Ever since the first nervous systems evolved, evolution has been striving to overcome "genetic determinism"—the direct coding of behaviors in genes. No scientist believes that genes preprogram every single behavior demonstrated by an organism during its lifetime. Evolution avoids such preprogramming by endowing animals with senses for registering what is going on in the environment, and reflexes for letting those senses influence movements. These senses and reflexes allow behavior to track environmental variables faster than genetic evolution can. One key variable is the location of food. A flatworm's eyes can notice that food is available in a certain location, without having to wait for the flatworm species to evolve the belief that food is there. If you believe in the existence of senses and nervous systems, you are not a genetic determinist in the strict sense.
Evolution did not stop with eyes and simple nervous systems. It took perfectly good simply nervous systems and expanded their first several segments into great bastions of antideterminism called brains, then added layer upon layer of thinking and feeling between sensory input and motor output. The job of evolutionary psychology is to analyze how evolution constructs these mental adaptations that turn environmental cues into fitness-promoting behaviors. The larger the brain, the more sophisticated the environmental cues it can use to guide behavior, and the more sophisticated that behavior. Into the grand, generation-long cycle of genetic evolution, brains insert millions of faster feedback loops. On a second-by-second basis, senses and brains track new opportunities to promote survival and reproduction. Their whole reason for existence is to keep genes from having to change every time the environment does.
Genes rarely determine specific behaviors, but they often determine the ways in which environmental cues activate behaviors. Many behaviors are fairly predictable if you know what an organism is perceiving at the moment. This predictability comes from the demands of optimality: for any given environmental situation, there is often one best thing to do. Animals that do the right thing survive and reproduce better; animals that deviate from optimal behavior tend to die. This pressure for optimal behavior makes many behaviors predictable.
However, there are situations in which it is a very bad idea to be predictable. If another organism is trying to predict what you will do in order to catch you and eat you, you had better behave a bit more randomly. Selection may favor brain circuits that randomize responses, to produce adaptively unpredictable behavior. The benefits of randomization were first understood in a deep way by game theorists. What they said about randomization will help us understand human creativity later.
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