Languages show periodic bursts of evolution in which many new words blossom, according to new research that treats linguistic evolution like its biological counterpart.

The research suggests that new words evolve slowly most of the time, but with spurts of diversification when two languages divide.

If all language evolved at the same stately pace, the distance between any two languages could be easily calculated by multiplying this constant by how long ago the two tongues parted ways. However, Mark Pagel, an evolutionary biologist at the University of Reading in the UK, and his colleagues have found that branches heavy with linguistic divorces evolve faster, suggesting "punctuational bursts" of language change when two languages split.

The authors calculate that the rapid change in these bursts accounts for between 10 and 33 per cent of total word differences between languages.

THE LANGUAGE TREE

The team used a common linguist's tool - a list of 200 everyday words - to create a family tree, or phylogeny, of hundreds of modern languages. These 200 words were presumably all identical in the first language but changed to new forms over time. Pagel says he used the words "almost like a set of 200 genes" in which a change to a new form is like a mutation.

The research shows modern languages that have split many times from other languages have accrued more "mutations", suggesting faster evolution. Pagel speculates that these mutations probably occurred in bursts of change right after the languages split.

"What we thought was quite remarkable is the effect that is causing between a tenth and a third of changes is associated with relatively short periods of time around these splitting events," says Pagel. "We think it is quite a powerful effect."

IN EQUILIBRIUM

The work recalls the notion of "punctuated equilibrium", an idea associated with Niles Eldredge and Stephen Jay Gould that suggests organisms go through short periods of rapid evolution from time to time, against a background of relative stasis.

Pagel and his team suggest two possible reasons for such bursts in language evolution: founder events, in which the idiosyncrasies of a small number of language originators colour the language ever after, and the social desire of groups that have split off to separate themselves from the original language. Such pioneers would presumably tend to develop their own new words to help establish a sense of affiliation to their newly founded group.

"It is really reflective of a new and emerging trend: the application of mathematical evolution to linguistics," says Erez Lieberman, a mathematician at Harvard University, US, who has worked on similar problems.

Different language families show different amounts of this punctuational word change. But this variation is no cause for concern, says Lieberman: "The very clean stories are in the extremely mature fields where everyone knows exactly how to do the analysis. Fundamentally, if you are going to be breaking new ground, you are going to get dirty."

CAUSE OR EFFECT?

Brian Joseph, a historical linguist at Ohio State University, US, and the editor of the journal Language, says the work is clever, but notes: "The idea that language change does not occur at a steady rate is nothing new; 'bursts' of change and periods of stasis have long been recognised."

He wonders whether Pagel can prove that the language splits caused the bursts, rather than the other way around. Could languages that evolve faster just be more prone to splitting?

Pagel concedes that the trees would look exactly the same with the causality reversed. But he says that while founder effects and social factors can be used to explain post-split bursts, "There are no external indicators that the causality goes the other way."

Just as trying to fit Earth's life forms into phylogenetic trees is the work of generations of careers, fitting languages into trees is no picnic. Pagel's team relied on decades of work by linguists to position the splitting events on which the work rests.

But, says Joseph, "There are no clear-cut criteria that tell you that two speech forms have diverged to the point of being considered separate languages; sometimes, as the authors themselves recognise, social factors play the strongest role in perceptions of there being a new language."

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