What Is a Species, Really?

You learned what a species is in school. Two animals that can mate and produce fertile offspring. Simple, clean, done. Except biologists have proposed 34 different definitions of "species" — and none of them fully work. Polar bears and grizzly bears are actively mating right now. Bacteria don't have species at all. And the lines we draw between living things determine what gets protected by law — and what quietly goes extinct. The concept of species is one of the most useful broken tools in all of science. And the real answer is stranger than your textbook ever admitted.