Tone and genes: New cross-linguistic data and methods support the weak negative effect of the “derived” allele of ASPM on tone, but not of Microcephalin

Abstract

While it is generally accepted that language and speech have genetic foundations, and that the widespread inter-individual variation observed in many of their aspects is partly driven by variation in genes, it is much less clear if differences between languages may also be partly rooted in our genes. One such proposal is that the population frequencies of the so-called “derived” alleles of two genes involved in brain growth and development, ASPM and Microcephalin, are related to the probability of speaking a tone language or not. The original study introducing this proposal used a cross-linguistic statistical approach, showing that these associations are “special” when compared with many other possible relationships between genetic variants and linguistic features. Recent experimental evidence supports strongly a negative effect of the “derived” allele of ASPM on tone perception and/or processing within individuals, but failed to find any effect for Microcephalin. Motivated by these experimental findings, I conduct here a cross-linguistic statistical test, using a larger and updated dataset of 175 samples from 129 unique (meta)populations, and a battery of methods including mixed-effects regression (Bayesian and maximum-likelihood), mediation and path analysis, decision trees and random forests, using permutations and restricted sampling to control for the confounding effects of genealogy (language families) and contact (macroareas). Overall, the results support a negative weak effect of ASPM-D against the presence of tone above and beyond the strong confounding influences of genealogy and contact, but they suggest that the original association between tone and MCPH1 might have been a false positive, explained by differences between populations and languages within and outside Africa. Thus, these cross-linguistic population-scale statistical results are fully consonant with the inter-individual-level experimental results, and suggest that the observed linguistic diversity may be, at least in some cases, partly driven by genetic diversity.

Rate variation in language change: Toward distributional phylogenetic modeling

“…For instance, it has been claimed that languages spoken in drier climates use fewer vowels (Everett 2017) and that the same pattern holds for languages spoken in colder climates (Maddieson 2018), due to the fact that dry air creates articulatory problems for phonation needed to produce vowels, and high temperatures degrade the high-frequency spectral information helpful in perceiving consonant clusters. In addition to environmental factors, genetic factors have been implicated in biases toward linguistic tone (Dediu 2021). Studies demonstrate robustly that tonal languages are spoken in regions of higher humidity (Everett et al 2015;Roberts 2018), due to the fact that lower jitter in fundamental frequency, a property of humid environments, makes it easier to stabilize fundamental frequency and exapt it for linguistic purposes.…”

Neighbours and relatives: accounting for spatial distribution when testing causal hypotheses in cultural evolution

“…We should therefore expect many different genetic variants to correlate with language variants. Genes associated with brain function show correlation with tonal languages (Dediu, 2021 ; Dediu & Ladd, 2007 ). So do mitochondrial genetic variants (Collins, 2017 ).…”

“…Instead, mitochondrial genes are common markers of population history which flow with the tide of people and cultures. Any test of the significance of association between genes and language must do so by comparison with the expected background level of covariation between genes, language and space, to discount the expected relationships between genes and language that come ‘for free’ from human population history (Barbieri et al, 2022 ; Dediu, 2021 ; Ladd et al, 2015 ).…”

Language follows a distinct mode of extra-genomic evolution

“…Most notably, changes in tooth use enabled new labiodental sounds (e.g. "f" or "v") [22,23] , and genetically-influenced changes in pitch processing decreased the use of pitch distinctions for meaning differentiation (known as linguistic tone) in some populations [24,25]. No other innovations have been reported for the faculty of language in reconstructible history.…”

“…But specific aspects of languages and specific aspects of human phenotypes have evolved within similar time depths. For example, words for 'leg' in Austronesian [46] (linguistic evolution) and aspects of pitch processing in Europe [25] (arguably genetically based, and thus biological evolution) apparently evolved within the same time span of roughly 10,000 years. The rate or speed of evolution is a property of the individual unit that evolves under a specific condition (with a lower bound set by the unit's generation time).…”