Abrupt and Gradual Sound Change in an Expanding Lexicon

The sound structure of language changes over time, but the process of change is not well understood. Sound change appears to occur abruptly as well as gradually, but it is not clear why, and it is not clear how the different rates of change affect the sound structures that emerge. This paper advances two hypotheses to answer these questions: (1) The Rate Hypothesis suggests that change occurs abruptly or gradually depending on how much an existing system is destabilized by social and cultural forces. (2) The Variation Hypothesis suggests that a greater diversity of sound structures emerge from abrupt change than gradual change because selection occurs on larger amounts of variation in more destabilized systems. These two hypotheses were tested in a computational model of sound change. The simulation results confirmed the hypotheses, and further suggested that abrupt change initially results in functionally suboptimal structure, whereas gradual change preserves good functionality. Overall, the study explains different rates of change in terms of a single framework and resolves a paradox in historical linguistics in which abrupt and gradual change are seen as incompatible, yet both exist.