Language change and layered systems

Systems often achieve robustness and evolvability through the architectural principle of using large and thin layers---the layering allowing largely modular processing and the thin occupancy giving rise to a `digital' error-correcting capabilities. Innovations in the different layers that maintain this structure, and changes at the interfaces between the different layers, both allow evolution while maintaining function. These changes, when viewed from the perspective of a lower layer, appear as non-independent, coordinated, changes affecting the entire system. Human language displays such a structure where the the different layers---syntax, morphology, lexical tokens, phonemes, phones---are structurally largely modular, and affects language evolution strongly. For example, a change of mapping of phonemes to underlying phones ultimately gives rise to regular correspondences. Such coordinated changes need to be, and can be, explicitly modeled in reconstructing histories, or `phylogenies', of languages. Language can also be viewed as a distributed communication system, where agents constantly propose and adopt changes consistent with layering and maintaining communicative intent. At the lexical level, these ultimately lead to word innovation and replacement. The adaptive aspect of this diffusive process may be studied as a distributed optimization problem running on an underlying layer of semantic network that is shared by the agents. Such a view allows one to build a state-process model of language change that can be exploited both to study human languages and as a model for artificial adapting distributed communication systems.