Even among those who accept the overwhelming amount of evidence in favor of the idea of evolution by natural selection, as laid out by Darwin in "Origin of Species" and elsewhere, there is often some hesitance at applying the idea to humans (since they became humans, anyway). Surely one reason for this widespread reluctance is the way in which, in the late 19th and early 20th centuries, evolution was used as a theoretical justification for class or race bigotry. In theory, the consequences of a theory should not sway our judgement as to the weight of the evidence in favor of it. In practice, it does.
But, after a post-WWII "breather" of half a century or so, science has in recent years begun to edge back towards investigation of whether or not evolution has wrought detectable changes in our own species. One of the first areas in which we have widespread consensus that it exists, is in the area of lactose tolerance. Drinking milk as an adult is, essentially, a neutral trait in terms of social structure or power. Unlike IQ, or even physical appearance, we don't think less or more of a person based on their ability to digest milk as an adult, and it cuts conveniently across racial and class lines (we all probably have at least one friend who can, and one friend who cannot). This made it a safe topic for putting our toe back in the water of this radical idea.
It is helped by the fact that the data are so conveniently distributed. If the raising of cattle were only a long-established practice in one region of the world, then the fact that people there can usually drink milk as adults would be suggestive but not convincing, but there are smaller regions in Africa and elsewhere that have a long history of raising cattle, and adult milk drinking is long-established there as well.
What makes this most interesting is that cattle farming is only 8-10,000 years old, making lactose tolerance only 400-500 generations old. In northern Europe, lactose tolerance is by far the dominant trait (compared to intolerance). This means that a factor not directly related to self-defense or procreation (i.e. not directly related to survival) can achieve dominance in a relatively short amount of time (presumably because it helps to buffer against malnourishment during hard times). Given that we can be pretty sure widespread lactose tolerance existed in northern Europe around the beginning of recorded history, it was probably even quicker than that to achieve widespread dominance.
This raises the question: what else might be happening in human evolution? In particular, what might be happening right now, perhaps only recently begun, that will change what the human species looks like?
One response might be: who cares? I won't live to see it. And yet, I think we might; in fact, I think we maybe already have.
Until about fifty years ago, there was a fairly strictly enforced limit on the size of a baby's head at birth. There were some obvious benefits to having a bigger head (bigger potential brain, hence bigger potential ability to think about complex problems), but if it gets too big at birth for the artfully termed "birth canal", it will kill both baby and mother. This was a quite draconian and unforgiving pruner of the human gene pool. We pushed the mean of the human population far up the scale in newborn head size, until the benefits of a bigger brain (in finding mates, keeping other humans away from same, avoiding predators, etc.) were more or less counterbalanced by the risks of childbirth. It is no accident that humanity has (in its pre-technological state) a very high rate of death in birth relative to other mammals (even primates).
In the last half-century or so, however, this upper restraint has been removed. Birth by c-section has become routine, and even prior to that the invention of forceps allowed for big-headed babies to be born alive in cases where they never would have before. Thus, some families have had several generations of genetic drift towards bigger heads, and thus the potential for bigger brains and bigger intellects. It is often lamented that it is not always the brightest who choose to have big families, but it should be remembered that until relatively recently, newborn brain size below a certain size was a requirement even to live; mothers who conceived children with brain sizes too large not only lost that child but often their own lives as well, making it a very powerful restraint on human evolution. For several generations now, this restraint has been lifted.
Another case, nearly the opposite, is that of human choice in reproduction. The ability of humans to decide whether or not they wish to reproduce, and if so how many times, is a relatively recent occurrence. One key point is that this has not resulted in a more or less random reduction in birthrates. Rather, it has impacted precisely those who are either so low-resource as to be unable to support children (who might in the past have had children and then given them up for adoption instead), and those who, while capable of supporting children, decide not to.
Whatever you may think of either group, this much is clear: they are not precisely identical to the rest of the population, genetically. I will not speculate as to exactly what personality characteristics might cause a person to be more or less willing to raise children, because I don't know of any research to quantify it, but I am confident that there is a difference, on average, between those who do and those who do not. When we add to this the fact that those who wish to have children now (if they are not low income) have access to medical help in having children, we find that there has in the last half century been a great shift away from physical and towards mental characteristics as a determinant of reproductive success. That is, whether or not you have children is no longer determined by biological fertility, but by whether or not you wish to have them.
If this is so, then it is worth a moment to consider what else these groups might differ in besides a willingness to have babies. For example, would their politics be different, on average? If we selected at random 100 individuals who were left-wing in their political views and in their late teens, and 100 individuals who were right-wing and in their late teens, and then checked again in ten years how many children each had, do you think the averages would be the same? There have been a number of twin studies (e.g. comparing genetically identical to fraternal twins) that conclude that our political views are, if not determined, then at least substantially influenced by our genetics. If people of a certain part of the political spectrum are willing to have more children than others, and political views are even partially influenced by genetics, we could expect a steady drift over time in the direction of the part of the political spectrum more likely to reproduce.
Which brings us to the biggest Elephant In The Room, religion. It is no secret that many world religions advocate having babies (typically only as part of a married couple). Moreover, many religious communities provide support (even to unmarried women, it should be added) in the form of daycare, school, and less formally but perhaps most importantly access to a community of other parents who are willing to provide guidance and emotional support in all of the aspects of how to be a parent. Non-faith communities rarely provide this level of support (it is the author's good fortune to have an exceptional group of friends in this regard), in large part because of an emphasis among non-faith subcultures on individual choices and individualistic lifestyles. If a person believes (perhaps justifiably) that having children will cut them off from contact with their peers, and that they will have little or no support or guidance from their peers in the many challenges of parenting, it is not surprising if they put off or forbear entirely the decision to have children. On the other hand, if they believe that having children will result in an avalanche of support and encouragement, it is not surprising if they decide to have children more early and more often.
Studies in the last 20 years or so, however, add something to this. It appears that religiosity itself is at least partially genetic. To be clear, the precise religion which a person has is (as far as is known) entirely cultural, but whether or not one chooses to believe and actively participate in religious activities is strongly correlated to genetic background (twin studies again form part of the research basis for asserting this).
Until about 50-100 years ago, a person was roughly equally likely to have children whether or not they wanted to, and equally likely to go to church and take part in religious observances whether or not they believed in the religion's central tenets. It was more or less like language; cultural, but so everpresent as to be de facto required.
The ongoing affect of the Enlightenment, however, in the forms of liberal political institutions and technological progress, has been to provide evolution with a new arena in which to discriminate between one set of genes and another. To be precise, we can expect that the genes which have become more common in the last century or so are those that favor:
In addition, we can expect a wider distribution in the genes which impact the following:
It is difficult to predict with any degree of certainty what the impact on society will be, for all of these changes taken together. It would suggest a more conservative and religious one than we have currently, but also one with an even greater disparity between the classes in terms of tendency and ability to plan ahead, especially to the extent that these traits influence income potential along with propensity to reproduce.
Specifically, if society comes to have a wider distribution in the ability to plan ahead, and also in brain size, it will be likely to split into at least two distinct gene pools. People with large brain size tend, on average, to be attractive to, and attracted to, other people with large brain size. People who plan ahead well and people who do not plan ahead well I would consider more of an open question. It seems plausible to me that high-forethought people may be drawn to more spontaneous, low-forethought people, and vice versa. However, most actual research on the matter finds that for virtually every trait studied, opposites do not attract, and on average people are drawn to others who are more similar to themselves.
Another factor here is that people are able to draw from a much wider geographic pool for potential mates, due to the many internet-enabled methods of finding one. Until recently, people chose a mate from among a few dozen potential partners in their hunter-gather group or peasant village; having the ability to discriminate on a basis other than gender and age is a recent phenomenon.
Available evidence is that they will typically choose people as similar to themselves as possible. This being so, we can expect that high-IQ, attractive, and/or high-forethought people will be unwilling to settle for mates who are low-IQ, unattractive, or low-forethought. So, if humanity (in advanced economies such as the U.S., anyway) splits itself into distinct reproductive populations, what are the results? How many generations would it take before interbreeding between groups becomes either very rare or nonexistent? It took several hundred generations for lactose tolerance to spread, but that did not directly impact on fertility, and only impacted survival in those cases where there was a shortage of food, but milk was available (surely an unusual case). The changes we have been considering here directly impact on mate selection and fertility; they will have a correspondingly rapid effect on the human gene pool, and we can expect it to split into more than one group. What would we call such groups?
If we were honest, we would call them "classes", or perhaps "castes". Most human societies throughout history have had them, and it is precisely the highly individualistic, liberal orientation personality type (so opposed to class biases) that is more or less refusing to reproduce currently, thus removing one of the only obstacles to the re-emergence of a caste system. Already, the U.S. has a more pronounced and established class system than it is willing to admit, and youngsters who choose to mate across class boundaries will find that their families are not exactly supportive. A few more generations, plus a great reduction in the relative numbers of those who oppose class biases most strongly, and the frowns will turn into prohibitions.
This is all food for thought, but now we come to the final and most radical factor impacting human evolution in advanced economies such as the U.S. In the last decade or two, it has become possible for high-income families to make choices about the genetics of their child. Even in low-income societies such as India and China, it has for years been evident that selection based on the child's gender is widespread. Any society which values sons above daughters to the point of using modern medical technology to alter the ratio of boys to girls, is choosing to reduce its own population growth rate (because the latter is strongly correlated to the population of adult women, not men). This process is self-limiting, though. The predictable result (already apparent in places like South Korea) is that daughters, because more rare, become more valued. After one or two generations, things should be back to normal, with a 1:1 ratio of boy and girl births, and all that will have changed is perhaps a greater cultural regard for girls.
It is also clear that certain genetic diseases, once susceptible to in utero testing, will be removed entirely from the high-income gene pool (and once the test is available cheaply enough, perhaps from the low-income gene pool as well). This raises the question of how far below the average something needs to be to be considered a genetic defect. Is an IQ of 50 a genetic defect? Most would say yes (albeit perhaps uncomfortably and with much use of euphemism). What about an IQ of 90? Add to this the fact that IQ is not just influenced (though not entirely determined) by genetics, but is also almost entirely a relative measure. What was an IQ of 100 thirty years ago is an IQ of 90 now (the so called Flynn effect, which in the U.S. is about 3 points increase in average IQ per decade). It is not difficult to believe that, as genetic testing becomes ever more powerful, high-income parents will select for IQ that is at least NOT in the bottom quartile of their peers, while low-income parents will have no such option. Add to this the fact that all manner of genetic defects have (typically large) income affects, either because they limit what job options are available or simply cost a lot of money to treat. A low-income status will be not just inherited (as it has been for most of history), but also genetic.
In those rare cases where a low-caste child is born with high IQ and no obvious genetic abnormalities, they will have a decent chance of making the leap up to a higher caste (taking their favorable genes with them). This is especially true if they are physically attractive. In those (even rarer) cases where a high-caste child is born with low IQ and/or an obvious genetic defect, and somehow is not screened prior to birth, they will often either not reproduce at all or have to settle for a lower-caste mate. The separation between groups seems inevitable to get larger over time.
In the past, this has been a self-limiting process, because once a caste system becomes well established, mating between castes is effectively prohibited. Once that happens, high-caste children with unfavorable traits still achieve reproductive success, as part of an alliance between powerful families. Low-caste children with especially favorable genetics remain low-caste, and the removal of favored genes from the lower castes ends. In addition, lower-caste children with serious genetic problems are more likely to be simply unable to survive, relative to higher-caste children of the same kind. Carried forward enough generations, with the addition of high-caste males taking advantage of their status and the vulnerable status of low-caste females, the difference between the genetic favorability of different castes will greatly reduce over time.
Looking forward, though, we can see the possibility of great differences in the near future. The higher-income caste will be able to continually "improve" its genetics. The lower-income caste might gain access to these capabilities eventually, but not if they are prohibited by law and only available to those who can travel overseas to the few places which allow it, in which case we can imagine that it may remain the exclusive province of the higher-income castes. This is exactly the sort of hypocritical half-measure that our society excels at.
So, the picture we are looking at is, to a person of egalitarian disposition, fairly horrifying. It would appear that our future is currently headed towards a more conservative, and less egalitarian society. It will also be more religious, which I consider not necessarily a problem (it depends on the kind of religion), but certainly history does not indicate this to be an obstacle to building a caste society. The only bright spot would appear to be that there may be an increase in the number of people we would call geniuses, but even this is tempered by the fact that there may be an increase in the spread of intellectual abilities (more geniuses, but also more mentally deficient), with a strong class bias. What's to be done?
Thus far, I don't know. Perhaps one of the geniuses to come can figure out a solution better than I can.
It has been pointed out that brain size is not the only thing which once was pruned ruthlessly, and lately has been unaffected by selection. Many other genetic diseases which once would have resulted in an early death (prior to having children), are now (owing to medical advances in the last century) survivable. Even if the person ultimately dies from a genetic condition, if they live well past adulthood they may pass on the genetic basis for their disease (or a vulnerability to it).
It's a good point, and we can add to it the many genetic factors which contribute to lower fertility. If we have medical help with infertility, plus a variety of birth-control methods that are sometimes used to reduce the birthrate of those without infertility problems, we have a dramatic reduction in the penalty for genes that lead to low fertility.
Taken together, these add to the phenomenon mentioned above in which the genetic distribution will become wider. Taken far enough, we might imagine a society in which having children the natural way is very difficult (because natural fertility has declined), and having children then becomes even more a result of mental and societal (income) factors rather than physical ones.
The other thing pointed out to me was that there is some evidence that parents may not WANT to have children smarter, better looking, etc. than themselves. I could believe this might be true for some, but I think (without anything other than anecdotal evidence, I admit) that they will be more concerned about having an imbecile than having a genius, and will be pruning the genetic distribution more at the bottom end than the top. It does bring up, though, whether or not there may be a problem with parents trying to have children with characteristics undesirable for society. We see a foreshadowing of this in the nations which have problems with too many boys and not enough girls.
One response to this may be that parents become judged by their children's characteristics even more than today. If you have a child with a physical abnormality, a very low IQ, or physically unattractive, people may look at the parents with contempt rather than sympathy, thinking that they chose to have children like this in order to more easily control them. This also could vary from one society to another, so we could see the direction of evolution diverge in different societies, depending on the culture.
[click here to return to 1] There is a good survey of the data on this here, although a good search engine can show you many others.
[click here to return to 2] For example, see here.
[click here to return to 3] For example, see here. Note that when searching for papers on this topic, it is important to distinguish between "religion" (almost entirely cultural) and "religiosity" (large genetic factor), with the difference being that the latter is about how often one takes part in religious ritual, attends a religious service, etc.
[click here to return to 4] For example, see here.