Both language and genes evolve by transmission over generations with opportunity for differential replication of forms1. The understanding that gene frequencies change at random by genetic drift, even in the absence of natural selection, was a seminal advance in evolutionary biology2. Stochastic drift must also occur in language as a result of randomness in how linguistic forms are copied between speakers3,4. Here we quantify the strength of selection relative to stochastic drift in language evolution. We use time series derived from large corpora of annotated texts dating from the 12th to 21st centuries to analyse three well-known grammatical changes in English: the regularization of past-tense verbs5,6,7,8,9, the introduction of the periphrastic ‘do’10, and variation in verbal negation11. We reject stochastic drift in favour of selection in some cases but not in others. In particular, we infer selection towards the irregular forms of some past-tense verbs, which is likely driven by changing frequencies of rhyming patterns over time. We show that stochastic drift is stronger for rare words, which may explain why rare forms are more prone to replacement than common ones6,9,12. This work provides a method for testing selective theories of language change against a null model and reveals an underappreciated role for stochasticity in language evolution.
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We thank H. Bacovcin, T. Kroch, and M. Liberman. R.C. acknowledges support from the University of Pennsylvania Research Foundation. J.B.P. acknowledges support from the David & Lucile Packard Foundation, the US Defense Advanced Research Projects Agency (D12AP00025), and the US Army Research Office (W911NF-12-1-0552).
The authors declare no competing financial interests.
Reviewer Information Nature thanks R. A. Bentley and the other anonymous reviewers for their contribution to the peer review of this work.
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Extended data figures and tables
Each panel shows the time series of a polymorphic verb (black lines), repeated from Fig. 2a, and the frequency of similar-sounding monomorphic regular (orange) and irregular (blue) verbs in the Corpus of Historical American English. The tokens included are all tenses of those lemmas that possess a pronunciation known to the Carnegie Mellon University Pronouncing Dictionary in both the lemma and the simple past tense. The list of verbs incorporated in each time series is given in Extended Data Table 2. For 17 polymorphic verbs we find no similar-sounding monomorphic irregular verbs (all-orange panels). The title of each panel indicates the sign of the maximum-likelihood selection coefficient, either regular → irregular or irregular → regular.
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Newberry, M., Ahern, C., Clark, R. et al. Detecting evolutionary forces in language change. Nature 551, 223–226 (2017). https://doi.org/10.1038/nature24455
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