Generative accounts of loanword phonology typically focus on the role of an individual speaker's grammar and/or perceptual biases in generating the phonological adaptations seen in loanwords. While these types of generative models can be successfully used to explain static synchronic patterns in loanword adaptations in terms of the set of existing constraints in the native phonology, it is more difficult to use these models to explain changes in such patterns over time. In this paper I will propose a so-called "evolutionary" model of loanword borrowing which extends generative accounts by considering how the transmission of a loanword among the members of a speech community affects the establishment of adaptation patterns over time, allowing integration into the model of both the perceptual and social factors influencing adaptation patterns. I then use an agent-based implementation of the evolutionary model to model diachronic data on the adaptation of coronal obstruents before /i/ in Japanese loanwords from English, showing that the greater acceptability of [ti] and [di] sequences over time can be tied to the increase in contact with English speakers that took place during the late 19th and early 20th centuries.