  Languages and genes are both transmitted from generation to generation, with
opportunity for differential reproduction and survivorship of forms. Here we
apply a rigorous inference framework, drawn from population genetics, to
distinguish between two broad mechanisms of language change: drift and
selection. Drift is change that results from stochasticity in transmission and
it may occur in the absence of any intrinsic difference between linguistic
forms; whereas selection is truly an evolutionary force arising from intrinsic
differences -- for example, when one form is preferred by members of the
population. Using large corpora of parsed texts spanning the 12th century to
the 21st century, we analyze three examples of grammatical changes in English:
the regularization of past-tense verbs, the rise of the periphrastic `do', and
syntactic variation in verbal negation. We show that we can reject stochastic
drift in favor of a selective force driving some of these language changes, but
not others. The strength of drift depends on a word's frequency, and so drift
provides an alternative explanation for why some words are more prone to change
than others. Our results suggest an important role for stochasticity in
language change, and they provide a null model against which selective theories
of language evolution must be compared.
