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.”
That’s because the evolutionary tree is tangled, and many organisms on diverging branches can still interbreed. “Canids have, pigs have, mice have. You name it, and it has,” says University of Georgia evolutionary biologist Michael Arnold. “There’s a hybrid under every bush.”
