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'.

Keywords: cross-cultural research; cultural evolution; cultural macro-evolution; phylogenetic comparative methods.

Figure 1.

Phylogenetic methods that can be used to study cultural macro-evolution. Black arrows indicate that the preceding methodological steps are directly incorporated in later methods: (a) tree construction [5] is required for all subsequent steps; (b) testing for phylogenetic signal (e.g. [–8]) forms an integral part of phylogenetic regression (e.g. [–11]), which in turn forms the basis of phylogenetic path analysis which can identify causal relationships; (c) ancestral state reconstruction (e.g. [12]), estimated in conjunction with rates of trait change and transformation (e.g. [13,14]), is required for models of trait correlation [–17] and diversification ([18,19]; but see [20]). Red arrows indicate that suitable tests of phylogenetic signal (i.e. that the trait data fit sufficiently to the history inferred by the tree) should be conducted by the researcher before using methods detailed in (c); (see also §2). Shading: grey shading indicates methods that both assume and require inferred historical relationships between the cultural units (tree taxa) to sufficiently reflect the history of the trait; green shading denotes methods that detect and quantify tree-like structure in cross-cultural data; blue shading denotes methods that detect and control for tree-like data structure among societies, but do not require it.

Figure 2.

Incomplete lineage sorting in language evolution. The table in (a) shows words for ‘sun’ in Indo-European languages along with intermediate stages, with two different forms for nominative and genitive case in Indo-European (similar to the irregular plural of mouse/mice in English), reflecting linguistic variation, and two independent word forms postulated for Proto-Germanic, reflecting sociolinguistic variation; (b) shows the basic linguistic change scenario by which either the nominative or the genitive form is preferred as a base form in complex paradigms; (c) shows a parsimonious but wrong language tree for extant languages, and (d) shows a reconciled scenario.

Figure 3.

Histogram showing the phylogenetic fit of the lexical and grammatical/structural features of the languages from Greenhill et al. [73] as measured by the delta (δ)-score [50,92]. The grammatical/structural data show higher δ-scores, indicating a far worse fit to the overall phylogeny of Austronesian languages. The networks inset demonstrate the conflicting signal in these data visually, with the lexicon having a more tree-like pattern with fewer conflicts. (Figure is taken from [73] with permission.)