Séquence 3: Critères d'appartenance à la lignée humaine

Overview

	Similar appearance of children  from the same region of Asia  due to a shared gene pool

With the exception of monozygotic twins, every one of us is genetically different from every other human who ever lived.  Each of us is unique in terms of the combination of tens of thousands of genetically determined characteristics that we possess.  However, we clearly have some traits in common with other people.  Most of us have readily identifiable male or female sexual characteristics which we share with others of our gender.  People who are closely related to each other usually have even greater similarity in appearance because much of their genetic makeup is shared.  Unrelated people whose ancestors came from the same part of the world often are generally similar in terms of such body features as skin and hair color, facial characteristics, body shape, and stature.  Not surprisingly, these traits have a strong genetic component as well.  However, they can be affected by environmental influences.  For instance, skin color often can be darkened, or tanned, seasonally by prolonged exposure to ultraviolet radiation from the sun.  Likewise, stature can be affected by nutrition.  When young children do not receive sufficient calories in their diet, especially protein, their growth is likely to be stunted--they will not reach their full genetically programmed height.

Humans like to classify and use identity labels for people and things with which we come in contact.  It satisfies our apparent need for a sense of order.  In addition to gender and age, most of us readily classify each other into categories on the basis of what we consider to be races.  In North America, people usually think in terms of Black, White, Asian, Hispanic or Latino, and Indian or Native American.  These are all archaic concepts of physical types that have little biological reality.  Academics may use more sophisticated sounding terms for these perceived biological groupings, such as Negroid , Caucasoid or Caucasian , and Mongoloid .   Nevertheless, they are still bad science.  However, they are important to contemporary life in North America because they reflect culturally defined differences in our society.  They are essentially labels of ethnicity that are used for categorizing and discriminating.

We now know that clearly distinct human races do not now exist.  This does not mean that our species is lacking anatomical and physiological variation between populations.  Rather, the true nature of that variation is far more complex.  It does not correspond to commonly believed simple racial lines.

The physical traits that we think of as clustering together among particular peoples often have much broader distributions.  They continue well outside of the geographic areas in which a "race" is stereotypically supposed to exist.  For instance, dark brown skin is usually thought of as the key trait in distinguishing sub-Saharan Africans from people elsewhere in the world.   However, dark brown skin is also found in southern Asia, Australia, New Guinea and on the nearby islands of Melanesia, as well as in much of the Americas.

(Data for native populations collected by R. Biasutti prior to 1940.)

The non-African peoples with dark brown skin color (like the man in the photo from New Guinea) do not share a close common ancestry with Africans.  Their skin coloration is largely due to natural selection rather than recent shared descent.  The environmental factors that led to dark brown skin among Africans apparently led to it elsewhere as well.

Genetically inherited traits often have a clinal distribution.  That is to say there is a continuous, progressive gradation moving from one geographic region to another.  The frequency of yellow-brown hair among Australian Aborigines illustrates this trend (as shown by the map on the left below).  This trait generally becomes more common with distance from the coast.  Such patterns can result when selective pressures differ from one region to another and when people mate mostly with their immediate neighbors.  Selective pressures favoring or discriminating against a trait may come from several sources.  There may be natural selection resulting from environmental constraints.  At the same time, there also may be patterns of culturally defined discriminatory mate selection that vary from region to region.

Sometimes, the distribution of genetically inherited traits does not follow a pattern of gradual change from one geographic region to another but has a discontinuous distribution.  The frequency of red hair in Britain illustrates this sort of pattern.  Note in the map on the right above that there are several relatively isolated pockets where there is a high frequency of people with red hair.  Such a pattern can result when groups of people migrate into a new area or when there are closed breeding groups that select mates based on such a trait.