Most biological concepts exhibit striking clarity and intuitive accessibility at the highest levels, but engender confusion when you drill down to the details. You can see this in an understanding of evolution. Most people can grasp the idea of common descent with modification relatively easily. But when it comes to getting an good intuitive grasp of evolutionary process, and what that might entail, people are often left grasping at straws. Consider the reality that many people believe that evolution works by benefiting the species, even though the mainstream position within the discipline is that operates through variation in individual fitness. What is likely happening is that our cognitive intuitions are slipping into our understanding of science. We imagine evolutionary process as changing an entire species, rather than the mass action of individuals within the species.

This is clear when you look at the research on “folk taxonomy.” Humans have an idea of what a species is, and it often corresponds relatively closely to the biological species concept. Though higher and lower taxonomic scales such as genus and subspecies reflect genuine information about the structure of reality, species is considered special by many in that it is a clear and distinct level of organization where groups are of organisms are neatly encapsulated from other groups of organisms. It’s “real.” Here intuition and folk taxonomy align with our understanding of biology. The problem is that species is neither so general, nor so neat and airtight even in cases where one might think it applies. First, the biological species concept obviously makes sense only in the context of sexual organisms. Asexual organisms are living things too., and important ones at that. There are after all an order of magnitude more bacterial cells in our body than somatic cells. Second, even many sexual organisms (e.g., plants) engage in hybridization quite regularly. Species are useful semantic sugar, but they have no atomic Platonic reality. They are not one of the fundamental units of organization of the universe around us, but rather a term which maps upon a dynamic which is of great interest to humans (i.e., the fission of complex eukaryotic sexual organisms into distinct populations over time).

The reality that for most biologists species are viewed in instrumental terms is not always clear to the broader public. The constant war of words with Creationists which revolves around speciation and macroevolution is actually somewhat beside the point, because the very idea of species is not coherent or consistent, despite its genuine utility. And isn’t just Creationists who have drafted our cognitive Platonism toward their ends. Disputes over the Endangered Species Act get bogged down in minutiae as to what a species is, and what sort of diversity is worth preserving (or not). That’s what happens when you take a scientific concept which is not airtight on the margins and introduce activists, lawyers, and various assorted interests with no compunction about using casuistical arguments in the service of their preferred ends.

The same general confusion seems to crop up when you move below the level of species. In my post below, Fear of Race Mixing in Biodiversity, I make a pass at the fallacy of the blending theory of inheritance, which was overturned by a little known field which goes by the name of Mendelian genetics. Nevertheless in the comments the blending theory of inheritance pops right back up, even though I dismissed it in the post itself:

What about the modern Auroch? It has been reconstructed through selective breeding though no geneticist would consider it the same as an ancient Auroch. It may look similiar but it is no more Auroch from a genetic standpoint than a spanish fighting bull. Once a breeding population is lost in its isolation and genetic uniqueness. It can not be reassembled any more than two cans of paint can be unmixed. Even if the original genes still exist in the mixed population. Your viewpoint also assumes that all phenotypes have the same reproductive and behavioral tendencies. Which does not seem to be the case for nothern euros in comparison with more tropicaly evolved peoples. So unless blondeness or ruffocity convey some selective advantage over other phenotypes they will dissapear in time, especially the platinum or ash blonde varieties.

First, let me say that I really don’t understand about half of this comment, and told my interlocutor exactly that. But the portion relating to the analogy of mixing with paint is obviously leveraging the intuition about the blending theory of inheritance, and it turns out to just be false (also, a rule of thumb might be to not engage in analogies with a geneticist about genetics; just get to the point in plain language). In the early 20th century Mendelian genetics, which traces patterns of inheritance of traits across generations and sieves it through a particular model, turned out be exactly what was needed to allow for the persistence of the variation which is the raw material of evolution. The necessity and maintenance of this variation was a paradox which confronted Charles Darwin in the 19th century, and he never quite resolved it. Basically, if offspring are the blended mix of their parents, then each generation should be progressively more blended and uniform. That uniformity removes the variation which is necessary for adaptation through natural selection. Attempts to maintain variation through processes such as high mutational rates were simply not plausible. The genius of Mendelian genetics was that inheritance was transformed into a process mediated by discrete particulate units. Genes. In sexual organisms the patterns of inheritance are governed by the law of segregation and the law of independent assortment.

When I teach undergraduates basic genetics and these laws I always tell them to conceptualize them physically, because we know now that genes are actually embodied in strings of base pairs. Mendelian genetics, and the abstract understanding of the genome in its essence, long predate the discovery of DNA. But through a grasp of both the abstraction of genetic inheritance, as well as its concrete manifestation in the sequence of DNA, one can infer the dynamic of microevolutionary process. Because of segregation there is variation in the ancestry inherited from the grandparents. Because of recombination the law of independent assortment holds beyond a certain genetic distance even on the same chromosome. Evolution can be thought of now not as simply phenotypic change, but rather as fluctuations in allele frequencies over generations, unto extinction and fixation. Finally, the nature of quantitative traits due to polygenetic architectures becomes much more transparent if one simply imagines the combined effect of numerous genes producing a final outcome.

The above explains why racial admixture of modern populations will not lead to uniformity and homogeneity. Consider the case of skin color, where variation in ~10 genes accounts for ~90 percent of the inter-continental variation in complexion. Populations where many of these genes are segregating, such as in Brazil or India, are not of uniform coloration, but manifest the full range of ancestral complexions. On average the complexion lies at the midpoint, but the underlying allelic variation remains. In fact, because admixed populations exhibit combinations of multi-locus genotypes not found in the ancestral populations they’re likely to be more diverse overall than the total variation in the summed ancestral groups (e.g., which group has more phenotypic diversity, Spaniards and Amerindians separately, or mestizos?). From the standpoint of anti-racists who may hope for a post-racial world where amalgamation leads to the abolition of race, that will not happen. First, the data from Latin America is clear that phenotypic race remains even after genetic admixture because individuals vary a great deal in appearance. Second, most modern races themselves are almost certainly the product of admixture events over the past ~10,000 years. Racial categorization can be useful, and reflects real history, but it is not a fundamental unit of special genetic structure. Races are neither primal nor Platonic. But given rather conventional conditions they seem to emerge out of folk taxonomies. They’re an evoked part of human culture.

Following the comments of my interlocutor I don’t really believe the issue at heart was scientific. It’s patently false that once genotypes are scrambled they can’t be unscrambled. The law of segregation and the law of independent assortment offer up exactly the manner in which you can reassemble ancestral types from admixed populations. The root concern derives not from biology at all, but psychology, and an intersection between folk taxonomy and ideas of Platonic essences and contagion. Once the category of the Northern European phenotype is sullied in some way, it is lost forever. Rather than conceiving of Northern Europeans as a biophysical expression of discrete alleles, which are a very recent event in history due to admixture from diverse lineages and in situ evolution, the implicit cognitive assumption is that they have a particular unitary racial spirit, and that their racial category is fundamental and primordial. These might seem to be antiquated early 20th century concerns, but though the sentiment is sublimated I’m pretty sure it’s still very common, because that’s the only reason I can think that phenomena such as the “disappearing blonde gene hoax” often go viral so quickly.

When conceptualizing genetic and evolutionary processes one can frame it as a spectrum, from gradual change on polygenic characters to the emergence of single gene traits. The human default frame seems to veer erratically between the two. On the one hand blending inheritance theory implicitly underpins many intuitions about the nature of how characters change over time. Yet there is also a craving for a specific and singular concrete gene for a given condition. The latter likely emerges from the reality that DNA and modern methods, starting with linkage analysis, zero in on very specific positions in the genome and are localized to a particular gene for many Mendelian diseases. What gets lost is the Mendelian framework, which is a conceptual model which can integrate both the quantitative genetic characters and the monocausal traits and diseases conditioned upon a very specific change in one region of DNA.