Gene flow and selection.
<P>
   The bent grass in the foreground is growing on an abandoned mine where
   the soil is rich in copper.
<P>
   The  population  adapted  to  the  mine  soil  has a high frequency of
   copper-tolerance  alleles, while the population growing in normal soil
   has a low frequency of these alleles
<P>
   Since  grass is wind-pollinated, the gene flow between the populations
   tends to reduce genetic differences between them.