Phylogenetics, Cultural Evolution and Horizontal Transmission

ResearchBlogging.orgFor some time now, evolutionary biologists have used phylogenetics. It is a well-established, powerful set of tools that allow us to test evolutionary hypotheses. More recently, however, these methods are being imported to analyse linguistic and cultural phenomena. For instance, the use of phylogenetics has led to observations that languages evolve in punctuational bursts, explored the role of  population movements, and investigated the descent of Acheulean handaxes. I’ve followed the developments in linguistics with particular interest; after all, tracing the ephemeral nature of language is a daunting task. The first obvious road block is that prior to the invention of writing, the uptake of which is limited in geography and history, language leaves no archaeological record for linguists to examine. One particular note I’d like to make is that when Charles Darwin first formulated his theory of natural selection, he took inspiration from linguistic family trees as the basis for his sketch on the evolutionary tree of life. So it seems rather appropriate that phylogenetic approaches are now being used to inform our knowledge regarding linguistic evolution.

Like many other attempts applying evolutionary thinking in culture, phylogenetic approaches are, at times, met with contempt. This stems from assertions that cultural evolution and biological evolution differ greatly in regards to the relative importance of horizontal transmission, as evinced in these two quotes:

The course of organic evolution can be portrayed properly as a tree of life, as Darwin has called it, with trunk, limbs, branches, and twigs. The course of development of human culture in history cannot be so described, even metaphorically. There is a constant branching-out, but the branches also grow together again, wholly or partially, all the time. A branch on the tree of life may approach another branch; it will not normally coalesce with it. The tree of culture, on the contrary, is a ramification of such coalescences, assimilations, or acculturations – Alfred Kroeber (1948, pg. 138).

Human cultural evolution proceeds along paths outstandingly different from the ways of genetic change. Trees are correct topologies of biological evolution. In human cultural evolution, on the other hand, transmission and anastomosis are rampant. Five minutes with a wheel, a snowshoe, a bobbin, or a bow and arrow may allow an artisan of one culture to capture a major achievement of another – Stephen Jay Gould (1987, pg. 70).

Both Gould and Kroeber provide an interesting entry point into some of the criticisms of phylogenetics. Cladistic (phylogenetic) theories approach the problem of modelling culture from a historical perspective: languages, cultures and populations are primarily derived from a parent group. A visual representation of this would be a bifurcating tree. However, what the above quotes emphasise is a rhizotic (reticulate) approach, in which the focus is more geared towards a non-treelike representation of transmission: here, linguistic and cultural features are shaped by several different, antecedent groups. In considering these different mechanisms of transmission we are faced with two broad themes. The first thought is in regards to the relative influence of horizontal and vertical forms of transmission. Is there an overarching form of transmission that dominates the variation found in cultural and linguistic features? Or do we need to examine this on a case by case basis? So, for instance: some cases are dominated by vertical transmission, whilst other cases are dominated by horizontal transmission. Secondly, even if there is a reasonable level of horizontal influence, then does this invalidate the use of cultural phylogenies?

Addressing these issues are two papers. The first provides an overview of the relative importance of both horizontal and vertical intergroup transmission in human culture, and then applies these findings to chimpanzee cultural diversity. In the second, the authors test the robustness of phylogenetic inferences in regards to horizontal transmission.

The first paper by Lycett, Collard & McGrew (2009) uses data from long-term field studies to investigate behavioural differences among groups of wild Pan troglodytes. A revealing aspect of this paper, and testament to the power of phylogenetic methods, is their use of cladistics to shed light on three sets of hypotheses: 1) that genetics explains the behavioural diversity seen in chimpanzees; 2) evaluating “the importance of vertical intergroup transmission in the evolution of chimpanzee diversity” (my emphasis); 3) to investigate whether chimpanzee culture is adaptive. I’m not going to discuss points (1) and (3), mainly due to limitations of space, time and relevance, but suffice to say that having established the first point can’t account for chimpanzee behavioural variation, the authors move onto investigating the role of vertical intergroup transmission. To do this, they carried out two analyses:

First, we reasoned that if vertical intergroup transmission has been the dominant process, the multiregion MP cladogram should be statistically indistinguishable from a cladogram in which the eastern and western populations form separate clades. Next, we first used the RI [Retention Index] to assess how tree-like are patterns in comparative samples of human cultural data sets and biological data sets. Given that the biological data sets can be confidently assumed to have been structured by speciation, which is a branching process, our rationale was that if the human cultural and the biological RIs are not significantly different, then it is reasonable to conclude that the human cultural data sets have been structured by vertical intergroup transmission. We also reasoned that if the chimpanzee cultural RIs fall within the range of human cultural RIs, then whichever process is found to have structured the human cultural data sets is likely to have structured the chimpanzee data sets.

They found the first analysis revealed no significant differences between the multiregion MP cladogram and a cladogram in which eastern and western populations form separate clades. Furthermore, the returned RIs show human cultural data sets are not significantly different to biological data sets. In fact, the authors note that:

[...] the fit between the bifurcating tree model and the human cultural data sets is little different from the fit between the bifurcating tree model and the biological data sets. Not only are the averages similar, but also the ranges are comparable. The RIs for 25 human cultural data sets range from 0.42 to 0.80. The mean RI for the human cultural data set is 0.60. The RIs for the biological data sets range from 0.35 to 0.94. Their mean RI is 0.61. Thus, on average, the human cultural data sets appear to be no more reticulate than the biological data sets.

Thus, based on the aforementioned data sets, it appears vertical intergroup transmission is more salient in generating human cultural diversity than horizontal intergroup transmission. This also appears to be the case for the chimpanzee data sets, with both the multiregion chimpanzee cladogram (RI: 0.44) and the regional chimpanzee cladogram (RI: 0.53) falling within the range of RIs produced by the human data sets. As you can see, the regional chimpanzee RI is close to the mean in the human data sets. All of this suggests a tree-like appearance for chimpanzee and human cultural data sets, and supports the notion that cultural behaviour is primarily the product of vertical intergroup transmission. Of course, as the authors note: if we look closely at the human data sets, then there are individual instances where horizontal transmission is the dominant evolutionary process. One example being the transmission of basketry traditions amongst Californian Indians. Still, it appears in the majority of cases, the dominance of horizontal transmission is the exception rather than the rule. Time will tell whether these observations hold up to further scrutiny; as more experiments into the transmission of cultural variants allow us to produce larger analyses of the data sets.

The second paper by Greenhill, Currie & Gray (2009) asks two specific questions relating to phylogenetics and culture: 1) What are the effects of different levels of horizontal transmission on the accuracy of phylogenetic estimates? 2) Are the problematic levels of horizontal transmission actually representative of real situations? Another major point in this paper is the use of Bayesian inference in phylogenetics. Basically, it’s the use of a likelihood function as a method of phylogeny estimation in generating a posterior distribution for the parameter (the phylogenetic tree and a model of evolution). This is in contrast to Maximum Parsimony which, besides being non-parametric, is not statistically consistent.

Using computer models, the authors simulate the evolution of languages under a natural model of language change:

First, we generated data by simulating the evolution of linguistic traits on two different tree topologies under varying degrees of horizontal transmission. Then, we evaluated how horizontal transmission affects the ability of these methods that do not account for horizontal transmission to recover the ‘true’ tree topology. Finally, we explored how borrowing affects inferences taken from the tree structure by attempting to estimate the age at the root of the trees, and how this varies from the root age of the true trees.

The general conclusions of the paper is that phylogenetic inference is surprisingly robust, even when high levels of borrowing are introduced. This is based on instances of a local borrowing scenario (where horizontal transfer takes place between geographical neighbours) over a range of 0-30 per cent of unidentified borrowing. Under these conditions, there are few differences between true and estimated tree topologies. Discrepancies do arise in the root time estimates, with an increased amount of borrowing tending to yield younger age estimates. This is important if your hypothesis is dependent on the phylogenetic dating of historical events:

For example, there has been considerable controversy over the use of phylogenetic methods to infer the age of Indo-European language family [...] Gray & Atkinson support an older farming-based dispersal from Anatolia ca 8500 BP rather than the ‘Kurgan’ hypothesis that dates this family to ca 6000 years BP. Our results suggest that if unidentified borrowing has affected these divergence time estimates, then the real age may be older than that suggested by Gray & Atkinson, making the Kurgan hypothesis even less probable.

They also explore how the shape of tree topologies offer different levels of robustness. For instance, a tree with a ‘balanced’ topology — in which most nodes reflect an equal number of offspring cultures and have relatively long internal branches — are more robust to the effects of borrowing than a tree composed of shorter internal branches and a chained pattern of descent (an unbalanced topology). Lastly, some types of borrowing will adversely affect the trees. The authors identify two major categories of borrowing: non-systematic and systematic. Simply put, in a non-systematic scenario the levels of borrowing increases the amount of noise found in the data. However, this is not enough to introduce any systematic bias. An example here would be the English language borrowing the word ‘taboo‘ from Tongan, where the influence is relatively minute in shaping the historical trajectory of English. A systematic scenario then, is when systematic biases are introduced into the data. An extreme example of this is the Oceanic language Yapese, which has several different sources of vocabulary. As the authors note:

This systematic borrowing will perturb the topology by drawing the interacting languages together, making them appear to be more similar. This will have the further effect of making any time-depth estimates shallower. However, these types of borrowing tend to occur within a small subset of the taxa being examined and will not necessarily affect other parts of the tree or the broader scale inferences.

I guess the general points to take away from this post are: 1) Do not necessarily assume horizontal transmission is dominant in shaping culture; and, 2) Even with certain levels of reticulation, it does not necessarily invalidate a phylogenetic approach in investigating cultural and linguistic evolution.

Citations:

Lycett SJ, Collard M, & McGrew WC (2009). Cladistic analyses of behavioural variation in wild Pan troglodytes: exploring the chimpanzee culture hypothesis. Journal of human evolution, 57 (4), 337-49 PMID: 19762070

Greenhill, S., Currie, T., & Gray, R. (2009). Does horizontal transmission invalidate cultural phylogenies? Proceedings of the Royal Society B: Biological Sciences, 276 (1665), 2299-2306 DOI: 10.1098/rspb.2008.1944