LANGUAGE EVOLUTION IN A NUTSHELL
I use the term language evolution to describe any of the following: 1) changes in the structural system of a language or dialect and in the pragmatic principles determining the felicity of its speakersí utterances; 2) the diversification of a language or dialect into different varieties; and 3) increase or decrease in the vitality of a language or dialect, including, in the latter case, experiences of endangerment or death. Aspects of language evolution apply to creole and non-creole languages alike. Contact plays a role in all such cases, provided one takes seriously the fact that idiolects are real and differ from one another even in the same dialect or household. (For a relatively more explicit discussion, click here for The Feature Pool Idea.)
A lot of changes happen simply through the efforts of speakers to accommodate each other. Many others occur through speaker-based innovations that spread within speech communities, and the spread itself occurs through the mutual accommodations of speakers. Assuming that languages are species, the real contacts that matter even in the case of the development of creoles are those occurring among the idiolects of individuals who interact with each other. One must really understand the dynamics of population behavior to understand how changes occur in a language or dialect as a communal phenomenon extrapolated from the coexistence of idiolects. For me, creoles are among the latest instantiations of language speciation under the specific ecological conditions of the European colonization of the rest of the world. These speciations are indeed reminiscent of the diversification of Vulgar Latin into Romance languages, of the evolution of English under different contact conditions, and of the speciation of Indo-European and Bantu languages (among a host of all conceivable language families and subfamilies). All these conditions involve some form of colonization and I invoke variation in colonization style to account for variation in kinds of population contacts, which produced different kinds of language restructuring. Aside from traditional cases of structural change and of the emergence of new varieties, I take "language evolution" to also subsume cases of language endangerment and loss, as explained in my book The Ecology of Language Evolution (2001, especially Chs. 6 & 7).