Genetic  drift.  This small wildflower population has a stable size of
   10 plants.
   Only  5  plants (those in white boxes) of generation 1 produce fertile
   offspring.
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   By  chance,  only  2  plants  of  generation 2 manage to leave fertile
   offspring.
   The  C^W allele frequency (q) increases in generation 2, then falls to
   zero in generation 3.
   Genetic variation in this population has been reduced.