Cline (population genetics)

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.

Ring species


In biology, Ring species present an interesting problem for those who seek to divide the living world into discrete species. Ring species are a distinct type 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.

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", et cetera with a myriad intermediates. Because of 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.

Some scientists prefer to conceptualize and analyze human genotypic and phenotypic variation in terms of populations and clines instead.

Some scientists however, have argued that this position is motivated more by political than scientific reasons. Several have argued that race is a biologically valid and useful concept because self-identified races correspond to measurably discernable clusters of genes.

At the beginning of the 20th century, anthropologists questioned, and eventually abandoned, the claim that biologically distinct races are isomorphic with distinct linguistic, cultural, and social groups. Then, the rise of population genetics led some mainstream evolutionary scientists in anthropology and biology to question the very validity of race as scientific concept describing an objectively real phenomenon. Those who came to reject the validity of the concept, race, did so for four reasons: empirical, definitional, the availability of alternative concepts, and ethical (Lieberman and Byrne 1993).

The first to challenge the concept of race on empirical grounds were anthropologists Franz Boas, who demonstrated phenotypic plasticity due to environmental factors (Boas 1912), and Ashley Montagu (1941, 1942), who relied on evidence from genetics. Zoologists Edward O. Wilson and W. Brown then challenged the concept from the perspective of general animal systematics, and further rejected the claim that "races" were equivalent to "subspecies" (Wilson and Brown 1953).

One of the crucial innovations in reconceptualizing genotypic and phenotypic variation was anthropologist C. Loring Brace's observation that such variations, insofar as it is affected by natural selection, migration, or genetic drift, are distributed along geographic gradations; these gradations are called "clines" (Brace 1964). This point called attention to a problem common to phenotypic-based descriptions of races (for example, those based on hair texture and skin color): they ignore a host of other similarities and difference (for example, blood type) that do not correlate highly with the markers for race. Thus, anthropologist Frank Livingstone's conclusion that, since clines cross racial boundaries, "there are no races, only clines" (Livingstone 1962: 279). In 1964, biologists Paul Ehrlich and Holm pointed out cases where two or more clines are distributed discordantly&mdash;for example, melanin is distributed in a decreasing pattern from the equator north and south; frequencies for the haplotype for beta-S hemoglobin, on the other hand, radiate out of specific geographical points in Africa (Ehrlich and Holm 1964). As anthropologists Leonard Lieberman and Fatimah Linda Jackson observe, "Discordant patterns of heterogeneity falsify any description of a population as if it were genotypically or even phenotypically homogeneous" (Lieverman and Jackson 1995).

Finally, geneticist Richard Lewontin, observed that 85 percent of human variation occurs within populations, others have found similar results, that between group variation (measured by Sewall Wright's population structure statistic FST) accounts for as little as 5% of human genetic variation². Lewontin argued that human populations could not be correctly defined due to this observation (Lewontin 1973). A. W. F. Edwards claimed in 2003 that this conclusion is unwarranted because most of the information that distinguishes populations is hidden in the correlation structure of the data and not simply in the variation of the individual factors. While it makes Lewontin's argument regarding classification unwarranted, Edward's paper only discusses a statistical analysis of human genetic variation, see Lewontin's Fallacy.

Also, it has been argued that the calculation of within group and between group diversity has violated certain assumptions regarding human genetic variation. Calculation of this variation is known as FST and Long and Kittles (2003) have questioned the validity of this reproducible statistic. The first problem is that effective population size is assumed to be equal in the calculation of FST, if population sizes vary, then allele relatedness among alleles will also vary. The second problem is that FST calculation has assumed that each population is evolutionarily independent. Calculation of FST can therefore only be made for the set of populations being observed, and generalisations from specific data sets cannot be applied to the species as a whole.

Long and Kittles tested four models for determining FST and concluded that the model used most often for estimating this statistic is the simplest and worst fitting. Their best fit model was still a poor fit for the observed genetic variation, and calculation of FST for this model can only be made on a population by population basis. They conclude that African populations have the highest level of genetic diversity, with diversity much reduced in populations outside of Africa. They postulate that if an extra-terrestrial alien life form killed the entire human species, but kept a single population which it preserved, the choice of population to keep would greatly effect the level of diversity represented. If an African population were selected then no diversity would be lost, whereas nearly a third of genetic diversity would be lost if a Papuan New Guinea population were chosen. Indeed within population genetic diversity in African populations has been shown to be greater than between population genetic diversity for Asians and Europeans. They conclude that their findings are consistent with the American Association of Physical Anthropologists 1996 statement on race

They also state that none of the race concepts they discuss are compatible with their results.

These empirical challenges to the concept of race forced evolutionary sciences to reconsider their definition of race. Mid-century, anthropologist William Boyd defined race as:

Lieberman and Jackson (1994) have pointed out that "the weakness of this statement is that if one gene can distinguish races then the number of races is as numerous as the number of human couples reproducing." Moreover, anthropologist Stephen Molnar has suggested that the discordance of clines inevitably results in a multiplication of races that renders the concept itself useless (Molnar 1992).

The distribution of many physical traits resembles the distribution of genetic variation within and between human populations (American Association of Physical Anthropologists 1996; Keita and Kittles 1997). For example, ∼90% of the variation in human head shapes occurs within every human group, and ∼10% separates groups, with a greater variability of head shape among individuals with recent African ancestors (Relethford 2002).

A prominent exception to the common distribution of physical characteristics within and among groups is skin color. Approximately 10% of the variance in skin color occurs within groups, and ~90% occurs between groups (Relethford 2002).

Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness. Sub-Saharan Africans, tribal populations from southern India, and Indigenous Australians have similar skin pigmentation, but genetically they are no more similar than are other widely separated groups. Furthermore, in some parts of the world in which people from different regions have mixed extensively, the connection between skin color and ancestry has been substantially weakened (Parra et al. 2004). So, taking them in isolation would appear to get you nowhere. In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups (Parra et al. 2003).

Considerable speculation has surrounded the possible adaptive value of other physical features characteristic of groups, such as the constellation of facial features observed in many eastern and northeastern Asians (Guthrie 1996). However, any given physical characteristic generally is found in multiple groups (Lahr 1996), and demonstrating that environmental selective pressures shaped specific physical features will be difficult, since such features may have resulted from sexual selection for individuals with certain appearances or from genetic drift (Roseman 2004).

Alongside empirical and conceptual problems with "race" following the Second World War, evolutionary and social scientists were acutely aware of how beliefs about race had been used to justify discrimination, apartheid, slavery, and genocide. This questioning gained momentum in the 1960s during the U.S. civil rights movement and the emergence of numerous anti-colonial movements worldwide.

In the face of these issues, some evolutionary scientists have simply abandoned the concept of race in favor of "population." What distinguishes population from previous groupings of humans by race is that it refers to a breeding population (essential to genetic calculations) and not to a biological taxon. Other evolutionary scientists have abandoned the concept of race in favor of cline (meaning, how the frequency of a trait changes along a geographic gradient). (The concepts of population and cline are not, however, mutually exclusive and both are used by many evolutionary scientists.)

In the face of this rejection of race by evolutionary scientists, many social scientists now view races to be social constructs (Gordon 1964).