How can we accurately classify people in a manner that corresponds to real biological differences rather than culturally defined stereotypes?  The answer to this question is not simple.  There are three basic ways in which anthropologists have tried to do it in the past.  These are generally referred to as the typological, populational, and clinal models. 

Typological Model

During the 19th and early 20th centuries, anthropologists naively divided all people into geographical groups on the basis of what they saw as the regular occurrence together of selected traits.  They were using the typological model which has the same basis as our folk categories of race.  This approach focuses on a small number of traits that are readily observable from a distance such as skin color, hair form, body build, and stature.

The typological model is based on what is now known to be a false assumption concerning the nature of human variation--that is, that we can be unambiguously assigned to a "race" on the basis of selected traits.  In fact, when we look at specific individuals, we often run into difficulty trying to categorize them.  For example, on the basis of skin color, we might put them into one "race" and on the basis of nose shape, body form, or blood type, they might go into others.

Another major problem with the typological model is that the number of "races" you end up with depends on the number and kinds of traits employed in the classification.  The more traits used, the fewer people in the world there are who share them.  For example, light skin color is considered to be a defining characteristic of Europeans.  However, when you add the criteria of narrow noses, straight hair, and tall stature, many Europeans would be excluded altogether or the European racial category would have to be further subdivided into several smaller "races."  Since the number of "races" can be so easily changed by the way they are defined, it is clear that they do not really exist as distinct biological groupings of people.  Instead, they are arbitrary creations that reflect our ethnocentric views of ourselves and other people.

Populational Model

By the 1940's, anthropologists recognized these problems and adopted the populational model as an alternative.  This is based on the idea that the only significant groups, in evolutionary terms, consist of people whose ancestors have more or less exclusively mated with each other over long periods of time.  Individuals in such distinct breeding populations would be expected to share many genetically inherited traits and to have a similar appearance.

This approach to understanding the patterns of human biological diversity differs radically from the typological model.  The latter starts by defining traits that presumably characterize a "race" and then looks around the world to see who has them.  In contrast, the populational model looks for breeding populations first and then considers the anatomical and physiological traits that may distinguish them.

While the populational approach makes theoretical sense, it is undermined by the fact that humans rarely mate within a single group for long.  Physical anthropologists have found only a few moderately distinct breeding populations because cultural and geographic barriers to intergroup mating most often break down over time.  As a result, the populational model is of little help in understanding most of human variation.  However, it is of value in studying the few relatively isolated communities that still do exist.

Clinal Model

Since the 1950's, physical anthropologists have gathered sufficient data to understand that a clinal model more accurately reflects the true nature of human biological variation.  This model is based on the fact that genetically inherited traits most often gradually change in frequency from one geographic area to another.  For instance, the allele for type B blood generally increases from west to east in Europe.  We can record different frequency zones, or clines, as shown in the map below.   Unlike the typological and populational models, the clinal model does not result in the definition of distinct groups or races of people.

Clinal distribution of the B blood allele in Europe

Gradual changes in gene frequency from one region to another are mostly due to the simple fact that the chance of our mating with someone is usually directly related to the distance they live from us.  People whose ancestors have lived close to ours for many generations are more likely to share genetically inherited traits with us than are people who live further away.  However, as long distance transportation systems have become more accessible and dependable over the last 1½ centuries, the distance from home that we travel and potentially find mates has increased.  Despite this change, most people still usually end up marrying others who live within a few hundred miles of their home.

Unfortunately, the pattern of human variation around the world cannot be entirely understood by the clinal model alone.  The distribution of some traits is partly discontinuous.  The example of red hair in England (described in the first section of the tutorial) is not unique.  There are other traits that have non-clinal distribution patterns.  These also can be understood as results of historical migrations or exclusive breeding within more or less closed communities.  For example, the map below shows that the frequency of people who have the B blood allele generally increases from southeast and northeast Asia to central Asia.   Within this more or less continuous cline, there are isolated pockets of relatively low B allele frequency.   Therefore, the distribution of this genetically inherited trait appears to be mostly clinal but, in part, it is also discontinuous .

Clinal and discontinuous distribution of the B blood allele in Asia

How then can human variation be best described?  It is clear that all of the models fail to adequately carry out the entire job.  The typological model is the most unsound because presumed racial traits are not found exclusively within defined races.  In addition, focussing on new sets of traits often results in assigning people to different races, despite the fact that they were lumped into the same race before.  The populational model makes sense theoretically but fails to account for most of the distribution patterns around the world because we do not limit our breeding to isolated populations.  The clinal model comes the closest to grasping the real nature of human variation.  However, it is undermined by the occasional discontinuous distribution resulting from migrations and the few remaining small isolated communities.

The patterns of human variation around the world are not only highly complex but also are constantly shifting through time.  Furthermore, the rate of change has been accelerating as our numbers grow and as long distance travel and migration become more routine.  In the final analysis, it is important to understand that despite our superficial differences, all humans around the world are biologically quite similar.  When we are compared to many other kinds of animals, it is remarkable how little variation exists within our own species.  Most physical anthropologists would agree that this human variation is not now sufficient to warrant defining separate biological races, varieties, or sub-species.

In order to better understand the true patterns of human variation, physical anthropologists have gathered detailed data about genetically inherited traits.  Most of this work has been done with blood typing.  The next section of this tutorial presents some of this evidence.  In the near future, DNA sequence comparisons will provide an even more detailed understanding of our human biological diversity.

Information provided by: http://anthro.palomar.edu