of culture—now called meme—be the same as the node of semantic memory and its correlates in brain activity. The level of the node, whether concept (the simplest recognizable unit), proposition, or schema, determines the complexity of the idea, behav- ior, or artifact that it helps to sustain in the culture at large.
I realize that with advances in the neurosciences and psychology the notion of node-as-meme, and perhaps even the distinction be- tween episodic and semantic memory, are likely to give way to more sophisticated and complex taxonomies. I realize also that the assign- ment of the unit of culture to neuroscience might seem at first an attempt to short-circuit semiotics, the formal study of all forms of com- munication. That objection would be unjustified. My purpose in this exposition is the opposite, to establish the plausibility of the central program of consilience, in this instance the causal connections be- tween semiotics and biology. If the connections can be established empirically, then future discoveries concerning the nodes of semantic memory will correspondingly sharpen the definition of memes. Such an advance will enrich, not replace, semiotics.
I CONCEDE that the very expression "genes to culture," as the con- ceptual keystone of the bridge between science and the humanities, has an ethereal feel to it. How can anyone presume to speak of a gene that prescribes culture? The answer is that no serious scientist ever has. The web of causal events comprising gene-culture coevolution is more complicated—and immensely more interesting. Thousands of genes
prescribe the brain, the sensory system, and all the other physiological processes that interact with the physical and social environment to pro- duce the holistic properties of mind and culture. Through natural se- lection, the environment ultimately selects which genes will do the prescribing.
For its implications throughout biology and the social sciences, no subject is intellectually more important. All biologists speak of the interaction between heredity and environment. They do not, except in laboratory shorthand, speak of a gene "causing" a particular behavior, and they never mean it literally. That would make no more sense than its converse, the idea of behavior arising from culture without the in- tervention of brain activity. The accepted explanation of causation from genes to culture, as from genes to any other product of life, is not heredity alone. It is not environment alone. It is interaction between the two.
Of course it is interaction. But we need more information about interaction in order to encompass gene-culture coevolution. The cen- tral clarifying concept of interactionism is the norm of reaction. The idea is easily grasped as follows. Choose a species of organism, whether animal, plant, or microorganism. Select either one gene or a group of genes that act together to affect a particular trait. Then list all the envi- ronments in which the species can survive. The different environ- ments may or may not cause variation in the trait prescribed by the selected gene or group of genes. The total variation in the trait in all the survivable environments is the norm of reaction of that gene or group of genes in that species.
The textbook case of a norm of reaction is leaf shape in the arrow- leaf, an amphibious plant. When an individual of the species grows on the land, its leaves resemble arrowheads. When it grows in shallow water, the leaves at the surface are shaped like lily pads; and when sub- merged in deeper water, the leaves develop as eelgrasslike ribbons that sway back and forth in the surrounding current. No known genetic dif- ferences among the plants underlie this extraordinary variation. The three basic types are variations in the expression of the same group of genes caused by different environments. Together they compose the norm of reaction of the genes prescribing leaf form. They embrace, in other words, the full variation in expression of the genes in all known survivable environments.
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