VII.1 A large majority of species create a large number of more or less genetically isolated populations within their range.
Each species has a particular geographic range. Within that range, it exists in individual populations, some of which can be neighbours in terms of space while, on the other hand, others can be more or less isolated. Some populations are permanent, some gradually appear and disappear and some re-locate in space both in the long and in the short term, depending on how the natural conditions evolve in time. Members of these populations interact, including reproduction, mostly within their own population, less frequently with the members of the neighboring populations and least frequently with the members of the most distant populations. However, in many species, an even subtler structure can be discerned within each population, leading to the formation of subpopulations of individuals that are most likely to breed amongst themselves. These subpopulations are usually called demes. Thus, species tend to have a rather complex hierarchical structure, culminating in a metapopulation, i.e. the largest population unit whose members still share a common gene pool and can exchange genes with populations in their range via migrants, and a deme at the other end, whose adult members are most likely to breed amongst themselves.