The best summation I've ever heard of the most pernicious of all misconceptions concerning what evolution is and how it works is provided by eminent punk rockers NOFX:

Tell me why in hell are all the stupid people breeding?
Watson, it's really elementary....
The industrial revolution
Has flipped a bitch on evolution
The benevolent and wise being thwarted, ostracized
What a bummer
The world keeps getting dumber
Darwin's rolling over in his coffin
The fittest are surviving much less often
Now everything seems to be reversing
And it's worsening

The Idiots Are Taking Over

This notion that somehow evolution has been subverted by the progress of technology, and that now persons who are somehow "unfit" are managing to reproduce, is completely wrong. It fails on moral grounds, it fails on logical grounds, and it fails empirically as it regards the specific mechanisms of hereditary intelligence in human beings.

The idea people have in their minds is that at some point in the past "survival of the fittest" required the strongest, fastest, meanest members of early humanity be the ones who successfully managed to reproduce and to pass on their genes to the next generation. They think that the advent of technological control over the environment made it possible for the "unfit" to survive and to have children. Further, many (perhaps most) people believe that this process has accelerated over recent generations and that evolution no longer even operates on the human species.

In some sense, this is true. At some point in the past it was undoubtedly the case that being the strongest, fastest or most aggressive was one of the dominant factors affecting the selective value of individual proto-humans. It is also true that many individual human beings survive and have children today who no doubt would not have in the past. But to draw the conclusion, as so many seem to, that this means that the "fitness" of humanity is trending down over time is untenable.

This notion, which has been used to justify some of the most reprehensible eugenic theory and practice ever implemented (including the forced sterilization of hundreds of thousands of Americans, some as young as a few months old at the time of their diagnosis of "feeblemindedness", between the early 1900s and the 1960s), fails because it ignores the most important feature of evolution--the fact that the environment is not only not assumed to be static, it is assumed to be in a constant state of flux. While it is true that people who survive and reproduce today would not necessarily have successfully done so in the past, it is tautologically not the case they have failed to survive and reproduce in the present environment. That the traits that lead to reproductive success today are different from those that led to success 500,000 years ago on the savanna is no more interesting than the difference between what traits are successful today and what traits were successful in the primordial soup of the pre-Cambrian world when the atmosphere was mostly methane, the day was half as long, and the sun was much dimmer than it appears today. Evolution is concerned with this generation--it has neither memory nor foresight. It is impossible to be both a parent and evolutionary unfit. In the phrase "survival of the fittest", "fittest" means precisely "has kids", until it means "has grandkids" and then "has great-grandkids".

It is not only the notion that the unfit are somehow surviving that is broken. The idea that the industrial revolution has removed the effects of natural selection upon the human race is also unsupportable. The industrial revolution has unquestionably altered what specific selective pressures are operating on the human genome, but it has not magically removed the existence of such pressures. So long as some people breed and some do not, selection is happening. Some genes are propagating to the next generation and some are disappearing. This is evolution via natural selection.

What undoubtedly has changed is the environment in which selection is operating. Ability to successfully reproduce today is far more dependent upon having sufficient verbal fluency to convince someone of the other sex to dally with you than it is on raw physical power. The advent of modern medicine has removed the advantage that a genetic resistance to smallpox once carried, but the advent of modern manufacturing has created a significant survival benefit for people somewhat more resistant to PCB contamination.

There is yet a third misconception embodied in the quoted lyric above. It is based on the undisputed fact that the best educated among us tend to have children at a much lower rate than the least educated and that with each step up the educational ladder it becomes less and less likely that you will have children. It seems as if the most intelligent amongst us are being outbred by the least intelligent. Simple logic would seem to dictate that this means the average level of intelligence must be trending down.

This conclusion is very easy to draw, and is the most intuitive result to reach from the facts given. That it is intuitive, though, doesn't save it at all from being wrong. There are three primary problems with the idea; the particular combinations of alleles that lead to greater intelligence can be inherited from average parents, there is a longterm general upward trend in most measures of intelligence across most segments of society, and a reduction in environmentally induced damage with negative cognitive effects (though this last is at least in some respects the cause of the second).

The first point is the most damning to the idea that we are trending downwards intellectually as a species. Intelligence clearly has a genetic component. Smarter people, on average, tend to have smarter kids than dumber people. But this is only on average. Intelligence isn't determined by one allele for one gene that you either get from mom and dad or you don't. It is, instead, a combination of an enormous number of specific genes, each of which improves some small portion of mental functioning a little bit. Maybe memories are laid down a little bit better, adding a tiny intellectual advantage. Maybe spatial imaging is just a tad faster, adding its own minuscule advantage. Draw enough of these good genes in the genetic lottery and you end up being a super-genius. Draw less good alleles at each of these genes, and you're average. Draw a couple of really bad ones, especially ones that interact badly with eachother, and you end up with serious mental deficiencies.

Understanding that intelligence is going to be controlled by a host of genes, rather than by just one, lets us see both why smarter people tend to have smarter kids as well as why less intelligent people having more kids isn't going to ruin our society. Smart people are smart precisely because they have more of the good alleles than bad. This obviously means that their children have a higher likelihood of getting good alleles at those genes. But it isn't a guarantee; the genetic lottery is mostly random and not only could those smart parents end up passing along their genetic weaknesses rather than their strengths, the nature of recombination ensures that at least some of the time they are going to.

The opposite situation holds on the other side of the curve. Less smart people are still going to be carrying the better alleles at some of those genes. While most of their kids will tend to be carrying the less good alleles as well, some of their kids are going to land way over on the far right side of the distribution through sheer genetic chance. The transcendentally brilliant and the hideously cognitively damaged will both continue as they are now--the occasional result of a random world where, if you wait long enough, everything will happen. The middle 50% of the population will continue to march along, passing alleles to and fro in the endless Darwinian parade.

The second reason to not worry about a general loss of intellectual capability is that empirical data indicates that the exact opposite is happening. Not only is education spreading--literacy rates on a global scale are better than they've ever been--but long term studies of human populations indicate that most segments of humanity have been showing major and ongoing improvements in intelligence as measured by most testing methods. Israel, France, the Netherlands, and other nations with mandatory military service requirements have been conducting IQ testing on basically every male member of society for generations now. The Brits have data covering every incoming batch of soldiers since before World War I broke out. As far as the tests indicate, we're getting smarter as a species, at least in the modern Western democracies. The causes of these improvements are, at best poorly understood. We know that the next factor is a big part of it, but the effect is so pronounced that it seems unlikely to be all of it.

As mentioned, the third reason to not worry about humanity converging to the mental level of a Jerry Springer audience is probably a major cause of the steady improvement in intelligence as measured by intelligence testing performance. From conception through birth through early childhood and on into adolescence, our children's health has been steadily improving for at least a century. Mothers take better care of their bodies during pregnancy than they once did; children suffer from fewer debilitating diseases; nutrition is better (yes, really, McDonald's notwithstanding, most children in most of the world for most of time have not had enough calories per day, much less a properly balanced diet).

All of these health factors play into the general increase in intelligence. The more often and more seriously ill a child is, the worse they perform on cognitive tests for the rest of their lives. Immunizations have all but eliminated the most severe of childhood illnesses from the developed world, and progress continues to be made at improving the plight of children in the developing world. We all know the damage that a negligent mother can wreak upon a baby's mind through irresponsible behavior during pregnancy, but pro-actively good behaviors during pregnancy including more and better neo-natal care are helping even responsible women have smarter babies.. Just getting enough calories every day improves all aspects of childhood development and is bringing enormous benefits to third world nations and to the remaining pockets of deep poverty in the developed world.

On the whole, the notion that we are breeding ourselves into a somehow less fit state as a species is completely wrong. It fails on moral grounds, justifying coercive eugenic policies without offering real benefit. It fails on conceptual grounds by ignoring the fundamental rules under which evolution by natural selection operates and by misunderstanding the term "survival of the fittest". Finally, it fails on empirical grounds in regard to how intelligence is transmitted in the specific case of Homo Sapiens.

The idea has nothing to recommend it save being easy, elitist, and alarmist.