Biologists no longer consider evolution a steady or gradual process. New traits often spread suddenly, and a new species may crop up in tens of millennia or less. These “punctuated” or fast evolutions happen when a population faces a new stress or opportunity, particularly when a group stumbles into a new environment and gets isolated from the rest of the species. (I’ve blogged about a couple examples: the abrupt Stone Age appearance of a new subspecies of wolf, the dog, as well as the even faster rise of dog-like domestic foxes during the late 20th Century.) Biologists disagree about what happens between these bursts of evolution — how much the species changes — but they agree that any change then is slow, often noticeable only after millions of years. The resulting theory is punctuated equilibrium: the idea that evolution alternates between sudden change and stability. I think history works the same way. Continue reading “Punctuated Equilibrium: Natural History’s Pattern, and History’s Too” →