Scientists for decades have clashed over whether evolution takes place gradually or is driven by short spurts of intense change.

In the latest chapter in this debate, researchers report in Science this week that it appears that when new languages spin-off from older ones, there is an initial introductory burst of alterations to vocabulary. Then, the language tends to settle and accumulate gradual changes over a long period of time. The team believes this discrete evolutionary pattern occurs when a social group tries to forge a separate identity.

Study co-author Mark Pagel, an evolutionary biologist at the University of Reading in England, says that the latest study grew out of an earlier finding in which he and colleagues determined that about 20 percent of genetic changes among species occur when they first split off, whereas the rest happen gradually.

"It was very natural for us to wonder if a similar process [of evolution] happens in cultural groups," Pagel says. "We treat the words that different languages use almost identically to the way we use genes: … The more divergent two species are, the less their genes have in common, just as the more divergent two languages are, the less their words have in common."

The team focused on three of the world's major language families in its study: Bantu (Swahili, Zulu, Ngumba, for example), Indo-European (English, Latin, Greek, Sanskrit) and Austronesian (South Pacific and Indian Ocean languages such as Taglaog or Seediq). They constructed genealogical trees—similar to those they had created previously in their 2006 species-related study—albeit this time the trees traced existing languages back to their common roots; the length of a "branch" indicated the extent of word replacement that took place as each old language morphed (possibly with new languages splitting off) into its current form.

"If vocabulary change is accumulating gradually and independently of how many times new languages arise, then all of the paths—the routes from ancestor to tips—in the tree should be the same length; that is, there should be the same amount of evolution in each path," Pagel says. "But, if the number of language-splitting events along a path predicts the path length, this tells us that change has not been gradual but that language-splitting has added to the change."

The researchers determined that 10 percent to 33 percent of divergence between languages stemmed from key vocabulary changes at the time of language splitting.

He offers a few examples of these sorts of events, such as the sudden emergence of American English when Noah Webster published his American Dictionary of the English Language in 1828. More recently, he says, black American English could fit the bill as an emerging idiom.

"It's plausible to think of black American English as having diverged from standard English as a way of establishing a distinct identity," he says. "I think everybody's impression is those differences [between the two languages] are greater than one would expect for people who live in the same area."