And yet, as this New York Times Magazine article with the cool graphics explains, there appears to be a secret backdoor to species adaptation: hybridization, when individual animals of related species cross paths, have sex and produce offspring.

Sometimes hybrids are evolutionary dead ends, as today’s This Planet episode, Bears! And Other Hybrids, points out. The most common example of cross-species hybridization, the mule, is almost always infertile, and is likely why hybridization has such a bad rep.