Malaria exemplifies this. Within the Plasmodium family tree, as revealed by molecular phylogenetics over the last two decades, the four human-afflicting kinds don’t cluster on a single branch. They are each more closely related to other kinds of Plasmodium, infecting nonhuman hosts, than to one another. In the lingo of taxonomists, they are polyphyletic. What that suggests, besides the diversity of their genus, is that each of them must have made the leap to humans independently.
