When it comes to species, says biological anthropologist Rebecca Ackermann, “forget everything you learned in high school.”

The classic textbook definition, known as the biological species concept, is a group of organisms that only produce fertile offspring with one another. By this rule, domesticated dogs are a single species — whether dachshund or Great Dane — but a donkey and a horse are not.

Ackermann, a professor at South Africa’s University of Cape Town, favors a different definition that’s not dependent on successful sex: a group of organisms sharing a mix of anatomical, behavioral and genetic traits that distinguishes them from other groups. But, she adds, “Many, many evolutionary biologists I know often avoid the word species entirely.”