Saturday, April 4, 2009

Has Human Evolution Ceased?

I dwell quite frequently on the forces of evolution, particularly on how they have shaped human social behavior. This has been a life-long curiosity and a topic of more formal study and extensive reading over the past decade. For most of my adult life, I have naively accepted the notion that human evolution had essentially ceased.

The argument supporting this is easy to understand and has been almost universally accepted in expert circles during my lifetime. It goes like this:

In the past, mutations conferring a survival advantage were propagated in the gene pool while those hampering survival were pruned by the cruel forces of nature. This process wrought stark changes in human intelligence, posture, and demeanor (specifically with regards to an affinity to cooperate and to trust) and kept the wheels of evolution turning toward higher and higher standards of fitness.

Now, so goes the argument, agricultural abundance and modern medicine ensure that humans, even those who are “weaker”, enjoy survival advantages conferred by a benevolent society. As such, the gene pool is no longer driven toward increasing fitness because “last year’s DNA”, though not competitive with the latest mutations, stays in business with the help of agro-medical subsidies. Social forces have brought evolution (for our species) to a rather abrupt halt.

That is a nice little argument with a tidy conclusion. I cannot believe I never noticed the screeching logical flaw it abides.

The blind spot comes from the framework (inferred above) that “surviving to a reproductive age is a major challenge and the ability to meet that challenge is the primary driver of adaptive mutation. According to that framework, when survival is no longer difficult, then the engine driving change loses its power.

Here is a better framework that exposes the fallacy. During most of the 4 million year history of homo sapiens, death has been a serious constraint on our ability to propagate characteristics of weakness. Now that a large percentage of us gets all the food and medical care we need, all sorts of unhealthy mutations can persist in the human gene pool. From this perspective, we should be (and by many accounts are) evolving faster than at any other time in history.

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