Thursday, December 27, 2007

Evolution (and biology in general) defines an arrow of time

In a post earlier this year, I pointed out that evolution is not a reductionist explanation. By that I meant that the evolutionary mechanism isn't dependent on its implementation. Evolution is (a) the generation of variation and (b) the selection (or at least favoring) of some of the variants for reproduction in a manner that passes on aspects of the variation that led to the selection.

The second law of thermodynamics says in effect that entropy inevitably increases. This defines the arrow of time. But the second law is like dirt in the gears of physics because all other laws are blind to time. They work as well backwards as forward. There is no apparent reason in physics why time moves as we experience it.

So the point of this post is to notice that evolution is like the second law of thermodynamics in that it is not blind to time. It moves in a defined direction. It doesn't make sense to say that evolution could run just as well backwards as forwards.

This is a lot less creepy than the second law, however, because evolution is defined on a much higher level of abstraction. For one thing, it assumes the notions of survival, reproduction, and the passing on of properties. So just that assumption seems to require a direction of time. It doesn't make sense to think of reproduction working backwards. One can't get from an offspring to a state prior to the offspring. So perhaps it's not as significant as I first thought that evolution implies a direction in time. Both reproduction and survival until reproduction seem to have an assumption of a direction of time built into them.

But then doesn't all of biology — or at least all of biology that is understood functionally — also have a direction in time? I don't know if it was on one of these blog pieces, but I've defined functionality as the change a mechanism has on its environment. Biology is all mechanism and functionality. So almost by definition, biology in general defines a direction in time.

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