The uses and abuses of tree thinking in cultural evolution

Abstract

Modern phylogenetic methods are increasingly being used to address questions about macro-level patterns in cultural evolution. These methods can illuminate the unobservable histories of cultural traits and identify the evolutionary drivers of trait change over time, but their application is not without pitfalls. Here, we outline the current scope of research in cultural tree thinking, highlighting a toolkit of best practices to navigate and avoid the pitfalls and ‘abuses' associated with their application. We emphasize two principles that support the appropriate application of phylogenetic methodologies in cross-cultural research: researchers should (1) draw on multiple lines of evidence when deciding if and which types of phylogenetic methods and models are suitable for their cross-cultural data, and (2) carefully consider how different cultural traits might have different evolutionary histories across space and time. When used appropriately phylogenetic methods can provide powerful insights into the processes of evolutionary change that have shaped the broad patterns of human history. This article is part of the theme issue ‘Foundations of cultural evolution'.

Historical, archaeological and linguistic evidence test the phylogenetic inference of Viking-Age plant use

“…[4,21]). However, they do not represent all aspects of language history, masking, in particular, the effects of horizontal transmission via language contact [8,9,44]. For example, in the North Germanic clade, Norwegian would historically cluster with Icelandic and Faroese as West Nordic varieties, but due to later close contact with East Nordic languages, it has now diverged from insular Nordic languages and converged with neighbouring Scandinavian varieties, Danish and Swedish [45].…”

“…Although contact effects like the historical convergence of Norwegian with other Scandinavian varieties are not adequately captured by any single tree, trees remain the best available general model for most biological and historical linguistic relations [46]. By taking a Bayesian approach and calculating the fit over a posterior probability distribution of trees rather than a single tree, any uncertainty in the tree topology caused by horizontal transmission will be partially incorporated into the trees in the posterior, and our results will include this uncertainty in the estimates of traits [8,47]. Importantly for our approach here, while linguistic and plant-use borrowings to and from neighbouring cultures are not retrievable with PCMs, we can supplement and update our inferences with broader linguistic and historical analysis.…”

“…Finally, environmental constraints should be accommodated in PCM analyses [8,9]. Positive inference of ancestral plant uses will always be concomitant to the plant's presence in the environment-as endemic, cultivated or traded.…”

The potential to infer the historical pattern of cultural macroevolution

“…Ideally, the tree will reflect diverse sources of data across history, linguistics, genetics and archaeology (see also [41]). (iii) Investigators should consider carefully how the trait of interest is transmitted, using ethnographic or historical materials where available, taking care to differentiate mechanisms of transmission for between and within populations; this will inform about the potential rate of change, and whether the horizontal transmission is likely to be prevalent (see also [88]). (iv) Investigators should explore how missing data, horizontal transmission and/or differential lineage extinction might influence their inferences for their particular sample (see our code: https://github.com/dieterlukas/Cul-turalMacroevolution_Simulation).…”

Foundations of cultural evolution

“…These methods can be very powerful and illuminating, yet their application is not without pitfalls. Evans et al [13] discuss the scope and the limitations of applying phylogenetic methodologies to cultural data, thereby providing recommendations for their appropriate use. In particular, the choice of phylogenetic methods and models requires a careful consideration of the nature of the traits to be studied, and the inclusion of multiple lines of evidence.…”