Evolution and human skin color: how do Jablonski and Chaplin explain skin color variation?

Jablonski, N. G. and G. Chaplin (2002). “Skin Deep.” Scientific American October: 74-81.

I have written about Jablonski and her work on evolution of skin color and roles of Vitamin D before (here).  This article published in Scientific America in 2002 is today widely read by undergraduate anthropology students.  In this article, Jablonski and Chaplin reject that skin cancer is the major evolutionary factor for dark-colored skin in Equatorial areas.  They suggest that
Vitamin D and folic acid are two important factors of biological adaptation responding varying level of UV radiation, and cultural adaptation and recent migration were also important factors understanding the observed geographical pattern of skin color.

The people living in equatorial areas, whether they have dark colored or light colored skin, can get enough sun light to synthesize Vitamin D.  However, they have to have dark colored skin to protect them from harmful UV radiation.  Overexposure to the UV radiation can cause skin cancer, and skin cancer is deadly to the individuals, but it does not reduce reproductive success of individuals, because people usually have skin cancer after reproductive age.  Instead, overexposure to UV ration may lead to reduced reproductive success of individuals, because overexposure to UV radiation may reduce the level of folate, an essential Vitamin B, in their blood.  The low level of circulating folate is associated with higher risk of babies with spina bifida and low sperm counts.

For those people living in high latitude areas, where they can get enough sun light to synthesize Vitamin D only in limited time of the year, having light-colored skin is advantageous.  In high latitude areas, people with dark colored skin tend to have low level of circulating Vitamin D, and low level of circulating Vitamin D is associated with problems with bone development and maintenance, and immune system, etc (see here).

Probably, her major contribution is to publicize how evolution can explain skin color variation around the world and to demonstrate that the field of anthropology can help people appreciate the genetic and biological variation that exists in the world.

Persistence of racial thinking in the online communities

There are arguments on Afrocentrism, Eurocentrism, and criticisms against these ethnocentrisms in the online communities, such as blogs and YouTube.  For example, some argue whether ancient Egyptians were black Africans or not, while others question if the first Europeans looked more like modern Africans or not.  Despite anthropologists, geneticists, and educators’ efforts to eradicate racial thinking among the public, I believe that these arguments exist because of persistence of racial, or typological thinking among the human population geneticists as well as the public.

Keita and Kittles (1997) argue that racial thinking, not racist thinking, persists in the studies of human evolution through use of phylogenetic trees to show evolutionary relationship of human groups and by estimating divergence time between major racial groups(e.g. Cavalli-Sforza et al., 1994; Nei and Roychoudhury, 1993).  Weiss and Long (2009) also argue that some human geneticists have replaced ‘old racial classification’ with more sophisticated scientific methods identifying human population clusters using multilocus genetic data and Structure-like population genetics methods (e.g. Rosenberg et al., 2002; Li et al., 2008). 

One underlying assumption that racial thinking and clustering approach is based on is relative reproductive isolation because of lack of gene flow (e.g., Andreasen, 2004; Cavalli-Sforza et al., 1994; Risch et al., 2002).  People, including human geneticists, with racial thinking believe that human populations have had very limited gene flow, where there are geographic barriers and linguistic, cultural, and political differences.  They also focus on biological, genetic, linguistic, and cultural differences between different groups, while assuming that there are genetic, cultural, and linguistic similarities within a human group.

However, many anthropologists and human geneticists believe that gene flow between different human groups is common and there are great biological, genetic, cultural, and linguistic variations even within small human populations (e.g., American Association of Physical Anthropologists statement on race; Livingston, 1962; Tishkoff et al., 2009).  Therefore, the clusters that human geneticists identified should not be equated with racial groups. 

So, where did the ideas of Black Africans, Europeans, Asians…. come from?  These ideas were developed based on stereotypes of people living in different parts of the world, probably very recently, after the colonial era (American Anthropological Association statement on race).  Ancient Egyptians and the first Europeans probably did not have self-identities as Africans, Caucasians, or Europeans.  Ancient Egypt was multi-ethnic state. 

Note: This also posted on the blog section of AnthroGenetics website.

Reference

American Anthropological Association Statement on Race

American Association of Physical Anthropologists – AAPA Statement on Biological Aspects of Race

Andreasen, R. O. (2004). “The cladistic race concept: a defense.” Biology and Philosophy 19: 425-442.

Cavalli-Sforza, L. L., R. Menozzi, et al. (1994). The History and Geography of Human Genes. Princeton, NJ, Princeton University Press.

Keita, S. O. Y. and R. A. Kittles (1997). “The persistence of racial thinking and the myth of racial divergence.” American Anthropologist 99: 534-544.

Li, J. Z., D. M. Absher, et al. (2008). “Worldwide Human Relationships Inferred from Genome-Wide Patterns of Variation.” Science 319(5866): 1100-1104.

Livingstone, F. B. (1962). “On the non-existence of human races.” Current Anthropology 3: 279-281.

Nei, M. and A. K. Roychoudhury (1993). “Evolutionary relationships of human populations on a global scale.” Molecular Biology and Evolution 10(5): 927-943.

Risch, N., E. Burchard, et al. (2002). “Categorization of humans in biomedical research: genes, race and disease.” Genome Biology 3(7): comment2007.2001-2007.2012.

Rosenberg, N. A., J. K. Pritchard, et al. (2002). “Genetic Structure of Human Populations.” Science 298(5602): 2381-2385.

Tishkoff, S. A., F. A. Reed, et al. (2009). “The Genetic Structure and History of Africans and African Americans.” Science 324(5930): 1035-1044.

Weiss, K. M. and J. C. Long (2009). “Non-Darwinian estimation: My ancestors, my genes’ ancestors.”Genome Research 19(5): 703-710.

American Anthropological Association Statement on Race

Two years after AAPA’s statement of race, American Anthropological Association (AAA) also published their statement on the race.  Like AAPA statement of race, AAA statement of race follow the tradition of anthropologists and rejects the biological and genetic basis of racial classification.  There are also differences between AAPA and AAA statement of race.  While AAPA focuses on explaining the non-existence of biological race, AAA statement of race focuses on the historical, social, and cultural aspect of race.  In this post, as I did for AAPA statement of race, I will evaluate AAA race statement with new multilocus genetic data in mind.

First, it addresses that science does not support biological and genetic basis of the race.  A great amount of genetic variation exists within each racial group.  There is a great deal of overlapping of phenotypic variation, because the gene flow between different groups of humans is common.   Classification of humans based on physical characteristics is arbitrary and subjective.

Second, it reviews that historical context of how racial classifications are used and justified in the Western Societies.  Race as a way of categorizing people is developed during the colonial era and used to rationalize social and political relationship between Europeans and conquered indigenous groups and to legitimatize the socio-political power of Europeans.  Historical examples are numerous, including slavery in the U.S. and the Nazi Germany.

Finally, it stresses that today anthropologists understand that there is a great variation in human behavior, not because of genetic makeup, but because of culture, learned behavior.  

The basic argument is that concept of race is socially and culturally constructed.  However, they have to address why many genetic studies keep showing the genetic differences among human racial groups and why and how genetic and biological differences are maintained, if race is socially constructed.

American Association of Physical Anthropologists – AAPA Statement on Biological Aspects of Race

AAPA published their statement on biological aspects of race in 1996.  Following the tradition of anthropologists, such as Boas and Livingstone, the race statement rejects the biological and genetic basis of racial classification.  However, since the publication of the statement, many human genetic research projects using multilocus genetic data have shown genetic differences among human groups.  In this post, after reviewing what is in the 1996 AAPA statement, I will evaluate the AAPA race statement with new genetic data in mind.  I believe that major revision the race statement is not necessary.  There is no new genetic data that I am aware of that significantly contradicts with the statement, but the statement on biological aspects of race needs to address more thoroughly why a significant genetic differences among the human groups in molecular level are observed and genetic ancestry of individuals can be inferred, when many locus are used for analyses, but the data do not support the racial classification.

The AAPA race statement states that all human belongs to one single species and a significant amount of genetic variation is found within each population.  The genetic difference between populations and human groups is product of their unique evolutionary history and environment, and is created and maintained through genetic drift, natural selection, and socio-cultural factors.  However, the statement emphasizes on lack of genetic boundary due to frequent migration, gene flow, and admixture.

The statement address the issue of race to wide audience, including anthropologists, other scientists, students, and the general public.  Since common sense race or folk concept race is based on the phenotypic variation, it explains that there is no clear concordance between one phenotypic trait and other traits (for example, skin color and ability to digest milk) or between phenotypic/genotypic variation and socio-cultural-linguistic groups.  It also explains that distribution of many phenotypic traits can be explained by clinal changes.   

Contrary to the thorough explanation of phenotypic variation among humans, there are minimal discussions on genetic variation observed in molecular level.  The world-wide human population genetic research projects using multilocus data show genetic differences among the human racial groups (I reviewed Rosenberg et al. and Li et al. here, and Risch and his colleagues here and there).  The results are used to defend the existence of cladistic race (Andreasen, 2004) or questioning non-existence of biological race (Sesardic, 2010).  Many anthropological geneticists (e.g., Weiss and Long, 2009), on the other hand, criticize the multilocus genetic studies that show world-wide human population structure.  These authors argue that clustering or geographical/racial categories do not explain the human genetic variation, but as addressed in the AAPA race statement, they argue that cline explain the human genetic variation.

American Association of Physical Anthropologists needs to update their race statement and address some of new findings in genetic research as well as growing interests among the public, though statement is still supported by genetic data.  Since the publication of the race statement, there are many important genetic research projects showing genetic differences among human groups, including biomedical research showing differences in disease causing allele frequencies.  Also, there is increasing interests and demands among the general public for acquiring genetic information on their ancestry and disease risk.

Race: a social destruction of a biological concept

Sesardic, N. (2010). “Race: a social destruction of a biological concept.” Biology and Philosophy 25(2): 143-162.

Anthropologists, other social scientists, philosophers, and human population geneticists have argued that there is no genetic basis for racial classification, but in this article, Sesardic (2010) argues that non-genetic basis of human race arguments are not supported by the recent multilocus genetic data.  The point that he is making is not existence of human biological race, but questioning the scientific basis for the non-existence of biological race arguments.

Like Pigliucci and Kaplan, Sesardic starts out with a problem of defining race, but he mainly focus on examining how philosophers and others, who argues no genetic differences among human groups, define race to illustrate the way they define race are not supported by recent genetic data showing genetic differences among human groups. 

Sesardic argues that if frequencies of alleles on one locus are used for racial classification, individuals cannot be classified correctly into right racial categories, but if multilocus genetic data are used as demonstrated by Risch and his colleagues and Rosenberg et al (2002), many individuals can be classified into racial or geographical categories correctly.  Similarly, if forensic anthropologists look at many skeletal traits, they can accurately infer the racial identity of individuals. 

As Sesardic suggested, no genetic difference argument is not supported by many genetic and osteological studies.  However, we should avoid a naïve conclusion.  The multilocus genetic data showing genetic differences among human groups should not be used to argue the existence of human biological race (note that Sesardic is not arguing this).  We have to consider evolutionary and historical process as well as sampling and statistical effects that cause the clustering of human groups illustrating genetic differences.

The cladistic race concept: race and biological reality

Andreasen, R. O. (2000). “Race: biological reality or social construct?” Philosophy of Science 67: S653-666.

Andreasen, R. O. (2004). “The cladistic race concept: a defense.” Biology and Philosophy 19: 425-442.

In these two publications, Andreason argues that human race is biologically real and cladistic concept explains the human biological race.  Cladistic races are “ancestor-descendent sequences of breeding populations that share a common origin” (2004:430) and she views cladistic races as “geographically circumscribed breeding populations” (2004:436), or subspecies.  While there are similarities between cladistic race and common sense race (for example, shared ancestry), there are significant differences as well.  Common sense racial classification is based on similarities in physical characteristics, but two individuals who have similar physical characteristics can belong to different cladistic racial groups. 

She believes that phylogenetic trees of human populations accurately represent evolutionary relationships between different human populations and objectively categorize people into racial groups using cladistic approach.  She cites Cavalli-Sforza and Nei, and says

…many evolutionalists agree that it is possible to accurately represent human evolution as a branching pattern.  As long as this is possible it is possible to define race cladistically (2004:427).

She is a philosopher of science, but it seems that she is not familiar with more recent human population genetics literatures that discuss how the gene flow and admixture have shaped the genetic variation of many human populations.  She does not address the issues raised by anthropological geneticists, how the phylogenetic trees do not accurately represent the evolutionary relationship of human populations.

Following Cavalli-Sforza, she also assumes that human populations are relatively reproductively isolated due to geographic barriers and socio-cultural factors.

A ‘breeding population’ is a set of local populations that exchange genetic material through reproduction and are reasonably reproductively isolated from other such sets.  For example, a tribe of bushmen might constitute a local population….Separation often results from the introduction of geographic barriers; however, in the case of humans it can also be due to socio-cultural differences.

These statements clearly suggest that she has modernist, colonialist, racial/typological thinking and in her publications, she tries to defend this view.

Human genetic variation and population structure: statistical construct or real?

Rosenberg, N. A., J. K. Pritchard, et al. (2002). “Genetic Structure of Human Populations.” Science 298(5602): 2381-2385.

Rosenberg, N. A., S. Mahajan, et al. (2005). “Clines, Clusters, and the Effect of Study Design on the Inference of Human Population Structure.” PLoS Genet 1(6): e70.

Li, J. Z., D. M. Absher, et al. (2008). “Worldwide Human Relationships Inferred from Genome-Wide Patterns of Variation.” Science 319(5866): 1100-1104. (The article is also covered by Anthropology.net and Mathilda’s anthropology Blog)

To examine the how human world-wide population structure are identified with Structure-like programs using HGDP-CEPH samples, I compared the results of three studies. 

Number of individuals and genetic markers used

Rosenberg et al., 2002 (1056 individuals; 377 autosomal microsatellite loci)

Rosenberg et al., 2005 (1048 individuals; 783 autosmal microsatellite and 210 insertion/deletion)

Li et al., 2008 (938 individuals; 650,000 single nucleotide polymorphisms)

Types of Structure-like Programs

Rosenberg et al., 2002 (STRUCTURE version 1)

Rosenberg et al., 2005 (STRUCTURE version 2)

Li et al., 2008 (frappe)

Results

K=2 to K=5 show similar patterns in all three studies.  K=2 shows gradual change from Africa to the New World.  K=3 has three clusters (Africa, Europe/Middle East/South Asia, and Asia/Oceania, Americas).  K=4 separates Amerindians from the Asian cluster and K=5 separates Oceania from the Asian cluster.

Figure 1 from Rosenberg et al. 2002

Three different studies shows different pattern for K=6.  Rosenberg et al. (2002) separate Kalash (Pakistan) from European/Middle Eastern/South Asian cluster, but Li et al. (2008) have a separate cluster for South Asians.  Contrary, Rosenberg et al. (2005) have two clusters for Amerindians (North American and Latin Americans).  Moreover, Li et al. (2008) have the 7th cluster, the Middle Eastern separated from Europeans, though there are great levels of admixture among the Middle Eastern.

Figure 2 from Li et al. 2008

Interpretations

Rosenberg et al. (2002) and Li et al. (2005) say that five clusters identified correspond to major geographic groups.  They also recognize the importance of the recent gene flow and admixture.  However, all of them argue that there are clear patterns of human population clustering or population structure, because geographic boundaries and socio-cultural practices isolated populations, and the understanding of human population structure is important for disease causing gene mapping.

My comments

I do believe that there are great levels of genetic differences among the major geographic groups and the understanding of that is very important for disease causing gene mapping studies.  However, we should not use any population clusters identified in these studies as an evidence of biological basis for racial categories for many reasons (three are discussed here).  First, these authors probably have racial/typological thinking and over-emphasize the differences among human groups, without carefully considering how admixture and gene flow have played important roles.  It seems that Rosenberg et al. (2002) and (2005) arranged the populations on the STRUCTURE output to illustrate the genetic differences.  As human population geneticists, their major concerns seem to be technical aspects, not social, cultural, and humanistic aspects to understand the role of gene flow. Second, the problems of the HGDP-CEPH collections should have been mentioned, but only Li et al. (2006) very briefly mentions the limitation of the HGDP-CEPH collections.  We have to think how inclusions of more population sample, especially potentially more admixed populations, affect the results of analyses and interpretations.  Also, numbers and types of genetic markers, and types of Structure-like program used may affect the results of analyses.  The three studies discussed here have slightly different results and results are interpreted slightly differently as well.