Darwin, Plotkin says, used the way language changes “to popularize his heretical theory and explain for a broad audience what natural selection means. The process wasn’t easy to observe in organisms, but it was easier to see in words.”

But natural selection is just one force of evolutionary change. Under its influence, genes become more (or less) common because their owners are more (or less) likely to survive and reproduce. But genes can also change in frequency for completely random reasons that have nothing to do with their owner’s health or strength—and everything to do with pure, dumb luck. That process is known as drift, and it took decades for evolutionary biologists to recognize that it’s just as important for evolution as natural selection.

Linguists are still behind. It’s easy to see how languages can change through drift, as people randomly pick up the words and constructions they overhear. But when Darwin wrote about evolving tongues, he said, “The better, the shorter, the easier forms are constantly gaining the upper hand, and they owe their success to their own inherent virtue.” That’s a view based purely on natural selection, and it persists. “For the most part, linguists today have a strict Darwinian outlook,” Plotkin says. “When they see a change, they think there must be a directional force behind it. But I propose that language change, maybe lots of it, is driven by random chance—by drift.”

To see whether that was true, he and his colleagues developed statistical tests that could distinguish between the influence of drift and of natural selection. They then applied these tests to several online repositories, such the Corpus of Historical American English—a digital collection of 400 million words, pulled out of 100,000 texts published over the past 200 years.

The team focused first on the past-tense forms of verbs, and found at least six cases where natural selection is clearly in effect. In some cases, the verbs were regularized, losing weird past forms in favor of more-predictable ones that end in –ed. Wove, for example, gave way to weaved, while smelt lost ground to smelled. That’s not surprising: Many linguists have suggested that verbs tend to become more regular over time, perhaps because, like Darwin theorized, these forms are just easier to learn.

But Plotkin found just as many instances where selection drove verbs toward irregularity: Dived gave way to dove, lighted to lit, waked to woke, and sneaked to snuck. Why? Perhaps because we like it when words sound alike, and we change our language to accommodate such rhymes. For example, dove began to replace dived at the same time that cars became popular, and drive/drove became common parts of English. Similarly, the move from quitted to quit coincided with the rise of split, which became much more widely used when it acquired a new meaning—to leave or depart. In both cases, changes in one irregular verb—drive or split—may have irregularized others. “We can’t definitively say that’s the reason, but it’s coincident,” Plotkin says.