John Whitfield has an interesting review article Across the Curious Parallel of Language and Species Evolution in PLoS Biology (PLoS Biol 6(7): e186) on language and species evolution.

One parallel between living things and languages is that their most important components show the least variation. In biology, this means that genes such as those involved in the machinery of protein synthesis change so slowly that they can be used to discern the relationships of groups that diverged hundreds of millions of years ago. Likewise, the most commonly used words, such as numbers and pronouns, change the most slowly. Looking at 200 of the commonest words in 87 Indo-European languages, Pagel’s team found that the frequency with which they are used in everyday speech explains 50% of the variation in the rate of word change [4]. Similarly, Erez Lieberman, an evolutionary theorist at Harvard University, and his colleagues have found that over the past millennium, English verbs have become regularized at a rate inversely proportional to their frequency [5]. The frequency effect means that some rates of lexical replacement are comparable to the evolutionary rates of some genes, says Pagel; he thinks that these words might allow researchers to build family trees showing the relationships between languages reaching back 20 millennia, compared with the 8,000 years or so that most linguists currently think possible.

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Earlier this year, Pagel and his colleagues uncovered another parallel between linguistic and biological change. Languages, they found, change slowly for a long time, and then undergo a sudden burst of change [6]—what biologists call punctuated equilibrium. These bursts seem to coincide with periods of linguistic speciation, when populations split and their languages diverge. Looking at trees of Indo-European, Austronesian, and Bantu languages, the researchers found that those languages that had gone through the most splits had changed more, with up to a third of changes being associated with split points. Pagel suggests that languages change when populations split because groups consciously or unconsciously use how they talk to define themselves and separate insiders from outsiders—as in the Old Testament book of Judges, when the men of Gilead identify their Ephraimite foes by their inability to pronounce the Hebrew word for an ear of grain, shibboleth, now a general term for a linguistic password.