Does horizontal transmission invalidate cultural phylogenies?

Abstract

Phylogenetic methods have recently been applied to studies of cultural evolution. However, it has been claimed that the large amount of horizontal transmission that sometimes occurs between cultural groups invalidates the use of these methods. Here, we use a natural model of linguistic evolution to simulate borrowing between languages. The results show that tree topologies constructed with Bayesian phylogenetic methods are robust to realistic levels of borrowing. Inferences about divergence dates are slightly less robust and show a tendency to underestimate dates. Our results demonstrate that realistic levels of reticulation between cultures do not invalidate a phylogenetic approach to cultural and linguistic evolution.

Figure 1

The ‘true’ tree topologies used to synthesize the data, showing branch lengths and calibration points. (a) Tree 1 has a root age of 5111.14 years and (b) tree 2 has a root age of 5013.12 years.

Figure 2

Demonstration of the SDM and the resulting binary output matrix. Data synthesis begins at the root of the input tree and moves towards the tips. The process is assumed to have been running infinitely in the past, and has generated the first language, containing trait 1. Trait 2 is born into this lineage. This is followed by a divergence event where the extant traits (1 and 2) are copied directly into both new lineages (Proto-AB and Proto-C). After this divergence event, trait 3 is born in the Proto-AB lineage, followed by the death of trait 1 in the top lineage (denoted by a cross). Trait 3 is then borrowed (denoted by a dotted arrow) into lineage C. The last divergence event leads to lineages A and B, where trait 4 arises in lineage B, and becomes borrowed into lineage A (dotted arrow). Finally, trait 2 dies in lineage A. At the end of the simulation, the lineages have the traits at the tips (e.g. lineage A has traits 3 and 4, while B has 2, 3 and 4). This can be recoded into a binary matrix as shown, to represent the presence or absence of the traits.

Figure 3

Mean quartet distances between the true tree topologies and the estimated topologies for both topology types across all borrowing levels ((a) tree 1 and (b) tree 2). The dotted line marks the quartet distance of the unresolved topology and the cross marks the mean quartet distance of the no-borrowing scenario ((i) local 1000, (ii) local 3000 and (iii) global).

Figure 4

Mean reconstructed root time for each simulation under all borrowing scenarios ((a) tree 1 and (b) tree 2). The dotted line marks the true root age and the cross marks the root age under the no-borrowing scenario ((i) local 1000, (ii) local 3000 and (iii) global).