What is a species?

The most famous definition of a “species” was proposed by the biologist Ernst Mayr back in the mid-1900s: species are groups of interbreeding natural populations that for one reason or another, don’t have babies with each other. But it turns out to be super hard to determine whether groups are actually reproductively isolated, so other folks have tried to define what a species is based on looks, or genetic similarity, or evolutionary history, or… well, it keeps going. These days, scientists across different fields regularly use more than a dozen different so-called “species concepts”. See, there’s a huge scientific debate about whether the red wolf is just a hybrid between a coyote and a gray wolf, or whether it’s actually its own separately-evolving species, OR maybe it used to be its own thing but that thing went extinct and the current thing is actually a new hybrid? And that doesn’t even get into how weird plants are about this stuff. Like, some plants can literally double their genome when they reproduce – creating a totally new species in a single generation – which wreaks all sorts of havoc on any attempts to clearly differentiate between species. So it is really difficult to define a species.