Mace, R.

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UCL Discovery

PART II10-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 19710-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 198CHAPTER 10INTRODUCTION TO PART IION THE USE OF PHYLOGENETIC COMPARATIVE METHODS TO TEST CO-EVOLUTIONARY HYPOTHESESACROSS CULTURESRuth MaceIn 1994, Mark Pagel and I proposed that phylogenetic comparative methods wereappropriate, and indeed essential, for testing co-evolutionary hypotheses incultural or bio-cultural evolution (Mace and Pagel 1994). It is worth clarifying thatwe do not mean the comparative method as used by linguists (which means thecomparison of words to identify common origins); we use the term ‘comparativemethod’ as do evolutionary biologists, who use it to refer to comparison acrosspopulations or species to test an adaptive hypothesis. Comparative methods are one of three main techniques that behavioural andevolutionary ecologists use to test functional hypotheses (Krebs and Davies 1993).The others are experimental manipulations and optimality modelling, generallytested by examining the range of individual differences in behaviour andoutcomes in terms of reproductive success, or some correlate thereof. Theopportunities for experimental manipulations are limited in human studies.Furthermore, because so many cultural traits are (almost by deﬁnition) a propertyof a whole society rather than just an individual (see Chapter 1 this book), thereare few opportunities for examining individual variation. For these reasons, cross-cultural comparison is always going to be an important tool for testing functionalhypotheses. Before introducing the three cross-cultural comparative studies in this sectionof the book, I will clarify several common misunderstandings of comparativemethods that persist in the literature. ‘USE OF PHYLOGENIES BRINGS LOSS OF STATISTICAL POWER’Historically in anthropology, quantitative cross-cultural comparison consisted ofseeking simple correlations across cultures of the coincidence of a cultural traitand some ecological or other cultural trait. Such correlations frequently suggestsigniﬁcant relationships; but they have not been popular with mostanthropologists, contributing to the already strong suspicion that many culturalanthropologists have of any quantitative analysis. One reason for the suspicion isthat many of the supposedly functional associations that have been reported fromsimple cross-cultural correlations do not seem to be very meaningful.10-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 199If cultures arise from mother cultures by descent with modiﬁcation, thencultures cannot be considered independent data points – this is known as Galton’sproblem. Whether or not cultural evolution is tree-like was a central topic in Part1, and from here on I shall assume that, broadly, it is. Figure 10.1 illustrates two ofthe many possible scenarios for the evolution of nine historically related cultures,in which there is a perfect correspondence between the occurrence of Awith B andA’ with B’. Asimple chi-squared test might suggest a signiﬁcant association acrosscultures, but any p-value would be suspect and at grave risk of a Type I error. Thisis because neither the ﬁve cases of A occurring with B, nor the four cases of A’occurring with B’ are necessarily independent instances of A changing to A’facilitating the evolution of B into B’. That could have occurred on four separateoccasions, as in Figure 10.1b. Alternatively it might only have occurred once, as in200 The Evolution of Cultural DiversityFigure 10.1 Two possible historical reconstructions of the evolution of twotraits in nine, hierarchically related cultures (from Mace and Pagel 1994).10-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 200Figure 10.1a. Any ahistorical (non-phylogenetic) analysis would not distinguishbetween these two cases. Whereas one single case of B changing to B’ after Aturned to A’ is not strong evidence of A’ and B’ co-evolving, this sequenceoccurring four times independently is. In the latter case it is far less likely that thetwo traits arose either coincidentally or because of some common third variable.We need to make every attempt to distinguish between these two differenthistorical scenarios in order to test adaptive hypotheses. Phylogeneticcomparative methods do this.Some have argued that by forcing investigators to count only independentevents, rather than cultures, phylogenetic approaches make it harder to ﬁndsigniﬁcant relationships – that ‘statistical power is reduced’. It may be true thatsimple cross-cultural correlations return a higher frequency of signiﬁcantrelationships, but this is not evidence of greater statistical power. ‘Power’ is theprobability of detecting a true relationship when it exists. The point about simplecross-cultural correlations is that an unacceptable proportion of the signiﬁcantones are Type I errors: that is, the null hypothesis is in fact true. So, the complaintabout loss of statistical power is erroneous and confuses two very different things.Fiona Jordan and I (Chapter 11) provide an example of a study where the useof non-phylogenetic statistics would have returned several ostensibly signiﬁcantcorrelations that would not be correct. We use a method that uses generalised leastsquares in a phylogenetically controlled correlation of a continuously varyingtrait – in this case the sex ratio at birth in human populations. The sex ratio iscorrelated with various biological and cultural factors. Across-cultural correlationwould have suggested that gene-culture co-evolution could be occurring betweenmarriage system and sex ratio at birth; but a phylogenetic correlation suggeststhat this is in fact just an artefact of the clumping of both low sex ratios andbrideprice (which co-evolves with polygyny) in the African cultures: sex ratio atbirth and brideprice are not correlated. Thus, from these data (albeit rather crude,country-level data), there is no evidence of genes and culture co-evolving,unfortunately, as the idea was rather appealing. However, there is evidence thatsex ratio in adulthood is correlated with brideprice – males have to pay to getmarried when the adult sex ratio is high (females are more scarce). This suggestscultural evolution in response to the demographics of the population.‘CONTROLLING FOR PHYLOGENETIC INERTIA’Using comparative methods that take into account the phylogenetic relationshipsbetween cultures is sometimes described as controlling for phylogenetic inertia orphylogenetic constraints. Phylogenetic inertia implies that evolution is weak,slow, or for some reason cannot change a trait, with the result that all of thedescendants of a particular species, or culture, inherit a range of traits from theirancestor even if those traits actually no longer serve any adaptive function.However, the fact that members of a clade (related cultures with a commonancestor) all contain the same trait is not evidence of phylogenetic inertia: theymay all have the trait for very good adaptive reasons. Consider that mostIntroduction to Part II 20110-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 201mammals have ﬁve digits at the end of their forelimbs and hindlimbs. This is notbecause they couldn’t have it any other way, but very likely because it is useful:the evidence ironically, lies in the exceptions. Where ﬁve digits have been lost ormarginalised, such as in the grazing animals, it is because a smaller number isbetter. The existence of this variation shows that the retention of ﬁve digits in theothers is not attributable to inertia. Indeed it was probably one of the adaptationsthat contributed to the success of the mammals. So, if lack of variation in a trait isn’t evidence of inertia, why use comparativemethods? The reason has been given above in the discussion of independentevents. Lack of variation may indicate that some trait or traits have evolved onlyonce or a small number of times. To be sure that any apparent correlation betweenthem is real, it is necessary to identify independent events of their co-evolution.Phylogenetic comparative methods recognise when apparent correlation is likelyto be an artefact of two traits appearing to be correlated but for reasons other thanthe two being functionally linked.‘NOT ACCOUNTING FOR HORIZONTAL TRANSMISSION’Sometimes it is argued that phylogenetic comparative methods cannot be appliedto cultural evolution, because so much of cultural evolution arises from thehorizontal transmission of cultural traits. The ﬁrst part of the book reportsanalyses of the extent to which cultural transmission is vertical. These analysesshow that in a wide range of cases, despite some horizontal transmission, it hasbeen possible to see a phylogenetic signal in data on cultural traits. This is mostespecially so in the case of language (eg, Holden 2002; Gray and Atkinson 2003),although phylogenetic signals can also be found in other aspects of culture too.Language trees often show strong and reliable phylogenetic signals, indicating apreponderance of vertical transmission. Equally, language trees can beinformative about when the hypothesis of vertical transmission is unlikely tohold.Assuming that it is reasonable to make a phylogenetic tree, then any questionthat horizontal transmission of cultural traits invalidates phylogeneticcomparative studies evaporates. The tree means that one can ask whether somecultural traits move around the tree horizontally (jumping from one branch toanother) or vertically (moving down the tree branches from mother to daughterculture). This would be impossible without a tree.Changes in cultures can arise from de novo innovations, or from borrowing(horizontal transmission). Both are potentially of equal signiﬁcance in testing anadaptive hypothesis. Why re-invent the plough if you can copy the idea fromsome neighbouring population? Similarly, if you have no use for the plough, thenwhy would you bother to copy your neighbour? To test the hypothesis that theplough is associated with some other changes in the human societies that adopt it,it is not necessary or indeed sufﬁcient to restrict analyses to instances of theindependent invention of a tool used as a plough. If a society from some verydifferent branch of the phylogenetic tree in which the plough originally arose,202 The Evolution of Cultural Diversity10-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 202adopted the plough and then they acquired some other trait that is commonlyseen in the other plough-using cultures, then this provides some of the verystrongest evidence in favour of co-adaptation (some functional link between theplough and that other trait). Horizontal transmission is not ignored, nor does itinvalidate the phylogenetic comparative method; it provides the method withmuch of its power to understand cultural co-evolution.If it were true that most cultural traits were simply infectious, spreading likeviruses without any beneﬁt to the individuals or cultures that adopted them, thenphylogenetic comparative methods could be suspect in some circumstances. Ifvery few traits behave in this infectious way, then the chances that both of thetraits under consideration in a co-evolutionary hypothesis were spreading in thatway would be very small – so the chance of spurious correlations between twotraits would be low. However, if this kind of ‘cultural infection’ is common, thentaking the adoption of two traits as being of statistical relevance to a functionalhypothesis could be suspect, because both could move together simply due togeographic proximity even though they were not functionally related. But what reasons are there to believe that maladaptive cultural traits are soinfectious? Richerson and Boyd (2004) have pointed out that the Amish of NorthAmerica, who wish to preserve a way of life that in many ways is reminiscent ofthe 19th century, have isolated themselves culturally, to prevent the inﬁltration oftheir culture by modern American customs and technology; for example, they arenot allowed to watch television. However, the need to do this has probably arisenbecause the use of modern technology would make their lives easier and is thusappealing to those exposed to it. Other Americans exposed repeatedly to Amishculture do not seem in a great hurry to make their daily lives more arduous byforgoing their motor cars, not to mention buttons and zips. It has been argued thatsome memes do act more like viruses, in that they parasitise our cognitivestructures to reproduce themselves in hosts to whom they give no beneﬁt. Someforms of religious belief might fall into this category. But the evidence that mostcultural change is of this type is simply not there.WHAT TYPE OF COMPARATIVE METHOD TO USEThere are a number of different statistical comparative methods that have beendeveloped to test co-evolutionary hypotheses. Any attempt to take into accountthe hierarchical relationships between cultures is better than ignoring thisproblem entirely, but some methods are more powerful than others. Often thenature of the method used is constrained by the nature of the data available.Sometimes we may have historical information from archaeological or othersources that can tell us about extinct cultures and help inform our understandingof cultural change over time. But such information is usually very limited. Thenmethods have to infer the likely historical pattern of cultural changes statistically,from the distribution of cultural traits observed in extant cultures (or whateverdata is at the tips of the tree), and the tree. All comparative methods use someIntroduction to Part II 20310-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 203form of optimising criteria to do this. Most early methods used parsimony, that is,a method which seeks the smallest number of evolutionary changes that couldhave given rise to the distribution of traits observed on the tree. Parsimony isuseful, but of course evolution in general and cultural evolution in particular maynot always be parsimonious. Reversals, when a trait switches and then switchesback again, possibly even more than once along a branch of the tree, might becommon. Many recent studies in evolutionary biology are now using comparativemethods based on maximum likelihood (Pagel 1999a, 1999b). In these methods,the full range of possible evolutionary pathways are explored, and weightedaccording to how likely they are to have occurred, given their current distributionand the tree. These methods are powerful, and have only been possible toimplement since powerful computers became widely available. The methodsused in the chapters in Part II of the book all use maximum likelihood to testevolutionary hypotheses. These are not the only usable methods by any means,but we have chosen them for a number of reasons which make them especiallyuseful for testing hypothesis in cultural evolution.Table 10.1 summarises some of the differences between parsimony andmaximum likelihood for testing evolutionary hypotheses. In comparativemethods using likelihood, evolutionary models are much more explicit andprecise so that it is possible to calculate whether some models of evolution aremore likely than others to have given rise to the patterns of diversity that weobserve in our data. Explicit estimates of the rates and patterns of evolutionarychange can be generated. It is possible to test the direction of evolutionary change,as well as simply whether or not two traits are correlated. Ancestral states can bedescribed as probabilities rather than just as one or other condition. Whilst thecertainty of a single ‘answer’ may be appealing, it is usually spurious. Most204 The Evolution of Cultural DiversityTable 10.1 Differences between comparative methods based on parsimony andbased on maximum likelihood.Parsimony approaches Likelihood approachesIntuitive Not so muchComputationally quick More intensiveTreats ancestral states as known: Incorporates uncertainty: good when may underestimate the amount of  trait evolution is rapid. Probabilitychange. Single value assigned to each  distribution of character states at character at internal nodes internal nodesBranch length information may not  Branch length information used to be incorporated into model calculate rates of character transition All types of change treated as equally Transition-rate parameters estimate likely all kinds of changeDifﬁcult to detect order of change.  Model of evolutionary change is Simply seeks correlations explicit. Can detect order of change10-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 204ancestral states are uncertain, and the ability to estimate and take into account thatuncertainty is one of the most important advantages of the methods that uselikelihood.Clare J Holden and I (Chapter 12) use a method developed by Mark Pagel(1994) to analyse the co-evolution of discrete characters, in this case matrilineal orpatrilineal descent, and the presence or absence of cattle-herding. The likelihoodof a model in which the two traits evolve independently of each other is comparedwith the likelihood of a model in which the state of one trait inﬂuences the rate ofchange in the other trait; in other words, the two traits are co-evolving. We ﬁndthat the dependent model, in which one traits inﬂuences the rate of evolution inthe other, is signiﬁcantly more likely than the model in which the two traits evolveindependently. The adoption of cattle means that the culture concerned is morelikely to become patrilineal. In both Chapters 11 and 12, we use trees that are inferred using parsimonymethods, either from language data (Chapter 12) or from genetic data (Chapter11); and we consider the trees to be correct for the purposes of our analyses. Butthe comparative methods used to test for the co-evolution of cultural or bio-cultural traits on the tree use likelihood to evaluate the different evolutionarymodels. However, Mark Pagel and Andrew Meade (Chapter 13) describe a newdevelopment of the comparative method which not only incorporates theuncertainty in the evolution of the cultural traits being examined, but alsoconsiders the uncertainty in the tree itself, using MCMC and Bayesian statistics. Inthis case they examine the co-evolution of dowry and monogamy in a sample ofIndo-Europeans. Over 500 possible Indo-European language trees, all weightedaccording to their likelihood, are used in the comparative analysis, so the resultscannot be considered a product of just one tree that could be incorrect. Theexample given takes a simple dataset, so the results themselves need to beconsidered as preliminary, but they show a signiﬁcant correlation between dowryand monogamy, as predicted by evolutionary theory. But the important point hereis that many of the problems associated with the inevitable uncertainty overhistorical relationships between cultures can be overcome by taking thatuncertainty into account in the analysis. The analysis in this last chapterrepresents the cutting edge of the ﬁeld.All the methods used in this part of the book are readily available (seereferences in the relevant chapters) and do not require users to have particularcomputational or mathematical expertise to apply them. Ultimately, all thesecomparative methods will be evaluated by their ability to tell us new things aboutour evolution. Introduction to Part II 20510-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 20510-evolution-ch10-cpp.qxd  13/5/05  12:26 PM  Page 206

Introduction to Part II: on the use of phylogenetic comparative methods to test co-evolutionary hypotheses across cultures

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