Both of my parents served in World War II, my father as a soldier (luckily, never deployed) and my mother as a dental hygienist. They told me that life in the military consisted of “hurry up and wait.” Others have described the military life as long periods of boredom interrupted by brief periods of terror.
And that is what evolution is like also. Most of the time, in most species, it does not seem to be occurring—fluctuating directional selection creates long-term stabilizing selection. But sometimes, rapid directional selection occurs. When? Just as you might expect, and as the Grants observed in the finch populations—it occurs when there is a major environmental change. The environment around a population may change, or a few members of the population might migrate to a new location that has different environmental conditions. When conditions change, directional selection gears up to make the population change along with it. On the Galápagos Islands, the conditions changed back and forth. But sometimes environmental conditions change and then do not change back. Or the migrants do not go back home, remaining in permanently altered conditions. Under these circumstances, directional selection has permanent and rapid effects. It may lead to the formation of a new species, and do so rapidly. Such bursts of evolution have been called punctuations.
Scientists have a name for the overall hurry-up-and-wait pattern of evolution: punctuated equilibria. That is, long periods of equilibrium, during which stabilizing selection is the main process, punctuated by brief periods of rapid directional selection. New species usually originate during these punctuational events. A species comes into existence by a punctuation of directional selection, and then stays fairly unchanged until it either becomes extinct or experiences another punctuation. This pattern of evolution was first pointed out by evolutionary biologists Stephen Jay Gould and Niles Eldredge in the 1970s. It is the punctuations that demonstrate just how rapidly evolution can occur.
This entry appears in my book Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out World, from Prometheus Books.