Literature

Two papers in Nature have documented patterns of change that have affected contemporary languages. "Both concern language change, and come from laboratories of well-established evolutionary theorists. Both analyse historical linguistic data to show that patterns of change depend strongly on the frequency with which words are used in discourse, as measured from large contemporary databases." The first paper finds that "verbs evolve and homogenize at a rate inversely proportional to their prevalence in the English language". The second paper looked at words more broadly and found that "less-frequently-used words evolved faster, and that frequency can explain half of the rate of evolution." Taken together, this is the same trend: "frequently used words are resistant to change".
As Fitch points out, "Documenting these relationships remains descriptive, not explanatory". Significantly, both these papers frame their reported research using evolutionary language and concepts. This is pointed out by Marris: "Both papers were written by teams with evolutionary biology backgrounds, and both call attention to the similarities between language change and the evolution of species. Leiberman even refers to early English as a "primordial soup" of verb forms in his paper." From a design perspective, we ask whether this is helpful or confusing.
The first thing we can say is that neo-Darwinism is not appropriate, because words are used by intelligent agents, and the changes occur because intelligent agents choose to change. Consequently, a more appropriate conceptual framework is provided by Intelligent Design.
Secondly, the changes being reported relate to vocabulary, not syntax or grammar. If we are looking for an analogy with Darwinism, it is with micro-evolution - with no significant change in complexity of the language. This is a relatively non-controversial area, where evolutionary theory and design theory often converge.
Third, the reported changes in regular/irregular verb usage are all in one direction: towards regularity. Thus, the language becomes simplified with time. Early languages are not like a "primordial soup" - they were more sophisticated than their descendants! Marris' report makes this clear: "Brian Joseph, a historical linguist at Ohio State University in Columbus and the editor of the journal Language, says that's going too far. Early English, he says, was "just as regular and rule-based" as modern language - it just had more complicated rules."
Consequently, whilst the reported research is interesting and thought-provoking, the Darwinian gloss is superficial. Stephen Pinker gets close with this comment: "The analogy with darwinian evolution is crude, although not useless".

Quantifying the evolutionary dynamics of language
Erez Lieberman, Jean-Baptiste Michel, Joe Jackson, Tina Tang & Martin A. Nowak
Nature 449, 713-716 (11 October 2007) | doi:10.1038/nature06137

Human language is based on grammatical rules. Cultural evolution allows these rules to change over time. Rules compete with each other: as new rules rise to prominence, old ones die away. To quantify the dynamics of language evolution, we studied the regularization of English verbs over the past 1,200 years. [snip] We study how the rate of regularization depends on the frequency of word usage. The half-life of an irregular verb scales as the square root of its usage frequency: a verb that is 100 times less frequent regularizes 10 times as fast. Our study provides a quantitative analysis of the regularization process by which ancestral forms gradually yield to an emerging linguistic rule.

Frequency of word-use predicts rates of lexical evolution throughout Indo-European history
Mark Pagel, Quentin D. Atkinson & Andrew Meade
Nature 449, 717-720 (11 October 2007) | doi:10.1038/nature06176

[snip] Here we use four large and divergent language corpora (English, Spanish, Russian and Greek) and a comparative database of 200 fundamental vocabulary meanings in 87 Indo-European languages to show that the frequency with which these words are used in modern language predicts their rate of replacement over thousands of years of Indo-European language evolution. Across all 200 meanings, frequently used words evolve at slower rates and infrequently used words evolve more rapidly. This relationship holds separately and identically across parts of speech for each of the four language corpora, and accounts for approximately 50% of the variation in historical rates of lexical replacement. We propose that the frequency with which specific words are used in everyday language exerts a general and law-like influence on their rates of evolution. Our findings are consistent with social models of word change that emphasize the role of selection, and suggest that owing to the ways that humans use language, some words will evolve slowly and others rapidly across all languages.

See also:

Marris, E., How 'holp' became 'helped', news@nature.com, 10 October 2007 | doi:10.1038/news.2007.152

Fitch, W.T. Linguistics: An invisible hand, Nature 449, 665-667, (11 October 2007) | doi:10.1038/449665a