Can Bayesian phylogeography reconstruct migrations and expansions in human history?

Abstract

Bayesian phylogeography aims to reconstruct migrations in evolutionary processes. This methodological framework has been used for the reconstruction of homelands and historical expansions of various language families, but its reliability for language diversification research has remained unclear. We contribute to this discussion with a simulation study where we distinguish two types of spatial processes: migration and expansion. By migration we denote long-distance movement of whole populations, leaving their previous habitat empty. Expansions are small-scale movements of speakers or inclusions of new speakers into the language community, cumulatively contributing to a gradual spread into new territories. We simulate migrations, in the form of directional random walks, and expansions, in the form of a grid-based region-growing process. We run both simulation scenarios with varying degrees of directional trends and evaluate the performance of state-of-the-art phylogeographic methods. Our results show that phylogeography fails to reconstruct migrations, but works surprisingly well on expansions, even under severe directional trends. We demonstrate that migrations and expansions have typical phylogenetic and spatial patterns, which in the one case inhibit and in the other facilitate phylogeographic reconstruction. Furthermore, we propose descriptive statistics to identify whether a real sample of languages (Bantu), their relationship and spatial distribution, better fits a migration or an expansion scenario. Bringing together the results of the simulation study and theoretical arguments, we make recommendations for judging the adequacy of phylogeographic models to reconstruct the spatial evolution of languages.