What makes two species different?

For most of the 20th century, scientists believed that the reproductive incompatibility between species evolved gradually as a by-product of adapting to different environments. New research has shown there are more factors at play -- specifically the presence of 'selfish genes,' whose flow among species may dictate whether two species converge or diverge.

from Latest Science News -- ScienceDaily http://bit.ly/2R4Zt3b