One species, two populations.
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   These  2  caribou  herds in the Yukon are not totally isolated - their
   ranges overlap.
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   Still,  a  caribou  is  more  likely  to breed with members of its own
   population than with members of the other population.
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   Thus  the gene pool (total aggregate of alleles) of each population is
   isolated, with little gene flow between them.