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.

Testing methods of linguistic homeland detection using synthetic data

“…The only other simulation-based study looking at the performance of Bayesian phylogeography (but excluding other approaches from its purview) [22] similarly finds directed, migratory steps to be problematical. In this study simulations were specifically designed to look at the performance of the relaxed random walk and constant directional random walk models as implemented in BEAST 1.10.4 in a situation where languages have spread in a directional fashion versus a situation more akin to a random walk.…”

“…As often done in biology, e.g. [23], as well as in [22], we devise tests using simulations. Since it is not clear how biologists' tests of, say, the ability of a particular phylogeographical model to trace the origin of a virus [24] carries over to languages, we will not attempt a review of all the work done in biology.…”

“…The lack of performance tests of linguistic homeland detection methods motivates this paper. The only other work of a similar nature that we are aware of is a recent manuscript still under review [22]. As often done in biology, e.g.…”

Testing methods of linguistic homeland detection using synthetic data

“…The lack of performance tests of linguistic homeland detection methods motivates this paper. The only other work of a similar nature that we are aware of is a recent manuscript still under review [24]. Even the biological literature is found wanting in comparative performance tests of phylogeographic methods, as has been repeatedly pointed out [24][25][26].…”

“…The only other work of a similar nature that we are aware of is a recent manuscript still under review [24]. Even the biological literature is found wanting in comparative performance tests of phylogeographic methods, as has been repeatedly pointed out [24][25][26]. Tests have focused on technical aspects of individual methods, mainly as applied to epidemiology [27][28].…”