From Creatures Wikia
For the first time norns can genuinely speciate. In speciation, a population of norns changes into two populations that have different specific mate recognition systems (SMRS's). Note that speciation is not the same as morphological divergence. It sometime happens in nature that two species, called sibling species, do not interbreed and yet are morphologically indistinguishable. Once speciation has occurred, genes will no longer mix and the populations can continue to diverge in other ways.