The number of races, or groups of people with a common ancestry, in the world varies according to the models of classification that anthropologists use. These include typological, population and clinal models of classification.
Anthropologists in the 1800s to the early 1900s made a mistake of dividing people according to geographical groups basing their categories on what they viewed as the routine occurrence of selected traits. They were using the typological model of racial classification that can be traced back to the 18th century when Swedish naturalist Carolus Linnaeus proposed that humans belong to four biological subspecies. These four biological subspecies are basically white people from Europe, black-skinned people from the African continent, dark-skinned people from the Asiatic continent and natives from the Americas who are usually classified as having red skin.
The Typological Model
The system for this classification relies mainly on human traits that are apparent and can be easily seen even from afar, such as overall skin pigmentation, hair type and other physical attributes. The typological model for human classification was highly dependent on a mistaken notion of human variation and the idea that humans can definitively fall into a particular "race" on the basis of anatomical traits alone. However, science has consistently proven that this assumption is highly flawed since recent DNA sequences show that more genetic variation exists within a particular "race" than between other races. Despite this fact being only one of the problems posed by the typological model, it is still widely used today such as in the United States where the official definitions for racial and ethnic categories are simply white, black or African American, Hispanic or Latino, Asian, American Indian or Alaska native and Hawaiian or Pacific Islander.
The Population Model
Biological anthropologists recognized the problems posed by the typological model by the 1940s and thus opted for the population model of racial classification as an alternative. This model is based on the idea that groups of human populations should consist only of people whose ancestors have mated exclusively within their population for thousands of years. In contrast to the typological model that defines physical traits and then identifies geographical populations that exhibit them, the population model observes a particular breeding population and then determines the traits that set them apart.
While the population approach to racial classification may have some merit, it faces problems of its own in view of the fact that humans have long interbred with other populations. By the 1960s, biological anthropologists agree that the clinal model more accurately represents the variation of human biology. Clinal model of classification recognizes that genetically inherited traits gradually change in frequency from one geographical location to another relative to the distance between human populations. The clinal patterns, however, are still inconsistent as physical and biological traits are discontinuous and are not exclusive to a particular cline.
A Better Classification System
Since most models for racial classification fail at some point, what is the best way to describe human variation? One easy way would be to categorize human variation according to the eight geographical genetic groups to which all human populations belong. These are African, Caucasoid, Northeast Asian, Arctic, American, Southeast Asian, Pacific Island and Oceanian.