There is considerable biological variation in between human populations across the globe, resulting in fairly variable phenotypes. Traditionally human phenotypical variation has been described as breaking down into large continental races, characterized by easily definable traits. Humans were then classified into one of four or five phenotypical groups often based on skin color, hair texture, and facial anatomy, and which were matched to a continent with which each group were associated. Often racial classification of humans was described in terms of essential characteristics, and came to serve as a way of naturalizing social and cultural stereotypes about racial groups, in turn justifying or motivating different forms of racism. As the study of human biological variation advanced it became clear that most variation is clinally distributed and blends gradually from one area to the next, with no clear boundaries between continents, additionally different traits have different clinal distributions. This realization made many anthropologists and biologists abandon the idea of major human races, instead describing biological variation in terms of populations and clinally distributed traits.
Today there is no scientific consensus on the biological relevance of race. While biological characteristics of an individual can give many clues about the geographical origin of their ancestors, anthropologists generally reject the notion of human "race" as a biological classification scheme. Instead they see it as a set of social constructions that map onto, but partly obscures, biological variation. Most anthropologists also maintain that the term "race" tacitly assumes that races are clearly bounded groups with essential characteristics, often ordered hierarchically and used to justify social inequality.[103][104][105] An opposing view has it that it is possible to talk about "races" without making essentialist or hierarchical assumptions, and some biologists and many forensic scientists use the word race to describe biological variation associated with continental ancestry. It is generally agreed upon that certain genetic traits, including some common illnesses, correlate with genetic ancestry from specific regions, and genetic ancestry as determined by racial identification is becoming an increasingly common tool for risk assessment in medicine.[106][107][108][109][110][111][112][113]
The use of the term "race" to mean something like "subspecies" among humans is obsolete; Homo sapiens has no existing subspecies (with the exception of Homo sapiens sapiens, the subspecies which includes all existing humans). In its modern scientific connotation, the term is not applicable to a species as genetically homogeneous as the human one, as stated in the declaration on race (UNESCO 1950, re-ratified 1978[114]).[115] Genetic studies have substantiated the absence of clear biological borders; thus the term "race" is rarely used in scientific terminology, either in biological anthropology and in human genetics.[116] What in the past had been defined as "races"—whites, blacks, or Asians—are now defined as "ethnic groups" or "populations", in correlation with the field (sociology, anthropology, genetics) in which they are considered.[117][118]
http://en.wikipedia.org/wiki/Homo_sapiens#Race