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http://anthro.palomar.edu/vary
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Overview
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
Similar appearance of children
characteristics which we share with others of our
from the same region of Asia
gender. People who are closely related to each
due to a shared gene pool
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.
Papua New Guinean
Clinal distribution of hair color among Australian Aborigines
Discontinuous distribution of red hair in Britain
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.