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Noahs_ArkMost 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.
519gldjJoALThis 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).

51D2RoDDkXL._SY344_BO1,204,203,200_ 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.

download (1) 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.

download (2) 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.

product_thumbnail 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.

 
• Category: Science • Tags: Genetics 
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  1. I must slightly demur with this excellent article. Species are not just semantic sugar. They are phenomena that exist in the real world. It’s just that they do not have any universal explanation or theoretical foundation…

  2. What the data from South America show is that phenotypes, typical of former cultural categorization as racial markers, will continue to appear. In some families with parents with a blended appearance, there may several “races” in a group of siblings.

  3. Very good post about the issue. I am afraid that general folks are not appropriate audience.

    Some time I wonder how to dumb down for average people to comprehend this.

  4. Razib,

    Imagine a population in Mexico that is on average 80% Spanish and 20% Mayan, and appears phenotypically “white”.

    If the average Spanish man is 180cm and the average Mayan man is 160cm, would we assume that their average height would be something like 176cm?

    And with other complex polygenic traits, would it be fair to assume that this population would fall on the Spanish-Mayan spectrum close to the Spanish norm, but a bit skewed towards the Mayan value?

  5. Imagine a population in Mexico that is on average 80% Spanish and 20% Mayan, and appears phenotypically “white”.

    this is not that far from the average in argentina actually.

    And with other complex polygenic traits, would it be fair to assume that this population would fall on the Spanish-Mayan spectrum close to the Spanish norm, but a bit skewed towards the Mayan value?

    that’s a good null assumption. might be some issues with genetic background confounding, but i bet it would be tail effects only.

  6. 3 points

    1) Realizing the opaqueness of my analogy I pointed out that genes are digital and changed it to grains of sand as opposed to particles of paint.
    2) The extinct Fuegians of the South American Cone are no longer with us or the Tasmanian tribes. Both have surviving descendents due to intermixing with other populations but these genes will not reassemble into the original population through random chance.
    3) Due to the esoteric nature of your writting I am sometimes at a loss of what the exact point is you are trying to make but I took your post to mean that “We should not worry about the loss of certain populations in humanity (like the Fuegians) or the Dingo regarding non human animals because these populations are not extinct so long as their genes exist in current populations.”

    I suppose I will see Nehanderthals walking the streets one day due to random sorting of genes then. I may not have an expected Phd in Feline Genetics but my laymen’s understanding of the world does not allow me to invision this. My background in human anatomy leads me to believe that (at least at a skeletal level) populations do not reappear in their original undiluted form (to allude to that ignorant analogy again).. I know because the fossil record tells me so.

    Respectfully

    Scythian

  7. I should add I only added the example of Northern Euros since you cited the disapearing blonde gene hoax. I am not a Northern Euro by ancestory and am making no ideological points other than that the conservationist movement is not all bad and it beats having freeways everywhere.

    Also your opinion of Teddy Roosevelt seems a bit off base he was the first US president to have African American dinner guests to the outcry of many racialists of his day.

    To cite an older post.

  8. scythian, you still have no idea what i was trying to say. don’t comment again.

  9. Anonymous • Disclaimer says:

    @Scythian
    I guess notions like “populations do not reappear in their original undiluted form” is part of the bloggers prime frustration here. In evolutionary time- there is no original, nor an undiluted form; this form exists only as an aesthetic construct of human thought.
    If you wish to make ethical arguments for the preservation of species, evolution will grant you none. That is, unless you route it through arguments such as intrinsic value (which would apply to -what you call- species by proxy of individual organisms), or if you link interconnectedness of biological systems and grand-scale disturbance of this, to detrimental effects on the welfare of (psychological) beings.

    Also look up esoteric in your dictionary; the above post is hardly it.

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