ONE of the unresolved—and rather bitter—disputes in evolutionary biology is between the creeps and the jerks. The creeps (so dubbed by the jerks) think that evolutionary change is gradual. The jerks (so dubbed by the creeps) think it happens in sudden jumps that are separated by long periods of stasis.

Probably, both are true. Work done a couple of years ago by Mark Pagel of Reading University, in England, suggests that about a fifth of evolutionary change happens jerkily at around the time new species form. The rest creeps in gradually over the millennia.

Species, however, are not the only things that evolve. Languages do too. And in the current edition of Science, Dr Pagel and his colleagues publish evidence that they do so in a way which looks intriguingly similar to what happens in species.

There was already some historical evidence for this. The English of Geoffrey Chaucer (born in the 14th century), for example, is incomprehensible to modern laymen, whereas that of William Shakespeare (born in the 16th) is not only comprehensible but held by some to be a model. Dr Pagel, however, wanted to examine the question systematically and to include languages with no literary history in his analysis.

To do so he looked at three well-studied parts of the linguistic family tree: the Bantu languages of Africa, the Indo-European group from Eurasia and the Austronesians of the Pacific. In all three cases it is pretty clear how the branches connect up, even if it is not always obvious when particular splits occurred.

Dr Pagel did not, however, need to know that. He only needed to know the shape of the tree. That was because his hypothesis was that if linguistic evolution is jerky, the jerks will happen at the points where languages split—the equivalent of species splits in biological evolution. The way to test that is to track back along the branches leading from each existing language, and count the number of splits on each path before you get to the common ancestor of all.

His hypothesis turned out to be correct. Languages are formed not, it seems, by a gradual drifting apart of two groups who no longer talk to each other, but by violent rupture. Around a third of the vocabulary differences between modern Bantu speakers arose this way, around a fifth of the differences between speakers of Indo-European languages, and around a tenth of the Austronesians. That compares with around a fifth for biological species.

All this suggests that the formation of both languages and species is an active process. For species, adaptations to novel environments and the need to avoid crossbreeding with those on the other side of the split are both plausible hypotheses. For languages, the explanation may be a cultural rather than biological need to distinguish populations. As Noah Webster, the compiler of the first American dictionary, put it: “as an independent nation, our honor [sic] requires us to have a system of our own, in language as well as government.” In other words, if you don't speak proper, you ain't one of us.