What is a species? It’s a question that has agonized scientists since well before Darwin. With some exceptions, the thinking has landed on an evidently firm reproductive barrier: Members of different species don’t mate. If they do, their offspring are sterile and can’t contribute to future generations. The reproductive barrier has thus created a useful demarcation of “what is a species”—until a deep dive into butterflies showed otherwise.
Source: Do Butterflies Challenge the Meaning of Species?, an article by Mary Ellen Hannibal.