In population genetics, a cline is a gradual change of a character or feature (phenotype) in a species over a geographical area, often as a result of environmental heterogeneity. The change in phenotype does not result in different species as long as the geographically spread populations can interbreed with one another. This meaning of cline was introduced by Sir Julian Huxley. Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ... The phenotype of an individual organism is either its total physical appearance and constitution or a specific manifestation of a trait, such as size, eye color, or behavior that varies between individuals. ... In biology, a species is the basic unit of biodiversity. ... Sir Julian Sorell Huxley, FRS (June 22, 1887 – February 14, 1975) was a British biologist, author, Humanist and internationalist, known for his popularisations of science in books and lectures. ...

A ring species is a special kind of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline. In this diagram, interbreeding populations are represented by coloured blocks. ...

A "cultural cline" is a gradual change of a cultural characteristic or feature over a geographical area. Source of definition unknown.

In linguistics, the term cline refers to a list of words dealing with the same concept, arranged in order of intensity. For example a list could be made with words like "freezing", "cold", "cool", "tepid", "warm", "hot", and "scorching". Linguistics is the scientific study of human language, and someone who engages in this study is called a linguist. ...

Rejection of "Race" for Cline

Because the term race has often been used synonymously with subspecies, some people incorrectly use cline instead of "race" to describe what they see as distinct groups of humanity (Caucasoid, Mongoloid, Negroid, Australoid, Capoid, and so forth). This substitution of "cline" for "race" or "subspecies" is technically incorrect because the word "cline" refers only to the geographic density gradient of a single feature, while the words "race," "subspecies," etc., assume replicable clusters of features. On the other hand, using the term "cline" in substitution for "race" may be appropriate if the speaker understands racial distinctions more in terms of groupable variations on a continuum. In line with the example above from linguistics, human skin color may be described in terms of the following cline: "white", "cream","olive", "tan", "brown", "black", and etcetera with a myriad intermediates. Because the controversies over the term "race" the term "cline" may be much more of an accurate description. See the main article Race for details on this point. For other uses, see Race (disambiguation). ... In zoology, as in other branches of biology, subspecies is the rank immediately subordinate to a species. ... Typical Caucasoid skull Caucasoid is a racial classification usually used as part of a phenotypal system, also including other classifications such as Australoid, Mongoloid, Negroid, and sometimes others such as Capoid. ... Typical Mongoloid Skull A portrait of the Mongol ruler Genghis Khan; the Mongolians, for which the term Mongoloid was named after, are an example of the prototype Northern Mongoloid. ... Skull of the classic Niggeroid phenotype, exhibiting a pronounced dolichocephalism and both maxillary and alveolar prognathisms Niggeroid is an obsolete term once used in physical anthropology to delineate everyone indigenous to sub-Saharan and West Africa and portions of North Africa. ... Australoid is a broad racial sub-classification of Australasian peoples having generally dark skin and hair which can be curly, straight, or kinky, defined by the anthropologist Carleton S. Coon. ... Main article: Khoisan One of the five macro-racial groups often recognized by physical anthropologists (along with Negroids, Australoids, Caucasoids and Mongoloids). ... For other uses, see Race (disambiguation). ...