Interindividual vocal tract diversity influences the phonetic diversification of spoken languages

Human vocal tracts, like other physical traits, exhibit extensive interindividual variation. However, little is known about how this variation translates into phonetic diversity between speakers, and ultimately, across languages. Here we develop a biocultural model of phonetic evolution, analogous to Russell Lande's model of multivariate phenotypic evolution. We show that individual vocal tract morphologies and associated articulatory strategies leave consistent acoustic signatures, resulting in a characteristic pattern of phonetic variation within speaker groups that correlates with the pattern observed between groups. Recalling a central tenet of evolutionary biology-that within-group variation feeds between-group diversification-our findings suggest that phonologically similar vowel systems can attain phonetically diverse realizations according to a neutral-like evolutionary process. Our model serves as a null hypothesis for disentangling the biological versus cultural mechanisms underlying phonetic diversification, where deviations from the neutral expectation may indicate culturally mediated language change.