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In a previous post I pointed out the fallacy in preformationism and contrasted it with epigenesis: the true paradigm for understanding how DNA directs development.
But I didn’t say anything about behavior. Here, more than anywhere else, the preformationist assumptions most people make about genes being “blueprints” for bodies or “programs” for behavior lead them astray. Certainly, such fallacies invite complete incredulity when you try to suggest that DNA can direct the generation of behavior in much the same way that it demonstrably does the development of bodies. Indeed, you sometimes see this ridiculous, preformationist view of the epigenetics of behavior illustrated by a hand labeled “genes” or “DNA” controlling the strings of a puppet intended to represent a person (left).
But, as I suggested in the previous post, DNA does not blueprint bodies in the sense of being a one-to-one model or plan of them: Siamese cats are colored according to how ambient temperature affects the synthesis of melanin. There is no coat blueprint or coloring diagram written in DNA. And clearly, if this could not be the case with the body, there is no way it could be with behavior. Genes do not and could not program organisms the way spot-welders are programmed in car plants because organisms cannot expect to encounter exactly the same environment every time and be able to act in exactly the same robotic way a spot welder can each time a chasse is presented to it on an assembly line!
But not all robots work this way. Robot vacuum cleaners, for example, run freely and do not rely on rigid programming of the kind seen in spot-welders. On the contrary, all your household robot vacuum cleaner needs is an on-board computer programmed with some simple rules such as:
- if you encounter the edge of a stair, stop immediately and back off; or
- if the battery is low, return to the dock and recharge; or
- if you get stuck, alert owner, then switch off to conserve power and await rescue.
Each of these rules can be encoded digitally and applied if and as necessary by the robot, and natural selection can do the same. Indeed, something essentially the same as the stop-and-back-off-the-stairs rule is programmed by the fear response in animals and humans. For example, it makes crawling babies who have never been taught to do so stop and back off from an unexpectedly encountered cliff edge (below) in much the same way that robot vacuum cleaners do. And of course, what will work with fear of cliff edges will work with a lot of other things that natural selection can program into organisms, such as fear of darkness, strangers, wild animals, fire, etc.
Nor is this all. The low-battery-return-to-base-and-recharge rule has an equivalent in what we call “feeling tired,” and the if-stuck-call-for-help-then-switch-off-and-wait rule is something like what happens when people get depressed.
OK, these rules are a little more complicated in the human case perhaps. But my point is that there is no reason why, if free-ranging robot vacuum cleaners can be programmed this way by software engineers, comparable free-ranging organisms cannot be similarly programmed by their DNA.
A prime case in human beings is to be found in things like the pleasure/pain mechanism. This simply associates behavior favorable to the organism’s survival and reproductive success with pleasurable sensations and behavior detrimental to the same with pain. Evolution can also program organisms’ brains to learn the right lessons quickly where serious threats to survival are concerned. For example, lab-reared monkeys have no fear of snakes, but can become as fearful as wild ones after one exposure to the fear-reaction of a wild monkey to a snake. However, lab reared monkeys cannot be conditioned in the same way to fear flowers.
Combined with the pleasure/pain and other similar mechanisms—such as those involved in choice of mate or social behavior—built-in behavioral biases of this kind can produce complex adaptive behavior well able to serve the organism’s survival and reproductive success in the vast majority of cases. The epigenesis of behavior is as simple as that! DNA cannot directly control behavior in real-time, but it can generate it according to simple rules in much the same way that robots can be programmed to clean your floors.
(With thanks and acknowledgement to my iRobot® Roomba® 770 robot.)
