Introduction

This paper describes a precise sense in which language change can be regarded as a form of evolution – not of the language itself, but of the individual words which constitute the language. Many prominent features of language can be understood as the result of this evolution. In a unification-based theory of language, each word sense is represented in the brain by a re-entrant feature structure, which embodies the syntax, semantics and phonology of the word. When understanding or generating a sentence, we unify together the feature structures of the words in the sentence to form a derivation structure. The feature structure for any word can then be learnt by feature structure generalization, which complements unification to replicate the word feature structure precisely in a new mind. By this replication, word feature structures propagate precisely from one generation to the next, just as DNA propagates precisely in cell replication. The precision and transparency of DNA replication underlies the structure and diversity of life. Similarly, the precise and transparent replication of word memes underlies the structure and diversity of language. As word feature structures propagate from generation to generation, they undergo slow changes from selection pressures which cause many types of language regularities. In this analogy, each language is an ecology and each word is one species in the ecology. In language as in nature, different species exert selection pressures on one another.