Editing Race (human categorization) (section)

==== Clines ====
One crucial innovation in reconceptualizing genotypic and phenotypic variation was the anthropologist [[C. Loring Brace]]'s observation that such variations, insofar as they are affected by [[natural selection]], slow migration, or [[genetic drift]], are distributed along geographic gradations or [[Cline (biology)|clines]].{{sfn|Brace|Montagu|1965|p={{page needed|date=October 2021}}}} For example, with respect to skin color in Europe and Africa, Brace writes:{{sfn|Brace|2000|p=301}}
{{blockquote|To this day, skin color grades by imperceptible means from Europe southward around the eastern end of the Mediterranean and up the Nile into Africa. From one end of this range to the other, there is no hint of a skin color boundary, and yet the spectrum runs from the lightest in the world at the northern edge to as dark as it is possible for humans to be at the equator.}}
In part, this is due to [[isolation by distance]]. This point called attention to a problem common to phenotype-based descriptions of races (for example, those based on hair texture and skin color): they ignore a host of other similarities and differences (for example, blood type) that do not correlate highly with the markers for race. Thus, anthropologist Frank Livingstone's conclusion was that, since clines cross racial boundaries, "there are no races, only clines".<ref name="Livingstone" />

In a response to Livingstone, [[Theodore Dobzhansky]] argued that when talking about race one must be attentive to how the term is being used: "I agree with Dr. Livingstone that if races have to be 'discrete units', then there are no races, and if 'race' is used as an 'explanation' of the human variability, rather than vice versa, then the explanation is invalid." He further argued that one could use the term race if one distinguished between "race differences" and "the race concept". The former refers to any distinction in gene frequencies between populations; the latter is "a matter of judgment". He further observed that even when there is clinal variation: "Race differences are objectively ascertainable biological phenomena&nbsp;... but it does not follow that racially distinct populations must be given racial (or subspecific) labels."<ref name="Livingstone" /> In short, Livingstone and Dobzhansky agree that there are genetic differences among human beings; they also agree that the use of the race concept to classify people, and how the race concept is used, is a matter of social convention. They differ on whether the race concept remains a meaningful and useful social convention.

{{Multiple image
| align =
| direction = vertical
| width = 280
| image1 = Unlabeled Renatto Luschan Skin color map.png
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| image2 = Map of blood group b.gif
| caption2 = Skin color (above) and blood type B (below) are nonconcordant traits since their geographical distribution is not similar.
}}

In 1964, the biologists Paul Ehrlich and Holm pointed out cases where two or more clines are distributed discordantly&nbsp;– for example, melanin is distributed in a decreasing pattern from the equator north and south; frequencies for the haplotype for [[HBB#Sickle cell disease|beta-S hemoglobin]], on the other hand, radiate out of specific geographical points in Africa.<ref name="ehrlich" /> As the anthropologists Leonard Lieberman and Fatimah Linda Jackson observed, "Discordant patterns of heterogeneity falsify any description of a population as if it were genotypically or even phenotypically homogeneous".<ref name="Lieberman 1995" />

Patterns such as those seen in human physical and genetic variation as described above, have led to the consequence that the number and geographic location of any described races is highly dependent on the importance attributed to, and quantity of, the traits considered. A skin-lightening mutation, estimated to have occurred 20,000 to 50,000 years ago, partially accounts for the appearance of light skin in people who migrated out of Africa northward into what is now Europe. East Asians owe their relatively light skin to different mutations.<ref name="weiss" /> On the other hand, the greater the number of traits (or [[allele]]s) considered, the more subdivisions of humanity are detected, since traits and gene frequencies do not always correspond to the same geographical location. Or as {{harvtxt|Ossorio|Duster|2005}} put it:{{blockquote|1=Anthropologists long ago discovered that humans' physical traits vary gradually, with groups that are close geographic neighbors being more similar than groups that are geographically separated. This pattern of variation, known as clinal variation, is also observed for many alleles that vary from one human group to another. Another observation is that traits or alleles that vary from one group to another do not vary at the same rate. This pattern is referred to as nonconcordant variation. Because the variation of physical traits is clinal and nonconcordant, anthropologists of the late 19th and early 20th centuries discovered that the more traits and the more human groups they measured, the fewer discrete differences they observed among races and the more categories they had to create to classify human beings. The number of races observed expanded to the 1930s and 1950s, and eventually anthropologists concluded that there were no discrete races.<ref name="Marks 2002" /> Twentieth and 21st century biomedical researchers have discovered this same feature when evaluating human variation at the level of alleles and allele frequencies. Nature has not created four or five distinct, nonoverlapping genetic groups of people.}}