http://www.bioone.org/doi/abs/10.1644/1545-1542%282002%29083%3C0437%3AMGAOTN%3E2.0.CO%3... > Here's a better one that discusses the full taxonomic difference. - As I read the abstract the two separated groups of W's are still the same species and can mate. There is some drift as the climates are different and offer different challenges.