What makes two species different?

Source: Science Daily

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.

https://www.sciencedaily.com/releases/2019/01/190103120852.htm