Deep Time In a Bird’s Beak

Evolution works on different scales. In a single day, HIV's genetic code changes as it adapts to our ever-adapting immune system. Over the course of decades, the virus can make a successful leap from one species to another (from chimpanzees to humans, for example). Over a few thousand years, humans have adapted to agriculture--an adult tolerance to the lactose in milk, for example. Over a couple million years, the brains of our hominid ancestors have nearly doubled. Sometimes scientists distinguish between these scales by calling small-scale change microevolution and large-scale change macroevolution. Creationists have seized on these terms and used them to build one of their central canards: that they accept microevolution but can then reject macroevolution. That's a bit like accepting microeconomics--how households and firms make decisions and interact in markets--but then denying macroeconomics--how entire societies produce goods, how inflation rises and falls, and so on. Evolutionary biologists debate fiercely about how macroevolutionary change emerges from microevolution. But they continue to find abundant evidence that the two are a package deal.