Some thinkers believe that differences between human cognitive function, such as intelligence, are due solely to environmental differences. This would be called the Blank Slate model of the human intelect. According to this view, the brain is an empty vessel ready to be molded by “nurture”. Any variance between individuals would be solely due to their individual experiences and not to any inherent differences between how those brains were constructed or how their day-to-day workings are influenced by genetic factors that vary within the human population.
Other thinkers believe that the genome is filled to the brim with functional units that are exclusively shaped by natural selection. This would be called the panadaptationalist, or Swiss Watch model. Just like a Swiss Watch, the genome is filled with complex gears that have all been exquisitely shaped by natural selection. Adherents of this model implicitly argue that the brain is a product of these countless genetic switches and functional units. They believe that there is no junk DNA, and that neutral evolution is irrelevant, especially when it comes to phenotypic differences.
Although I do not know this for a fact, I believe that many thinkers, including many life sciences researchers, hold both of these beliefs. What these individuals fail to recognize is that these two views are mutually incompatible.
If the genome was mostly functional, what are we to make of the great genetic diversity in the human population? If we believe that this diversity has no influence on intellect, then this would mean that most of this diversity is essentially neutral when pertaining to brain function. What are we to make of the great diversity in appearance in humans? Apparently, genetic diversity affects how any individual looks, but not the functioning of their brain. This is a very dangerous argument. Why would the brain be so special? It is apparently highly complex and dependent on a multitude of finely tuned genetic features, but at the same time immune to genetic differences? Given that the genome is constantly absorbing a barrage of mutations every generation, the more genetic payload a trait has, the more likely it is going to vary. This is because it is a greater target for mutagenesis. When functional parts of the genome are hit, and these affect brain function, well the great chance that brain function will be impacted - this is called mutational decay. So to summarize, either the brain is going to depend on quite a bit of genetic payload that is subject to mutational decay, and thus vary quite a bit in the population because of the onslaught of mutations we absorb every generation, or it does not depend much on genetic factors and is not subject to much negative or positive selection.
Some others may argue that the brain is a very plastic organ that can adapt to the environment - it is literally* a learning machine, and is thus subject to environmental factors. So maybe it is plastic enough to cope with small genetic changes and buffer their effects? This brings up the thorny issue of extremely small phenotypic changes. The Swiss Watch model of the genome would predict that any minute genetic change that tweaks our intellect would be under a tremendous amount of selection, both positive and negative. This would have to be true for even the smallest of changes. If, on the other hand, you claim that the brain function is not significantly affected by genetic changes, you have just decreased the power of natural selection to make a difference. After all, with the Blank Slate model, every brain in every individual has the same potential. If that would be true, then the smallest of differences in any trait would essentially be neutral, and no tweaking or fine tuning by selection would be possible.
So what are the alternatives?
1) If you chose to be a strict panadaptationalist (i.e. an adherent to the Swiss Watch model of the human genome), then you should abandon the Blank Slate model. I don’t think that you have a choice.
2) If you choose to be a strict Blank Slate adherent, then you have to admit a fair degree of neutral evolution. But to what extent do these neutral changes affect phenotype?
This position also has another problem. It would then beg the question of how the human intellect could evolve in the first place. Humans and chimps are noticeably different in ways that no one would think are compatible with the Blank Slate model. No amount of environmental change will ever make a the intellect of a chimp the same as that of a human, and most thinkers would agree that these differences are largely due to genetic divergence between the two lineages. If we accept this, then it becomes hard to justify the Blank Slate. You would have to believe that the human lineage currently lacks any genetic diversity that can shape how the mind works. Given how much genetic diversity there is in humans, this becomes a stretch. You may counter argue that humans have “stopped evolving” (although what you are really saying is that humans are no longer undergoing selective evolution). But is this reasonable? When did selection disappear? Was it incredibly strong in the past and then slowly vanished? Is it currently true that there are no intellectual traits that tend to increase or decrease reproductive success? (Remember, selection acts on reproductive success, not some arbitrary idea of progress.) Even if you did believe that in our modern world there is less selection, then this would mean that humans are under less adaptive selection and more importantly less purging selection. This would automatically imply that a greater number of slightly deleterious mutations are now effectively neutral! With a decrease in purging selection, then more and more severe mutations that have stronger impacts on intellect should be able to spread in the human population. These differences should exist if the power of selection were to be diminished. Yes, I’m arguing that the Blank Slate model is incompatible with neutral theory, although maybe less so than with the Swiss Watch model.
3) Abandon both views. What does this mean? The brain is subject to a certain degree of selection - there are some genetic mutations under positive selection, and other genetic alterations under negative selection. At the same time lots of evolution in the genome is due to genetic drift - the random fixation of nearly neutral mutations that are invisible to selection. Some of these neutral changes have no influence on phenotype, while others change the phenotype in ways that do not significantly affect “fitness”. Brains do vary between individuals. Some brains will be inherently better at certain tasks than others. Some of this variance affects reproductive success, and some does not. In other words, this variance is at times under selection, and at other times evolves in a random manner due to drift.
One last point. And this is very important, but poorly understood. The opposite of the Swiss Watch model is not anarchy. Neutralists do not believe that all genetic and phenotypic evolution is neutral. The same should be said for the Blank Slate model. The mind is undoubtedly plastic to a certain degree. We would all agree that not much evolution has occurred in the last 150 years, but transport yourself back to 1872, and I’m sure that you would be able to see that the typical person from back then is quite different from your modern day human. Increases in sanitation, medicine, standards of living, access to education can all deeply and profoundly affect how an individual’s intellect develops. Nevertheless, there are differences between any two people you may meet in the world today. Moreover, we have diverged from chimps, and our evolution (be it adaptive or via nearly neutral drift) is still ongoing. That is to say that our genomes are in flux - they are subject to a barrage of mutations, many altering how the brain works, for good, bad or in a neutral way.
The Swiss Watch and Blank Slate models are both extreme views. They are extreme in that they deny that any other alternatives co-exist. The Swiss Watch model denies the existence of any neutral evolution. The Blank Slate denies the existence of any inherent differences between intelects. To deny both of these two extremist views is to be open to a more pluralistic and nuanced stance on these issues. Neutral changes can happen. Some differences in the brain function between individuals do exist. Ultimately extremist views are based on an oversimplification of how the world works, and often times these views are held in the service of some other ulterior motive. Our motive should be a greater understanding of how the world is, not a justification of what we want the world to be.
Finally there is the other thorny issues of “racial” differences and eugenics. Everything that I’ve written applies to differences between any two individuals, regardless of “racial” backgrounds (whether “races” exist or not), geographic distribution, or any other concerns of that type. As for eugenics, this is all tied up in judgements about redirecting the course of evolution (favoring certain traits over others - in other words, so called artificial selection). I take no position in this post, for or against, artificial selection. In my mind, this is not a scientific question, but a political one. Personally, I am not in favor of eugenics, but this is not based on some ill-informed notion of biology (the Blank Slate). In fact, those who fiercely defend the Blank Slate, precisely because they oppose eugenics, are probably undermining their own position. What if it turns out that the Blank Slate is wrong? It would be best that they develop a new position that is not based on such a weak foundation.
*”literally” - I’m using the ancient version of this adverb, not the vacuous modern version.