The relation between language change and the process of language evolution is controversial in current linguistic theory. Some authors believe that the two processes are completely unrelated, while for others the evolution of language is (at least in part) a consequence of linguistic changes. Both models imply a very different assessment of what is changing when languages themselves change. I present an explicit model of what changes when languages change, and I show that the claim that language change is a crucial factor in explaining the evolution of human language, although suggestive and very popular, faces problems of a theoretical and empirical nature.