The End of Natural Evolution
Every so often I'll read some post about how human beings are still evolving as a species. Please stop this. I'd hate to be the one to break the news, but we've stopped evolving, at least in the strict darwinist sense, and we have for a while.
It's there in Darwin's autobiography, but natural selection was a theory inspired by economics, specifically, Malthus' observation that famine and poverty were due to to overproduction, or in Malthus' case, the overproduction of children. The argument was that more children were produced than the natural resources could support, and so there was bound to be a dying-off of excess children, either because they starved, or else they would live in such abject condition, that they would live short miserable lives.
This intellectual construct, adapted for plants and animals, beats at the heart of natural selection. A bunch of animals produce more young than will survive to adulthood. It is assumed that the new generation will have a certain variety of random differences. If it's a harsh environment then many of these animals will die. The ones that survive will do so because their individual differences helped them eat more or run faster or hide better. And thus new traits are sculpted from the last generation, thus laying down a glacial animation of morphological changes in the fossil record.
One sign that natural selection happening is that the population stays relatively steady over generations, or worse, declines. In the past, for pre-industrial human society, most death came in the form of child mortality, and in paleolithic times, it's been estimated that up to 25% of the population died from low-level warfare. The world population only increased slowly until the explosion that happened with the industrial evolution. Since then, global human population has been growing exponentially. Much of it is due to great sanitation, good sewage, and a general decrease in low-level warfare, except for blood-feuds in Scicily.
We are no longer seeing the great dying off of the young and infirm that drives natural selection. Pretty much any joker with intact reproductive organs gets to procreate. And thus, there is no particular pressure for the human population to evolve in any particular direction. This has resulted in the end of natural selection for humans.
Still, one of the assumptions of natural selections is that in any new generation there will be some kind of random variations. This is the natural genetic drift that alway happens, no matter what. By itself, it is not natural selection, just the precondition for natural selection. But without a great dying off, there won't be the kind of gross systematic changes that leads to natural selection.
Nevertheless, the puzzle remains that, if you separate the human population between the developed and undeveloped world, the growth rate in developed world has totally plateaued off. It looks like human beings in the developed world have managed population control without the mechanism of natural selection, which asks for having lots of children and then killing most of them off. Most adults in the developed world will have children, just not quite the 2.1 needed for maintaining the population. Humans have learnt to control population without killing our young, thus subverting the primary mechanism of natural selection.