Evolution And History

Studying Human Biological Diversity 289

environment. Clinal analysis of a continuous trait such as

body shape, which is controlled by a series of genes, al-

lows anthropologists to interpret human global variation

in body build as an adaptation to climate.

Generally, people long native to regions with cold cli-

mates tend to have greater body bulk (not to be equated

with fat) relative to their extremities (arms and legs) than do

people native to regions with hot climates, who tend to be

relatively long and slender. Interestingly, these differences

show up as early as the time of Homo erectus, as described

in Chapter 8. A person with larger body bulk and relatively

shorter extremities may suffer more from summer heat

than someone whose extremities are relatively long and

whose body is slender. But they do conserve needed body

heat under cold conditions. A bulky body tends to conserve

more heat than a less bulky one, because it has less surface

area relative to volume. In hot, open country, by contrast,

people benefit from a long slender body that can get rid of

excess heat quickly. A small slender body can also promote

heat loss due to a high surface area-to-volume ratio.

In addition to very long-term effects that climate

may have imposed on human variation, climate can also

genes become more common in subsequent generations.
Similarly, the polymorphism of the human species has
allowed us to thrive in a wide variety of environments.
When polymorphisms of a species are distributed into
geographically dispersed populations, biologists describe
this species as polytypic (“many types”); that is, genetic
variability is unevenly distributed among populations. For
example, in the distribution of the polymorphism for blood
type (four distinct phenotypic groups: A, B, O, or AB), the
human species is polytypic. The frequency of the O allele is
highest in American Indians, especially among some popu-
lations native to South America; the highest frequencies of
the allele for type A blood tend to be found among certain
European populations (although the highest frequency is
found among the Blackfoot Indians of the northern Plains
in North America); the highest frequencies of the B allele are
found in some Asian populations (Figure 12.1). Even though
single traits may be grouped within specific geographic
regions, when a greater number of traits are considered,
specific human “types” cannot be identified. Instead each of
these traits is independently subject to evolutionary forces.
As mentioned above, today anthropologists study
biological diversity in terms of clines, or the continuous
gradation over space in the form or frequency of a trait.
As mentioned in Chapter 2, the spatial distribution or
cline for the sickle-cell allele allowed anthropologists to
identify the adaptive function of this gene in a malarial

polytypic Describing the expression of genetic variants in dif-

ferent frequencies in different populations of a species.

Eskimos

Blackfoot

Indians

Chippewa

Indians Whites

(Michigan)

Blacks

(New York)

Bushmen

BO

A

World distribution of the A, B, O

gene frequency (selected studies)

Ghanans

Nigerians

Nilotes

Basques Swiss

English

Russians

Chinese

Japanese

Bengalis

Thai

Papuas SolomonIslanders

Cape York

Aborigines

Australian

Aborigines

Icelanders

Saami

Navaho

Indians

Maya

Indians

Pemon

Indians

Makiritare

Indians

Xavante

Indians

Mapuche

Indians

Aleuts

Figure 12.1 Frequencies of the three alleles for the A, B, and O blood groups for selected
populations around the world illustrate the polytypic nature of Homo sapiens. The frequency of
the alleles differs among populations.