Here’s the latest instance, Future Humans Will All Look Brazilian, Researcher Says:
The recourse to a blending analogy is unfortunate. Genetics is not a blending process, it is a discrete one, which reconfigures variation every generation. The underlying variation in the form of alleles is maintained, even if the genotype frequencies shift. This insight is implied in the article with talk about recessive phenotypes and nods to Hardy-Weinberg equilibrium. One of the key problems with Charles Darwin’s original theory of evolutionary process is that it did not account for how heritable variation could be maintained. If that variation melted away every generation through blending processes then the world would rapid equilibrate toward homogenization. Roughly half the variation would disappear per generation in an exponential decay process.
Meanwhile, many other physical traits will simply blend together. “Most of the traits that we think of as distinguishing different groups (hair colour, skin colour, hair curliness, facial features, eye shape) are controlled by multiple genes, so they don’t follow a simple dominant/recessive pattern,” McDonald explained. “In those cases, blending will make people look more similar over time.”
And yet variation remains! Though the phenotypes, the traits, may exhibit blending between parents, the underlying genetic variation is governed by Mendelian dynamics. This is why in populations where alleles for traits like pigmentation segregate in a polymorphic fashion, such as in India, it is not uncommon for complexion to vary within families. Though on the population wide scale there is some tendency toward clustering about the mean, variance remains within a random mating group at equilibrium.
Another major issue is that these discussions too often focus on single traits. When evaluated across loci the variation and range in possibilities due to admixture in fact results in greater diversity than is possible today. Mendel’s law of independent assortment implies that traits and variation will not be co-inherited. Before international travel and migration the possibility of someone with blue eyes, an epicanthic fold, and tightly curled hair, was a theoretical affair. Today there are almost certainly people who exhibit all these traits.
By coincidence these people are likely to be Brazilian, as this is a nation where there are large populations of African, Japanese, and Northern European (German) ancestry. And the example of Brazil itself illustrates empirically why homogenization will not proceed in the manner which intuition tells us. Brazil may be modally a brown nation, but its physical types run the gamut, expressing the underlying genetic variation. Among populations such as the Uygurs, who are fusion of eastern and western streams, individuals arise who reflect in near totality the physical types of only one of their ancestral populations, even if most individuals exhibit configurations in equipoise.
And so it was, and so it will be. The reality is that today is not the age of amalgamation, that age has passed. The most recent work in human genomics actually brings us to the conclusion that in fact most of the “pure” populations we see around us today are fusions of deeply diverged human evolutionary threads. The ancestors of Europeans in the Pleistocene were as differentiated as modern continental races (i.e., Fst on the order of 0.05 to 0.15 depending on the pairwise comparison). The same is true of South Asians, and most other groups you can think of. The “Great Mixing” after the retreat of the ice and collision of peoples may explain why there is so little evidence for hybrid inviability today in cross-racial pairings; it may have been purged from the genomes of modern groups through selection during that period.
The admixture of this age will be but a shadow of the past. The reality is that for centuries into the future huge numbers of people will persist who we might recognize as European, African, and East Asian, in totality of their form and genetic heritage. The amalgamation of the early Holocene probably occurred through the fusion of groups in the early stages of demographic expansion. They were tribal affairs, parochial in their scope, born out of desperation and chaos, even if the consequences were continental in their implications. The clans of yore became the mothers of nations, but those nations are mature and endless in their number now. The existence of Brazil as we understand it is exceptional, the product of racial slavery on a massive scale during a time of tumult. It is the exception, rather than the norm.In the next few decades international elites will no doubt enter into a period of intermarriage as old barriers fall, and new commonalities of class transcend ethnicity. But for the majority of the citizenry of the old nations such considerations will be theoretical. The initial period of synthesis and cross-fertilization will give way to stasis as all those open to the new possibilities of finding mates across old racial categories will have done so. Those who remain, the majority, will be more conservative in their preferences and tastes. The Holocene ushered in races which are extant across the world today through admixture; the anthropocene will usher in the post-national international race of global elite. Rather than twining a few threads of the human lineage, this new population will twist all the threads together in a radical new conformation. And it will be anything but homogeneous and uniform in its expression!