Charles Darwin pictured evolution as a grand tree, with the world’s living species as its twigs. Scientists identify 10,000 new species a year, but they’ve got a long, long way to go before finding all of Earth’s biodiversity. So far, they have identified 1.5 million species of animals, but there may be 7 million or more in total. Beyond the animal kingdom, our ignorance balloons. Scoop up some sea water or a cup of soil, and there will likely be thousands of new species of microbes lurking there. Fortunately, a lot of the species that scientists discover each year are fairly close relatives to species we already know about. There may be plenty of beetle species left to be discovered, for example, but they will all end up as tufts sprouting from the same beetle branch.

Making matters more complicated is that the tree is, in some ways, more like a web. Genes sometimes slip from one species to another, especially among microbes. There are lots of ways this can happen. Viruses can ferry these genes from species to species; in other cases, microbes may just slurp up naked DNA. In the process, they blur genealogy.

This can be a hard concept ...