L. L. Cavalli-Sforza (1969.  Genetic drift in an Italian population.  Scientific American 221(2): 30-37) examined variation in gene frequencies at several blood-group loci among towns and villages in the vicinity of Parma,
Italy.  In the mountains outside of Parma (see map below), villages are small and isolated and marriages usually take place between couples from the same village.  This population structure creates ideal conditions (small population size) for genetic drift to cause gene frequencies to diverge among villages (populations).  By contrast, in the plains around Parma, towns (including Parma itself) are larger and there is much more migration between towns than in the mountains.  These conditions are less conducive to differentiation by genetic drift.
 

    Cavalli-Sforza and his colleagues obtained blood samples from villages and towns in the Parma region, from which they calculated gene frequencies for the Rh, ABO, and MN blood-group loci for different towns and villages.  With these estimates, they were able to examine the degree of genetic divergence between populations (plotted as "Genetic Variation" in the figure) for localities in the mountains and plains (see figure below).  It is clear that villages from the mountains exhibited much more genetic divergence at these loci than towns from the plains.
 
 

 
Map and Figure from L. L. Cavalli-Sforza. 1969.  Genetic drift in an Italian population.  Scientific
    American 221(2): 30-37.

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