The evolution of language families is shaped by the environment beyond neutral drift

Abstract

There are more than 7,000 languages spoken in the world today 1. It has been argued that the natural and social environment of languages drives this diversity 2-13. However, a fundamental question is how strong are environmental pressures, and does neutral drift suffice as a mechanism to explain diversification? We estimate the phylogenetic signals of geographic dimensions, distance to water, climate and population size on more than 6,000 phylogenetic trees of 46 language families. Phylogenetic signals of environmental factors are generally stronger than expected under the null hypothesis of no relationship with the shape of family trees. Importantly, they are also-in most cases-not compatible with neutral drift models of constant-rate change across the family tree branches. Our results suggest that language diversification is driven by further adaptive and non-adaptive pressures. Language diversity cannot be understood without modelling the pressures that physical, ecological and social factors exert on language users in different environments across the globe. Present-day linguistic diversity is non-randomly distributed across the globe, forming patterns at multiple levels. For example, more than 7,000 languages are currently spoken, and these can be classified into a few hundred language families 1. Each family contains (ideally) all-and only-descendants of a single ancestral protolanguage. Given that languages evolve through time in a manner similar to the evolution of biological species-through splits, extinctions and horizontal exchange-a language family can be approximated by a structured family tree (or phylogeny) that comprises a set of languages spoken by actual human groups occupying geographical space. An intriguing observation is that not only individual languages are non-randomly distributed across the globe; language families are too: some families are huge, spanning vast areas, while others are much more circumscribed. It has been proposed that this patterning reflects ancestral historical events and processes, such as demographic migrations and spreads, or language shift through elite dominance 14. Additionally, there is an emerging view that language diversification cannot be fully understood except in the wider context of physical, cultural and biological variation 15-17. A fundamental question, then, is why and how do language family trees unfold? Is linguistic diversification a self-contained process, or do pressures related to geographic and demographic dimensions drive diversification and shape language family trees? The classic view holds that explanations of diversity have to be sought 'first on the basis of recognized processes of internal change' 18. Here, 'internal' changes are either seen as a 'rather directionless pursuit of individual forms down the branches of the family tree' 19 or as regular phenomena such as sound change and analogy 19. Internal changes are often associated with the term 'linguistic drift' 20 , which

The ecological drivers of variation in global language diversity

“…In order to untangle causal connections from incidental associations, we need to account for sources of covariation in our data. In particular, we need to address spatial autocorrelation and phylogenetic non-independence 32–34 . Grid cells that are located near each other are likely to have similar values of climatic and landscape variables, contain related human cultures and languages, and share much of their flora and fauna.…”

“…Although previous studies have reported correlations between language diversity and a number of environmental or landscape variables, there has yet to be a comprehensive global analysis. Many of these variables co-vary and tend to be clustered in space and more similar between related languages 32–34 . Here, we present a global-scale analysis of language diversity that comprehensively deals with these statistical complexities.…”

Language evolution research in the year 2020: A survey of new directions

“…There is now increasing evidence that a large number of features that vary widely across languages, such as specific speech sounds (e.g., Everett, Blasi & Roberts, 2015;Moisik & Dediu, 2017) or lexical or semantic categories (e.g., Language Dynamics and Change 10 (2020) 3-26 Brown & Lindsey 2004;Regier, Carstensen & Kemp, 2016) might not just reflect arbitrary cultural 'choices' , but are rather motivated by and sensitive to the specific local physical, sociocultural or technological environment (see Lupyan & Dale, 2016 for an overview). While this kind of 'linguistic adaptation' was initially suggested due to observed correlations, such as the inverse relationship between morphological complexity and population size (Lupyan & Dale, 2010), such findings are now corroborated by phylogenetic evidence suggesting that the historical diversification of language families is better explained by environmental factors such as a climate rather than neutral drift (Bentz et al, 2018;Hua et al, 2019). An ambitious paper by Blasi et al (2019) illustrates just how rigorous and transdisciplinary more recent approaches to linguistic adaptation have become: Combined results from biomechanical modeling of bite configurations with phylogenetic analyses of language change and ethnographic data suggests that changes in diet during the neolithic revolution have affected the distribution of labiodental sounds such as [f] and [v].…”

Environmental conditions do not predict diversification rates in the Bantu languages

“…The distribution of present-day global language diversity has been shown to correlate strikingly well with several ecologically relevant variables, including climate and altitude (Bentz et al., 2018), biodiversity (Gorenflo et al., 2012; Harmon, 1996; Loh and Harmon, 2005; Moore et al., 2002; Nettle and Romaine, 2000; Sutherland, 2003), the distance from the equator (Bentz et al., 2018; Breton, 1991; Currie and Mace, 2012, 2009; Mace and Pagel, 1995; Nichols, 1999, 1990), and the length of the growing season (Nettle, 1998). A possible causal mechanism underlying these patterns was proposed by Nettle (1998, 1996), who explained a strong correlation between country-level language diversity and the length of the growing season in terms of the environmental risk hypothesis.…”