What role do abstract representations play in language change? How do acquisition and usage interact to produce patterns of variation and change? How can acquisition and usage effects be detected in diachronic data? What role do frequency effects play in language change? What is the basic replicator in the cultural evolution of language? How can social network theory inform models of linguistic variation and change? What is the effect of individual lifespan change on variation and change on the level of the speech community? How can current mathematical and computational models of variation and change be scaled up towards more linguistic and sociological realism? Is a unified approach to language change, incorporating both a theory of representations and a theory of usage, necessary to fully explain variation and change? Is such an approach possible, in principle?