Deme (pronounced “deem,” from the Greek for “people” and originally referring to a political division within ancient Greece) has been used in biology since the 1930s as a term for a local interbreeding population within a species. As such, the recognition of demes can be confused with, and can appear to provide justification for, the existence of biological races or subspecies.
The rationale for naming population units below the species level comes from the simple fact that members of a species are seldom, if ever, evenly distributed throughout the species’ geographical range. Uneven distribution can result in clusters of individuals partially isolated from other such clusters—that is, with more interbreeding within the clusters than between them—simply because of proximity. It is to such clusters that the term deme is usually applied. Thus, the green frogs in an isolated pond, a town of prairie dogs, or a field of wild sunflowers might be examples of demes.
If demes inhabit different local environments, natural selection can operate in different directions in these populations with the result that there may be genetic and even physical variation in the characteristics of individual demes. Other processes of evolution, such as mutation and other forms of genetic change, can also enhance these differences, depending upon the extent of demic isolation.
The problem with the concept of the deme is that there is no definitive set of criteria for recognizing demes within species. Normally, some spatial separation or other obvious impediment to genetic exchange is a clue, with genetic or physical distinctions as an expected result. Demes, however, are populations within species, and by definition, exchange genes with other demes of the same species either directly, in the case of adjacent demes, or by a series of steps, in the case of widely separated demes.
Demes are also transitory. According to Stephen Jay Gould in The Structure of Evolutionary Theory, demes have “porous borders,” they do not function as a “discrete [evolutionary] entities,” and are “defined only by the transient and clumpy nature of … habitats” (p. 647). Richard Dawkins, in The Selfish Gene, likened demes to “clouds in the sky or dust storms in the desert … temporary aggregations” (p. 36). Moreover, even if demes are spatially distinct clusters, their phenotypic features might still grade into one another in response to environmental gradients, thus making the boundaries between the demes even less distinct. This is known as a clinal distribution.
Given the accepted general definition, it can certainly be said that demes exist within the human species, where they are identified as semi-isolated endogamous (literally, “marriage within”) populations. The isolation may be spatial, as in any of a number of societies inhabiting separate and secluded valleys in the New Guinea Highlands, or cultural, as in the religiously based isolation of groups such as the Hutterites, Mennonites, and Amish.
All human populations consist of members of a single species. Thus, by definition, they exchange genes with other populations, directly or indirectly. More specifically, even the most isolated human population has experienced gene flow with adjacent populations at some point in time. Outsiders have always married into the Hutterites, for example, and Hutterites have married out. Over time, human demes are as ephemeral as those of any other species, and probably more so, given the human proclivities for mobility and genetic exchange.
The question becomes, then, what is the relationship between the concept of the deme and that of the biological race or subspecies? Both are proposed populations below the species level. Traditionally, a biological race was considered to be a group of related demes inhabiting the same general region and sharing genetic and/or physical characteristics (Savage 1977; Mettler et al. 1988).
The limitations noted for the deme concept, however, provide a lesson for considering such larger groups as races or subspecies because those limitations become more pronounced with larger populations. Larger populations within a species contain more genetic and physical variation, and thus stand even less chance of having specific biological characteristics. Because there is more gene flow between large populations, the boundaries between such populations are further blurred and have less real biological meaning and utility. At best, as Lawrence Mettler et al. note, defining and naming subspecific groups is “purely subjective” and a “matter of convenience” for the purpose of “intelligible communication” (1988, p. 48).
Dawkins, Richard. 1976. The Selfish Gene. Oxford: Oxford University Press.
Gould, Stephen Jay. 2002. The Structure of Evolutionary Theory. Cambridge, MA: Belknap.
Mettler, Lawrence E., Thomas G. Gregg, and Henry E. Schaffer. 1988. Population Genetics and Evolution, 2nd ed. Englewood Cliffs, NJ: Prentice Hall.
Savage, Jay M. 1977. Evolution, 3rd ed. New York: Holt, Rinehart and Winston.
Michael Alan Park