Why do people speak different languages? The usual account is that languages diverge when an initial speech community disperses such that the language of the new speech communities begins to drift independently instead of together. However, it is becoming clear that many linguistic differences from vocabulary, to phonology, to grammar show a fit to the environment. Some aspects of vocabulary are predicted by factors as technological development, some aspects of phonology can be predicted from the climate in which the language is spoken, and some aspects of grammar can be predicted from the size of the language's population. This non-random variation calls for taking an adaptationist perspective on which languages adapt to the environments in which they are learned and used. I will discuss several examples of applying the perspective to linguistic differences and discuss some of the challenges that it faces. I will end by outlining some of the exciting new research questions that arise when we think about linguistic diversity through an adaptationist lens.