Race and Genetics
Dare I venture into this politically and emotionally charged issue? When researching the questions regarding our evolutionary biology as I do, ignoring the question of race as it relates to species would be negligent. On the other hand, trying to discuss it in a short blog such as this could be considered foolhardy. This sounds like a lose-lose situation. I’ll let you, the reader, be the judge.
Humans have existed for about two million years. In that time, there have been many human species. Homo sapiens emerged about 300,000 years ago and we have been the only human species still living for about 37,000 years. There is little debate today among taxonomists, evolutionary biologists and all the other “ists” that have an opinion on this subject: today all seven billion plus of us belong to one species, regardless of racial, geographic, ethnic or any other classification.
Why do we say that? There are many conflicting definitions of species—referred to in the literature as the “species problem.” There might be some room to argue that the different groups of today’s Homo sapiens that we call “races” could fit one or more of the definitions of species. Even more problematic is the definition of “subspecies”. If the different races don’t qualify as separate species, could they at least qualify as subspecies?
The answer is NO and NO.
A species consists of a group of organisms with a definable set of genetic characteristics or common gene pool that evolves independently of all other groups of organisms. A common gene pool is not a precise nucleotide-by-nucleotide definition of a set of genes. Rather, it a set of genes that perform all the same functions. There will be great variation within these genes among the members of the same species. The “evolving separately” component of the definition implies that there is some barrier to interbreeding of a species with other species such that when new genetic variants enter the gene pool, they are not intermingled with other species to a large extent. This does not mean that the barrier to interbreeding is absolute. Many species today interbreed with other species to some extent, but by and large over time, they continue to evolve independently. For example, we now know that Homo sapiens interbred in the past with at least two other human species. With today’s human mobility and facile intermixing of genes among all ethnicities and localities, clearly there is not a separately evolving subgroup among us. That is particularly true of the large groupings that we call races.
There have been at least a dozen or more human species identified in the fossil record that existed in the past 2 million years. Examples of other human species include Homo neanderthalensis, Homo erectus, and Homo heidelbergensis. Why do we consider them separate species while today’s Asians are considered the same species as today’s Caucasians? One could read books about various differences in anatomy, brain size, tool use and other features that distinguish these various human species and still come away unconvinced that these distinctions are real rather than subjective. To me, the best argument relates to the likelihood of genetic intermixing of the various groups. When those other human species evolved hundreds of thousands of years ago or even millions of years ago, the total number of individuals in any group were well under 1 million individuals and closer to 10,000 or less. As migrations in the huge continent of Africa separated these small groups physically from all other groups, they had a physical barrier to interbreeding that allowed them to evolve independently for hundreds of thousands or even millions of years.
One could imagine a similar dispersion and physical separation when the early Homo sapiens left Africa in relatively small numbers and spread out through the entire world. The same phenomenon of small numbers in each group in the huge world would allow independent evolution of those who migrated to Asia compared to those who migrated to Europe or other widely separated destinations. There are two major differences, however. First of all, it has only been about 50-100 thousand years since the Homo sapiens who are ancestors to all of us living today left Africa—not a lot of time to evolve separate species. More significantly, the people who occupy virtually all parts of the world today are not the same people who originally settled there. There have been continual waves of migration in virtually every direction, including back into Africa. In some cases, this led to the replacement of original populations, and in all cases it led to a mixing of the various migrants and their genetic pools. As David Reich states in his book, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past, “We now know that nearly every group living today is the product of repeated population mixtures that have occurred over thousands and tens of thousands of years. Mixing is in human nature, and no one population is – or could be – ‘pure.’” Thus, there are no real or pure Spaniards, Brits or Indians, just as there are no genetically definable races.
The notion of subspecies is even more vague and difficult to define. The subspecies level is sometimes equated with “races”. In taxonomy, subspecies are designated with three Latin terms rather than the two that designate a species. There is only one subspecies of Homo sapiens alive today, called Homo sapiens sapiens, and it includes all present day humans. The only other subspecies of Homo sapiens, called Homo sapiens idaltu, is assigned to an extinct group of fossils thought possibly to represent the immediate precursor to today’s modern humans.
With that admittedly superficial background, let’s consider human races. If not separate species or subspecies, is there any genetic basis for categorizing people as African, Caucasian, or any other racial designation? That is, is there any genetic basis for race? One can find virtually any opinion on this subject in the legitimate scientific literature. In a publication in the New England Journal of Medicine, Robert Schwartz states that “race is a social construct, not a scientific classification” and that race is a “pseudoscience” that is “biologically meaningless.” On the other hand, in the same journal, Neil Risch states that today’s humans cluster genetically into five continent-based grouping that are biologically and medically meaningful.
Are these two points of view really different answers to the same question about genetics and race, or are they answers to different questions? Specifically, can one state that there is no genetic basis for race and, at the same time, state that there are some genetically measurable differences between self-identified racial categories? I think the answer is yes.
Let’s take, for example, sickle cell trait, which is much more prevalent in people who consider themselves African compared to those who consider themselves Caucasian. Yet sickle cell trait exists in all races and one could not use it to define African vs. non-African people. In fact, when one looks at the genetic variation within any racial category, it exceeds the variation between racial categories. There is no genetic profile that can define any race. Are there clusters of genetic traits that have higher probabilities in one race or another? Certainly. That would be true of other classifications of humans as well such as classification by size or athleticism or musical ability. Yes, certainly those who consider themselves African have, on average, darker skin than those that consider themselves Caucasian, but the variation in skin color is great in both groups. For example, the paleogenomic profile of the earliest human fossil found in Great Britain shows that it had dark skin in a geographic area that today consists primarily of Caucasians.
This comes back to the question of species. Aren’t there great variations within species as well? Yes, but they are far less than the variations between species. That is, today’s genomic variation between the various racial groups is less than the variation between Homo sapiens and Homo neanderthalensis. All of today’s human races, no matter how you define them, are clearly Homo sapiens and not Homo neanderthalensis.
This brings me to one final point that can either further clarify or further muddy this entire discussion of race and genetics. Generally, when we talk about genetic comparisons, we have been talking about comparing classical “genes” which are the DNA sequences that code for proteins (e.g. the hemoglobin protein coded by sickle cell trait). It is only in the past decade or so that we have learned that much of the 98% of the human genome that does not code for proteins has a profound effect on our phenotype. That is the epigenome, which regulates the expression of the classical genes.
One of the things we have learned about the epigenome is that it can change during the lifetime of an individual based on environmental factors such as diet, stress, toxins and other factors. These changes do not affect the DNA sequence of genes, but they do affect the expression of those genes. More significantly, some of these epigenomic changes are passed on to offspring and can effect generations into the future.
This raises the question of environmental factors related to racial groupings and their impact on genetics. There is evidence, for example, that African-American descendants of slaves have lower birth weigh children than African-American descendants of non-slaves, perhaps related to epigenetic factors of stress and diet during slavery. One can imagine many social-cultural factors that may vary by race that could impact the epigenome. Perhaps, when we have the ability to look at the full genome variation among racial groups, our knowledge of genetics and race will change.
References:
1. R. Schwartz, “Racial Profiling in Medical Research,” New England Journal of Medicine 344 (2001): 1392
2. E. Burchard, E. Ziv, N. Coyle, et. al., The Importance of Race and Ethnic Background in Biomedical Research and Clinical Practice,” New England Journal of Medicine 348 (2003): 1170
3. K. Lotzof, Cheddar Man: Mesolithic Britain’s Blue-eyed Boy, Natural History Museum website, Feb. 7, 2018, http://bit.ly/2o1pcIG
4. M. Meloni, Race in an Epigenetic Time: Thinking Biology in the Plural, The British Journal of Sociology 68 (2017): 389
5. D. Reich, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past, New York, Pantheon Books, 2018