Clines are gradations in biological features over geographic space. They refer to continuous degrees of difference in either phenotype or genotype across or within human populations. A given cline consists of the gradient in frequency of a single trait over space. This graded change is often associated with a gradually changing environmental factor. Thus, gradients in the appearance or function of a trait represented by a cline can correspond with graded alterations in the environment. Clines are useful to biological, medical, and other anthropologists interested in depicting and analyzing human variation.
When a cline is graphically portrayed, portions of the distribution of the trait that have the same value are connected by a line. On either side of the line appear other lines representing progressively greater and lesser frequency of the trait. Thus, the graph of the cline resembles a weather map, with bands of varying temperature or pressure occurring over space. A map of a cline effectively illustrates how the distribution of the trait is gradual and continuous rather than abruptly different from one area to the next.
Gene flow is one evolutionary process that can generate a clinal trait distribution. Population movements can result in a trait pattern of highest frequency where the population originated and decreasing frequency with greater distance from the ancestral home. Frequencies of the ABO blood group in Europe and Asia provide a good example of this effect of gene flow. The prevalence of Type B is greatest in Asia and declines as one moves west. This pattern is attributable to population movements by Asian nomads who spread to eastern and central Europe in several waves over the last 2,000 years. The dissemination of Type B was less with the smaller groups who migrated farthest, and the concentration of Type B remained greatest in the regions of origin. A second cline of the ABO blood group in Europe and Asia exists with Type A, which is greatest in Europe and decreases as one moves east, a pattern opposite to that of Type B.
In addition to gene flow, an evolutionary process that can produce clines is natural selection. If selective forces vary geographically, responding trait distributions can be clinally patterned. Ancestral skin color provides an example of clines that reflect natural selection. Average skin pigmentation reveals a notable pattern that varies with the selective force of ultraviolet radiation. Darkest skin color is seen at or near the equator, and amount of pigmentation becomes increasingly less as distance from the equator increases. Specifically, in the Old World, the cline of skin color shows gradients that decrease more readily as one moves north of the equator than as one moves south. The general pattern of pigmentation decline holds in both hemispheres, but it is more pronounced for a given latitude in the northern than in the southern hemisphere. A plausible explanation for this difference is that UV radiation is less in the northern than in the southern hemisphere at comparable latitudes. In the New World, pigmentation clines exist that do not reflect the forces of natural selection, but rather the historical effects of population movements. For example, the mass, forced withdrawal of West Africans during the period when slaves were traded placed human groups in new geographic areas. The resulting trait distributions were not linked to UV radiation as they had been in the ancestral homes.
Other clines revealing an influence of natural selection relate to body size and shape. Climate exerts strong selective pressures on certain body parameters.
As far as size, a cline for mean body mass demonstrates that body mass for a given stature is higher as latitude increases and average annual temperature declines. Concerning body shape, mean body breadth can be assessed by the width between the crests of the two ilia (hipbones). This body breadth measurement shows a latitudinal cline, with breadths wider as latitude increases and average annual temperature falls. A second latitudinal cline for body shape involves mean limb length. Arms and legs are shorter for a given stature or trunk size as latitude increases and average annual temperature declines. Collectively, these three gradients for body size and shape constitute adaptive strategies related to thermoregulation. Through alterations in body form, surface area per unit mass is decreased in colder climates to reduce heat loss, while it is increased in warmer environments to enhance heat dissipation.
With the accumulation of data on gene frequencies in various world regions, clines are sometimes redrawn. The increased knowledge of trait distributions enables clinal maps that have greater detail and smaller increments in trait frequency among neighboring areas. Another ongoing consideration with clines is that as human groups migrate and interbreed with others from different major geographic regions, connections between phenotypes and geography can be weakened.
As an approach to studying human diversity, clines are useful for investigating worldwide and regional distributions of traits and interpreting these distributions. A clinal orientation can be viewed as an alternative to a racial, or typological, perspective on human variation. For example, researchers of clines focus on only one or a few traits at a time, not on combinations of traits that are expected to differ together, as a racial orientation requires. Also, clinal analysis looks at the continuity of trait distributions, rather than the discontinuity that characterizes a racial viewpoint. Furthermore, in explaining the distribution of traits, a clinal approach focuses on evolutionary forces such as natural selection and gene flow that increase and decrease variation within and among human groups. Such an outlook contrasts with the static nature of human diversity assumed by the race concept.
Some anthropologists believe that the clinal approach is most valuable when it is applied in combination with a population focus. It is important to recognize that traits do not exist in isolation, but rather are part of the entire genetic endowment of humans. Thus, while single genotypes and phenotypes can demonstrate a clinal distribution, the overall patterning of traits is fundamentally linked to the behavior of individuals and populations.
- Cavalli-Sforza, L. L., Menozzi, P., & Piazza, A. (1994). The history and geography of human genes. Princeton, NJ: Princeton University Press.
- Molnar, S. (2002). Human variation: Races, types, and ethnic groups (5th ed.). Upper Saddle River, NJ: Prentice Hall.
- Ruff, C. B. (1994). Morphological adaptation to climate in modern and fossil hominids. Yearbook of Physical Anthropology, 37, 65-107.