Biology of Race
The biological definition of race is a geographically isolated breeding pop­ulation that shares certain characteristics in higher frequencies than other populations of that species, but has not become reproductively isolated from other populations of the same species. (A population is a group of organ­isms that inhabit the same region and interbreed.) Human racial groups compose a number of breeding units that in the past remained geographi­cally and perhaps temporally isolated, yet could interbreed and produce vi­able offspring within the species Homo sapiens sapiens. Paleoanthropological evidence suggests that these units have been interbreeding between popu­lations for at least the last two hundred thousand years or longer in what may once have been considered racial groups.
More recently, molecular techniques have developed to examine genetic differences between individuals and populations, including karyotypes pro­viding chromosomal number and patterns, deoxyribonucleic acid (DNA) hy­bridization, protein sequences, and nuclear and mitochondrial base sequences from ancient and modern DNA. From all this evidence, it is clear that populational, but not racial, differences do exist within the human species. Race should not be equated with ethnicity, which has a sociologi­cal meaning. Ethnicity is a self-described category that has three components—ancestry, language, and culture—that all have affinities to certain ancestral groups.
Early racial classification systems for humans used specific pheno­typic characteristics that occurred in higher frequencies in certain pop­ulations. Initially, three classes were identified by anthropologists: Caucasoids, Mongoloids, and Negroids; later, Australoids and Capoids (Bushmen) were added. Following this, even more classifications were made, with no consensus among biological anthropologists. Difficulties with these early classification systems stem from the immense genotypic and phenotypic human variation found in modern living populations. While the genotypic variation was not studied in great detail in the early part of the twentieth century, phenotypic variation in skin color, body height, hair type, nasal width, and other characteristics was studied in great detail.
Some genetic differences do exists between groups, but these by and large do not correspond to historical racial categories. For instance, there are populational differences in the frequency of ABO blood types. Native North and South Americans have an incidence of nearly 100 percent type O (less than 1 percent have type AB), while Asians have a lower incidence of O (60 percent) and higher incidence of type B (22 percent). Some char­acteristics, such as skin color and body height, are considered to be poly­genic traits. Skin color has a clinal distribution, with indigenous peoples with darker skin colors found in native peoples at the equator and lighter skin colors found in natives from higher latitudes.

The very large amount of variation within groups dwarfs the small differences between groups; therefore race in humans does not have a biological meaning.
Skin color is an adaptation to sunlight that provides protection from skin cancer, yet at the same time allows for vitamin D production for cal­cium absorption. Darker skin provides more protection, while lighter skin allows more penetration of the weaker sun in temperate regions. While body height is also considered a polygenic trait, it is very much affected by in­heritance, as well as environmental stressors (such as malnutrition and in­fectious disease).
Some differences between populations may correlate with historical ex­posure to different infectious diseases. For example, certain genetic variants of hemoglobin (for example, those causing sickle-cell anemia in people of African descent and thalassemia in people of Mediterranean descent) were strongly selected because they provide defensive mechanisms against infec­tion by the organism that causes malaria (Plasmodium). Such environmental selection pressures have caused more than three hundred variants of the he­moglobin molecule. Cystic fibrosis (CF), a disorder of a gene that produces a protein that forms a chloride pump in cell membranes, allows for the buildup of mucus in the respiratory tract, thereby leading to death from pathogenic invasion. Yet the heterozygous condition for CF protects against extreme dehydration due to cholera. Tay-Sachs disease, a disorder of an enzyme that breaks down a molecule in the myelin sheath of nerve fibers, is found more commonly in people of eastern European Jewish de­scent than in other populations. Whether the Tay-Sachs gene protects against an infectious disease is unknown, though some have made a con­nection to tuberculosis exposure.
The molecular techniques outlined above now allow anthropologists to study the migration patterns of ancient peoples. Genetic diversity has resulted from the extensive hybridization that has occurred in the last two hundred thousand years, hiding any clear evidence for typological classi­fication of race. Moreover, when selection pressures (temperature, alti­tude) are coupled with phenotypic variation, phenotypic expression defies taxonomic assignment of race. The genetic diversity within any historically defined race swamps the small amount of difference between such groups, making the boundaries of these categories entirely arbitrary. Therefore, race in humans does not have a biological meaning.
References
Brues, A. M. People and Races. New York: Macmillan Publishing Company, 1977.
Dobzhansky, T. Genetics and the Origin of Species. New York: Columbia University Press, 1951.
Jackson, F. L. “Race and Ethnicity as Biological Constructs.” Ethnicity & Disease 2, no. 2 (Spring 1992): 120-125.
“Sickle Cell, Thalassemia, Tay Sacs, Cystic Fibrosis.” HealthlinkUSA. http:// www.healthlinkusa.com/.