Tuesday, November 4, 2008

Are humans still evolving?

The other day I stumbled on a post about the question of whether or not humans are still evolving. To put this question on the table, you are talking about the Darwinian type natural selection where offspring are put out there and some are more fit to reproduce than others. Over time this leads to changes in the species.

An alternative or perhaps complimentary view is "punctuated equilibrium". The basic idea is that a species reaches an equilibrium state where it basically remains unchanged for a long period of time. Then some dramatic or catastrophic event happens separating members of the species into isolated populations, and from there new species evolve. This makes a lot of sense to me, I have no doubt this is at least part of the way that species evolve.

In modern humans I have a hard time believing traditional Darwinian evolution has much if any influence. Here is a basic fact: in the modern world, just about anyone can reproduce. The picture put forward by Darwin of more offspring than can possibly survive leading to changes in the species simply does not exist in humans anymore.

Medical advances have ensured that infant survival is much higher than it was in the past. Moreover, medical advances have also ensured that survival of mothers during and after birth is much higher than it was in the past. Infant mortality can certainly be improved (in particular in the United States among industrialized nations) but the plain fact is a baby in modern society has a very good chance of surviving to adulthood. The days of a mom having 7 kids where only 2 made it to adulthood are long gone.

Getting to my other point, like I said the bottom line is anyone can reproduce. Sure maybe some people have trouble "getting laid" in high school but this is a problem that goes away in adulthood. Its almost like a cliche-there is somebody out there for everyone. It doesn't matter if you're fat, skinny, tall, short, dark, light, blonde, brunette, bald or have all your teeth-if you want to you can find someone to have kids with-despite the difficulties a committed relationship might entail. You might have to hit your local trailer park if you're lacking in teeth-but the reality is even then you're going to get lucky sometime.

This brings me to another point. If Darwinian evolution is at work, then what traits are being selected for in human populations? Is it intelligence? I think we can agree that intelligence is certainly a quality that contributes (but does not guarantee) to success. A person with high intelligence is likely to attain higher education, leading to a higher paying job which in Darwinian terms contributes to the acquisition of more resources. But does this lead to more reproduction? The answer in modern humans is probably not. In fact its actually the people in the slums and trailer parks who are actually reproducing more. This may be partly due to choice-an upscale lawyer can choose to have sex while using birth control while a teenager living in low class circumstances may not bother with it. So the teenager might end up having 5 kids while the lawyer has 1, 2, or maybe none.

Medical intervention has made a lot of physical characteristics that might have contributed to Darwinian evolution in the past has now become irrelevant. Suppose that a person has a tendency to succumb to bacterial infections (say ear infections) more so than the general population. Two hundred years ago, that person might have died in childhood from a runaway bacterial infection. Now, in the vast majority of cases a quick trip to the doctor and you're cured. The person who would not have passed on their genes in previous generations now makes it to adulthood where they can reproduce. Eyeglasses are another example that is commonly given. If you could not see ten thousand years ago, well that could be a real problem. Not today where anyone in a modern society can get their eyesight corrected and survive quite well into old age. So near blindness is not a selection factor for reproduction anymore.

I think the bottom line is most of us born in a modern society can be assured of making it to adulthood if not to old age. Reproductive access is also reasonable for the vast majority of people, these days if people don't reproduce its usually by choice. So has the human species stopped evolving? I would say definitely yes.

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