Is Homo sapiens polytypic? Human taxonomic diversity and its implications
Introduction
Historically, the term race has been used in biology as a synonym for subspecies [1], [2]. Whereas the term subspecies was typically used in the description of infra-specific diversity in non-human animal species, the term ‘race’ tends to be employed exclusively in the description of diversity present within the human species. Despite this it is frequently asserted that humans are monotypic (belonging to one species and one subspecies – Homo sapiens sapiens), and that ‘racial’ diversity is either a socially constructed biological illusion or that it exists only at infra-subspecific scales and is therefore taxonomically trivial. In this manuscript a case will be made for the hypothesis that H. sapiens is in fact polytypic and that this has significant implications for fields such as anthropology and medicine.
Four major definitions of what constitutes a subspecies or race have been identified by Long and Kittles [3].
Table 1 illustrates the evolution of classificatory concepts of race from essentialist to lineage based. Although in each case the idea of ‘distinctness’ is invoked as a necessary criterion for the existence of a race there exists considerable disagreement over how to define that distinctness. The essentialist concept of Hooton places the emphasis on the existence of combinations of characteristics shared through common descent, whereas the taxonomic concept uses a combination of phenotypic similarity coupled with the idea of range restriction. The population concept of Dobzhansky on the other hand talks of race exclusively in terms of Mendelian populations whilst the lineage concept of Templeton requires races to have been subject to historical barriers to gene flow whilst simultaneously exhibiting contemporary genetic differentiation.
The table would seem to suggest that there is no universally agreed upon definition of race or subspecies and that the use of any particular race concept in the apportionment of human biological diversity is to a degree arbitrary. This situation has not been helped by inconsistent historical usage in the anthropological literature, where the term would frequently be used in the description of human populations at a variety of scales ranging from sub-continental to global [7].
This suggestion of arbitrariness has led many social scientists to claim that what is termed ‘race’ is in fact nothing more than a ‘social-construct’, devoid of any biological foundation. According to this view, which is known generally as social constructivism, the concept of racial classification is a recent invention (c. 18th century) and was developed as a means of grouping subjugated colonial peoples on the basis of arbitrary physical characteristics. By this logic the very notion of race therefore has inherently racist connotations as, it is inferred, the decision to use concepts of race in the ‘arbitrary’ grouping of humans is suggestive of a desire to delineate an out-group that is some way ‘inferior’ in contradistinction to a ‘superior’ in-group to which, it is presumed, the classifier would belong [8], [9]. As evidence of the pervasiveness of the view that races do not exist within the social sciences, a 1985 survey of 1200 academics who were asked whether they disagreed with the statement: “There are biological races in the species H. sapiens”, revealed that only 16% of biologists disagreed as compared to 53% of socio-cultural anthropologists [10]. The likelihood is that an even higher percentage of social scientists would disagree today. As evidence of this, one only needs to read the official position statements on race and ethnicity of major organizations such as the American Anthropological Association and the American Sociological Association.
The problem with social constructivism is that it attempts to engage racial classification on a normative rather than a scientific level. Using the idea that scientific race concepts stem from a desire to apportion people into ‘inferior’ vs. ‘superior’ categories as grounds for claiming that they are wrong is simply an appeal to motive and therefore is not a logical counter to scientific theories of race, which must be assessed purely on their merits. The notion of arbitrariness in the definition of race is a significant and legitimate scientific issue in need of redress however.
Prior to examining the race concept from a classificatory stand point it is necessary to demonstrate its validity as a biological construct independently of classificatory schemes. It was mentioned previously that all four of the major race concepts require races to be in some way distinct from one another, however it is frequently asserted that because the majority of genetic variation (85%) lies within the classically defined racial groups rather than between them (some estimates indicate that the number is as low as 6%), race is therefore a taxonomically meaningless category. Lewontin, who is the most influential promoter of this hypothesis, essentially assumed that because there is a 30% probability of misclassifying an individual’s race based on the variation in a single genetic locus, race must therefore be taxonomically invalid [11]. Lewontin’s claim was essentially a formalization of the old argument that human populations are too clinal (they share too much variance) to be clearly differentiable into races [12], [13]. Edwards has however countered these arguments with the observation that although Lewontin and others are correct when talking about a single locus or trait, concluding from this that race does not exist is fallacious as the likelihood of not being able to differentiate between racial groups rapidly approaches 0% as more loci or traits are considered. This is due to the fact that loci/trait frequencies within racial groups tend to be correlated [14].
Based on Fig. 1, race and synonymous concepts can be defined as populations expressing a composite number of traits whose distributions intercorrelate in such a way so as to give rise to a particular, distinct correlative structure. This basic definition allows for a potential reconciliation of the four major attempts at defining race listed in Table 1. Hooton’s essentialist definition, which requires the sharing of characteristics through common descent is clearly compatible with the observation that race is a correlation structure of traits, as is Mayr’s taxonomic definition, which sees races as phenotypically similar groups occupying different ranges. Eco-geographical distinctions between races would be to a degree congruent with respect to genetic and phenotypic traits, so would be expected to yield correlation structures similar to Fig. 1.
There is no reason why such correlation structures could not correspond to Mendelian populations as is required by the population definition of Dobzhansky, nor is there any reason why the distinct correlation structures could not have been subject to historical restrictions in gene flow, as is required by the lineage definition of Templeton. These last two would in point of fact be a prerequisite for the evolution of racial differences in the first instance. The four major race concepts can therefore be united within a common descriptive framework, the differences between them are purely a matter of where the descriptive emphasis is placed.
Section snippets
Races as biological subspecies
Demonstrating the biological construct validity of race does not necessarily address the issue of classification. Although it has been shown that the four major attempts at defining race differ only in terms of qualitative descriptive emphasis, the problem of taxonomic arbitrariness in terms of how diversity within species is classified is still an issue.
An old morphological method for determining the appropriateness of a subspecies classification is the 75% rule, which holds that if 75% of the
Are there multiple extant human species?
A minority of anthropologists in the past have held the view that human racial morphological differences are great enough in some instances to warrant being considered as species level differences [47]; however these views were often based upon the use of scientifically inappropriate morphological comparisons with extant primates (such as degree of prognathism). In this section, the two major definitions of species will be considered in addressing this question.
Overview of findings
There are strong grounds for suggesting that the hypothesis that H. sapiens is polytypic rather than monotypic is at least plausible: this argument is based upon the following lines of reasoning. Firstly, it has been demonstrated that there exists a considerable degree of diversity (as measured by morphology, heterozygosity and FST) within this taxon, which is structured in such a way that is suggestive of the existence of around five major clades (continental populations) corresponding to
Conflicts of interest statement
None declared.
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