Throughout the world, closely related species are found on landmasses separated by wide stretches of ocean. What explains these far-flung distributions? Why are such species found where they are across the Earth?

Since the discovery of plate tectonics, scientists have conjectured that plants and animals were scattered over the globe by riding pieces of ancient supercontinents as they broke up. In the past decade, however, that theory has foundered, as the genomic revolution has made reams of new data available. And the data has revealed an extraordinary, stranger-than-fiction story that...