Gamodeme

Gamodeme or deme refers to a local breeding population or mating group within a species, where random mating (panmictism) occurs. This means there are absolutely no barriers to sexual reproduction (gametes fusion) between individuals. Since complete random mating is rarely observed in nature, gamodemes instead are described as "more or less" panmictic units, where the majority of mating occurs in-group, with less (ideally very little) out-group mating (i.e. gene flow, or migration) between separate demes.

Virtually all living species consist of more than one deme (mating group) because there are geographical barriers that prevent or impede random mating between individuals.

Although demes are defined as "the most narrowly restricted group of individuals that interbreed a majority of the time" where 'majority' requires an in-group mating percentage to be higher than out-group (above 50%), this makes the delimitation of demes a bit problematic, especially for borderline cases where in-group mating % is only around half:

For example, suppose you travel to a village in a remote mountain region. You find that 99 percent of the people in the villages are married to others who were born in the same village. In this case, the village would appear to fit our ideal definition. But what if only 80 percent of the people chose their mates from within the village? What if the number were 50 percent? At what point do you stop referring to the population as a ‘breeding population’? There is no quick and ready answer to this question.

Examples of demes in the human species include clans, castes, tribes, rural villages or towns, and endogamous religious sects, whose mating boundaries are demarcated by relative geographical isolation, or cultural barriers. Some ethnic groups are also demes, since ethnicity can demarcate a mating group where intermarriage is greatly restricted via endogamy, however many ethnicities cannot function as reliable markers for demes - this is because they might contain different local mating groups, e.g. the ethnic group Hans Chinese are certainly not a deme since they contain many distinct smaller breeding populations, but the, in contrast are an ethnic group and deme.

Another problem is that many towns contain a diverse mix of demes, so geography is often in the 21st century a poor indicator of human mating group boundaries:

We must take care, however, to ensure that a local geographic unit, such as a town, is not composed of distinct subpopulations, such as groups belonging to different religious sects. A rural Irish village fits this criterion because most of its residents belong to the same religion, social class, and occupational group. New York City, on the other hand, clearly contains a number of subpopulations defined in terms of ethnicity, religion, social class, and other factors.

Anthropologist Stephen Molnar explains that human demes are now more complicated in identifying compared to the past:

Whereas formerly, small breeding units were restricted by economics, politics, or geography to a village community, today mating circles have expanded and the gene pool is much broader. In a world undergoing rapid urbanization, defining breeding populations and tracing patterns of gene flow are more difficult.