Polyploidy and sympatric speciation.

This example shows how polyploidy may have caused reproductive isolation between three natural species of hemp nettle. Research suggests that the diploid species Galeopsis pubescens and Galeopsis speciosa produced the allotetraploid Galeopsis tetrahit. If G. tetrahit is mated with either of the other two species, the resulting offspring would be monoploid for one chromosome set and diploid for the other set, making them sterile. Therefore, G. tetrahit is reproductively isolated from the diploid species, making it a new species.