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Geographic isolation leads to genetic isolation, and to eventual reproductive isolation.

This ecological differentiation most likely worked hand in hand with increased physical separation to promote reproductive isolation between populations.

It is smaller, and has paler, thicker fur than its lowland cousins and lives in reproductive isolation on that remote plateau in the north Sumatran jungle 1,000m above sea level.

In theory, the GRC could have created the reproductive isolation needed for new species to evolve by rendering those individuals that carried the extra chromosome unable to interbreed and produce fertile offspring with those that did not.

However, one issue that affects the chances of such reproductive isolation arising is the pattern of co-inheritance of the genes that affect mating preference and the genes responsible for the preferred trait.