By the first quarter of the 20th century, “A race was no longer a real series of populations, no longer a large cluster of real human groups, but an abstraction. It became, rather, a series of qualities or characteristics that were located in individual people”(Marks).
Thus, Harvard’s Roland B. Dixon was able to say, in 1923,
“The vast majority of living men must have a complex racial ancestry, and such a thing as a pure race can hardly be expected to exist. However distinct, therefore, races may once have been, the peoples of the world t-day are complex mixtures of these original types, in which we must see to discover, if we can, the constituent elements.”
Ronald B. Dixon had his own set of characteristics by which he distinguished races:
1) Cranial index (head shape)
2) Altiduinal index (face shape)
3) Nasal index (nose shape)
Races were considered various combinations of these characteristics:
“…his Alpine type contained a particular combination fo features: brachycephaly, hypsicephaly, and leptorrhiny (round head, high head, narrow nose). He was able to locate this type of skull among the aborigines of Switzerland, Hawaii, and China, though in varying frequencies. Likewise, his proto-Australoid (long head, low head, wide nose) could be spread through Australia, Egypt and California.”
As for Hooton, he seemed to regard “race” as a kind of ideal Platonic type which received no instantiation in the real world. Indeed, he acknowledged the admixture to which all humans are subject:
“I am of the opinion that racial characteristics are better defined in the skeleton than in the soft parts…Many individuals of mixed blood, who are fundamentally white, show characteristics of skin, pigmentation, and soft parts which would lead a superficial observer to classify them as predominantly negroid. But the skull and framework of the body may show a basically non-negroid morphology. On the other hand, it is equally true that some persons who appear to be white show definite negroid or mongoloid skeletal features”
Indeed, race was a “vague physical background, usually more or less obscured or overlaid by individual variations in single subjects, and realized best in a composite picture.” So Marks:
“Thus, very few persons could really be said to represent a race within this conception – the fact of variability within real human populations simply undermined the applicability of such an idea. So race was an abstraction, identifiable to varying degrees in people, but perfectly applicable to rather few of them. And what was the nature or source of this abstract racial type? Was it supposed to be the genotype of a distant progenitor? Hooton was never clear on just what the racial type represented. Mostly he seems to have regarded it as an artifact of statistics, an ideal Platonic image.”
Genetic ancestries are radically mutable, always in a process of change and subject to the continuous selective pressures of micro-evolution:
“Human populations, therefore, diverged from one another because of two forces: natural selection (adapting them to different environments) and genetic drift (genetically differentiating them in a non-adaptive manner. What had been identified as pure races were simply the most extreme human populations. But there is no justification for equating “most extreme” with “pure” or “primeval.” The most extreme human populations are simply those which have adapted most successfully to radical environmental circumstances. Thus, Frederich Hulse, in the intellectual generation that succeeded Hooton, recast the biological aspects of race in terms of microevolution: to the extent that it had any biological meaning, race became an “evolutionary episode,” a transient package of allele combinations and frequencies molded by natural selection and genetic drift. But ultimately, race is not a fundamental biological category at all, as those working within the Linnaean paradigm had assumed. Rather, it appears that human groups can be productively analyzed as populations, but not easily accommodated within a Linnaean framework – as Linnaeus’ rival Buffon maintained two centuries ago”(Marks)
Understanding genetic ancestries in terms of micro-evolution underwent a further advance with the advent of genetics (especially molecular genetics). The field of genetics would reveal large amounts of polymorphism, or variation within populations, rather than polytypy. To be sure, human populations consist of varying degrees of both polytypic variation, or variation among human groups, and polymorphism, or variation within human groups. An especially large degree of polymorphism, as noted before, is observable in many Hispanic populations, but especially Mexican populations, to say nothing of African Americans, who tend to exhibit high degrees of admixture with Europeans.
Racial typology according to discrete abstractions was no longer acceptable with the advent of genetics. Instead of static races, human populations became highly mutable and variable clusters of alleles, which exhibited varying degrees of polytypy and polymorphism depending upon interbreeding with outsiders, genetic drift, mutation, susceptibility to selective pressures created by environment, and so on. It was in the 1960s, with the advent of molecular genetics, that the folk understanding of heredity, naively centered upon phenotypes, became increasingly focused on genotype. Indeed, it is based on the insights of molecular genetics that it becomes evident, as noted before, how ridiculous, arbitrary, unscientific and unreliable a marker skin color can be in determining the sorts of differences that really make a difference among humans (it is one’s genotype that determines such really important differences) (See the link to the article at the bottom of the page, entitled “Race, genetics and ancestry,” for examples of how unreliable phenotypes such as skin color can be as guides of genotypic difference)
To be sure, humans tend to be more genetically related to one another based on their proximity. Homogeneity tends to increase with geographical proximity and tends to decrease because of geographical distance. But these differences are continuous rather than discrete, and there is no definitive criteria by which one group may be definitively distinguished as a distinct race. Microevolutionary processes such as natural selection differentiate produce subtle differences in one group relative to a neighboring population, genetic drift produces random and non-adaptive differences and gene flow may homogenize populations, relatively speaking.
It is for these reasons that “race” may be a convenient, though non-Platonic and fallible guide in certain circumstances. For example, it is certainly true that such categories may be useful to forensic scientists. Forensic anthropologists, for example, can identify skeletal remains according to certain “racial” categories. Dividing ancestors into groups such as African, Asian, European, etc. assists us in producing non-random guesses concerning skeletal remains of humans:
“Some of the distinguishing characteristics of the skull involve the wide and projecting cheekbones of “Asians”; nasal projection of “Europeans”; and wide distance between the eye orbits of “Africans.” These characteristics overlap between groups and are quite variable within each of the three groups; but armed with a list of such average differences, anthropologists can fairly reliably allocate skulls into those three categories, or more”(Marks)
But such measurements are based on entirely fallible (though still somewhat reliable) averages. In other words, as with other differences among populations, they are continuous differences, rather than guides by which one can pigeonhole populations into Platonic racial categories.
“None of the traits is perfectly diagnostic; these are average differences, and do not imply fundamental divisions of the human species into a small number of basic homogeneous types. Other criteria are also diagnostically useful, such as the shape of the femur, which tends to be more straight in “Africans” and more bowed in “Asians.” Again, however, it is crucial to appreciate that this does not mean that there are three discrete biological categories of people. It simply means that, given three categories, skeletal remains can reliably be assigned to one or another of them. This is a consequence of two facts: human populations differ from one another, and Americans are derived generally from large groups of immigrants from geographically distinct areas”(Marks)
What does all this mean? Does it mean that all humans and genetic ancestries are essentially the same? Not at all. Just because race is an arbitrary, proto-scientific construct does not mean that genetic ancestries do not exhibit medically important biological differences. One of the most well-known examples of this is the prominence of the allele which produces sickle-cell. The allele exists because heterozygotes are protected from malaria. Likewise, Ashkenazi Jews are 10 times more likely to possess an allele responsible for the devastating Tay-Sachs disease than other genetic ancestries. These are only a couple well-known examples, though many more could be added.
Source: Marks, Jonathan. Human Biodiversity: Genes, Race and History.