Oceanic barriers promote language diversification in the Japanese Islands

Good barriers make good languages. Scholars have long speculated that geographical barriers impede linguistic contact between speech communities, and promote language diversification in a manner similar to the process of allopatric speciation. This hypothesis, however, has seldom been tested systematically and quantitatively. Here we adopt methods from evolutionary biology and attempt to quantify the influence of oceanic barriers on the degree of lexical diversity in the Japanese Islands. Measuring the degree of beta diversity from basic vocabularies, we find that geographical proximity and, more importantly, isolation by surrounding ocean, independently explains a significant proportion of lexical variation across Japonic languages. Further analyses indicate that our results are neither a byproduct of using a distance matrix derived from a Bayesian language phylogeny nor an epiphenomenon of accelerated evolutionary rates in languages spoken by small communities. Moreover, we find that ...