A deme ia a locally occurring subset of a population.
Typically, members of a deme breed with each other more often than with members of other demes.
A deme can be considered a "local population" of a particular species in a given area.
The California Red-legged Frog (Rana draytonii) and the Northern Red-legged Frog (Rana aurora) are two closely related frog species native to coastal and inland valley river ecosystems in California.
The ranges of the two species are shown here (R. aurora in red; R. draytonii in orange and purple):
Frogs do not occur everywhere in their range.
They are restricted to fast-moving, sometimes ephemeral runoff streams with their sources in the coastal mountain ranges.
These streams are separated by sometimes large areas of dry, coastal chaparral or desert that are inhospitable to frogs.
Demes of Rana aurora and of Rana draytonii undergo little genetic exchange because their streams are separated by habitats they usually cannot cross. (Frogs cannot survive in salt water, so the ocean forms an impassable barrier to migration.)
Limited migration can take place where rivers meet or are very close to each other, allowing some gene flow.
Some of the demes of each species have been separated long enough for some genotypic/phenotypic divergence to have occurred, though they are still considered members of the same species. At least for now.
Check out the variation in
However, long-term isolation of demes from one another can eventually lead to speciation, since isolated populations will evolve independently of one another.