CHAPTER 13 Evolutionary Biological Methods and Cultural Data

MARK COLLARD, STEPHEN SHENNAN, BRIGGS BUCHANAN, AND R. ALEXANDER BENTLEY

As BENTLEY ET AL. note in chapter 8 of this volume, reproductive success or primarily through phenotypic Darwinian approaches to archaeology are rapidly plasticity. The former simply involves some individu­ gaining popularity, fueled in part by the recognition als outreproducing others; the latter is the ability ofor­ that cultural evolution can be fruitfully studied by ganisms to produce different phenotypes in response adapting the analytical tools that evolutionary biolo­ to different environmental conditions (Pigilucci 1996). gists and paleobiologists use to study biological evolu­ The schools of thought also differ over the role played tion. The goal of this chapter is to illustrate how two by population-level phenomena in cultural evolution. of these methods, population genetics modeling and These differences impact the way proponents of the cladistic techniques of phylogenetic reconstruction, three schools of thought envisage the relationship can be used to explain temporal and geographic varia­ between cultural evolution and genetic evolution, and tion in cultural phenomena as a result oftransmission, also affect their attitude toward adaptively neutral and innovation, and selection. We will begin by discussing maladaptive cultural behaviors. the conceptual foundations ofthis novel approach. We The central tenet ofevolutionary archaeology is that will then outline some of the ways in which models artifacts are as much a part of the human phenotype rooted in population genetics theory have been used as are bones, muscle, and skin (Dunnell 1989; Leonard to investigate cultural evolution. Lastly, we will discuss 2001; Lyman and O'Brien 1998; O'Brien and Holland some cultural evolutionary applications of the cladis­ 1995). The corollary of this is that evolutionary ar­ tic method of phylogenetic reconstruction. chaeologists seek to describe the archaeological record using classes amenable to evolutionary explanation CONCEPTUAL FOUNDATIONS (Cochrane 2001; Dunnell 1978, 1995; Tschauner 1994) Proponents of three schools of thought have applied and then account for the class distributions in terms of population genetics modeling and cladistic techniques what they consider to be the primary mechanisms of to cultural data. The first is evolutionary archaeology, evolution, namely natural selection and drift (Abbott which is also occasionally referred to as selectionist et aI. 1996; Braun 1987; Dunnell and Feathers 1991; archaeology (Abbott et aI. 1996; Dunnell 1980, 1989; Leonard 2001; Lyman and O'Brien 1998; O'Brien and Leonard and Jones 1987; Leonard 2001; Lipo et aI. Lyman 2000; Ramenofsky 1995). 1997; Lyman and O'Brien 1998; Neff 1992; O'Brien Explaining what evolutionary archaeologists mean and Holland 1995; O'Brien and Lyman 2000, 2002, by the terms "natural selection" and "drift" is not easy 2003a; Ramenofsky 1995). The second is gene culture because the definitions they have put forward differ in coevolution theory or dual inheritance theory (Ames subtle but potentially important ways. This point can 1996; Bettinger 1991; Boyd and Richerson 1985, 1993; be illustrated by comparing the definitions of natural Cavalli-Sforza and Feldman 1981; Durham 1979, 1982, selection and drift provided by Wilhelmsen (200 1) and 1990, 1991, 1992; Pulliam and Dunford 1980; Richer­ O'Brien and Lyman (2000), which seem to represent the son and Boyd 1992; Runciman 2002; Shennan 1991, extremes of a range ofviews. According to Wilhelmsen 2000,2002). The third is known as human behavioral (2001), natural selection is "the differential reproduc­ ecology (Borgerhoff Mulder 1991; Cronk 1991; Shen­ tion of organisms which, at the scale of population, nan 2002; Smith 2000; Smith and Winterhalder 1992; results in a directional change in trait frequencies over Smith et aI. 2001; Winterhalder and Smith 2000). time that correlates to external environmental condi­ These schools of thought differ over whether cultural tions" (p. 99), and drift is the differential reproduction evolution is effected primarily through differential of organisms as a result of "sampling error and the

203 stochastic patterns oftransmission" (p. 100). According where the forms are artifacts and the increase is brought to O'Brien and Lyman (2000), selection is the "process about through cultural transmission. In other words, it by which certain forms in a population that are better is possible that some evolutionary archaeologists take adapted to a particular environment increase in pro­ seriously the possibility that change in the archaeologi­ portion to less well-adapted forms" (p. 404), and drift cal record may be the result of phenotypic plasticity as is "random changes in trait frequency in a population well as differential reproductive success rather than just resulting from the vagaries of transmission" (p. 399). the result ofthe latter. In keeping with this, O'Brien and One of the differences between the definitions is that Lyman (2000) have noted that explanations of artifact Wilhelmsen's (200l) focuses on "organisms" whereas class distributions and human biological reproduction O'Brien and Lyman's (2000) definition of selection may be generated at different analytical scales. Likewise, and, by extension, their definition of drift, focuses on Leonard and Jones (1987; Leonard 200l) have drawn a "forms." The other difference concerns the mechanisms distinction between the replicative success of cultural of change involved in selection and drift. Wilhelmsen's traits and the reproductive success of individuals, and (2001) definition ofselection explicitly links it with dif­ argued that the former should not be assumed to be ferential reproduction. That is, his definition suggests reducible to the latter. that in the case ofselection, changes in trait frequencies However, having reviewed a good deal of the work over time are the result of some individuals producing that has been produced by evolutionary archaeologists more offspring than others. In contrast, O'Brien and over the past thirty years, we are ofthe opinion that the Lyman's (2000) definition of selection does not men­ differences between the definitions offered by Wilhelm­ tion differential reproduction. Indeed, no mechanism sen (2001) and O'Brien and Lyman (2000) are notin fact of change is specified. Selection is suggested to involve significant. O'Brien and Lyman (2000) and most other better adapted forms increasing in frequency at the evolutionary archaeologists normally dismiss the possi­ expense of less well-adapted forms, but the manner in bility ofchange occurring as a consequence ofindividu­ which this occurs is not specified. The situation is simi­ als copying behaviors and then modifying them in light lar with drift; Wilhelmsen's (200l) definition explic­ ofexperience before passing them on. They also usually itly links drift with differential reproduction, whereas treat transmission as if it occurs solely from parents to O'Brien and Lyman's (2000) definition does not men­ offspring, ignoring the various other forms of cultural tion differential reproduction. Rather, for O'Brien and transmission, which, as noted earlier, can in principle Lyman (2000) drift is the consequence of the "vagaries lead to the oblique and horizontal movement ofcultural oftransmission." traits among individuals and populations in the absence These differences are potentially important because, of gene flow (e.g., Abbott et al. 1996; Leonard 2001; together, they bear on the way in which evolutionary O'Brien and Lyman 2000; Ramenofsky 1995). Thus in archaeologists conceptualize the relationship between practice, evolutionary archaeologists treat cultural evo­ genetic evolution and cultural evolution. By linking lution as if it is effected primarily through differential selection directly with differential reproduction of or­ reproductive success, and therefore do not allow for the ganisms, Wilhelmsen's (2001) definitions of natural se­ possibility that genes and culture may have different lection and drift imply that culture evolves in exactly evolutionary dynamics. Significantly, our characteriza­ the same manner as genes, and that cultural evolution tion ofevolutionary archaeology seems to be in line with is therefore reducible to genetic evolution. In contrast, that of O'Brien and Lyman (2002:35), who note that O'Brien and Lyman's (2000) focus on forms rather than whereas "some behavioral ecologists argue that much of organisms and their decision to leave the mechanism of the change we see archaeologically is attributable to phe­ change unspecified opens up the possibility that O'Brien notypic plasticity ... most evolutionary archaeologists and Lyman (2000) recognize that genetic evolution and would argue that much ofit is attributable to selection." cultural evolution involve different mechanisms and In contrast to evolutionary archaeology, dual inher­ therefore potentially have different dynamics. Thus it itance theory treats cultural evolution as ifit is primar­ is possible that O'Brien and Lyman (2000) and some ily effected through phenotypic plasticity rather than other evolutionary archaeologists consider selection to differential reproductive success. In dual inheritance involve not onlyincrease in relative frequency ofadapted theory, genes and culture are viewed as two distinct forms where the forms are organisms and the increase but interacting systems of information transmission is brought about through differential reproduction, but (e.g., Ames 1996; Bettinger 1991; Boyd and Richer­ also increase in relative frequency of adapted forms son 1985, 1993; Cavalli-Sforza and Feldman 1981;

204 MARK COLLARD ET AL. Durham 1979, 1982, 1990, 1991, 1992; Pulliam and tion (figure 13.1). Multiple models ofbiased transmis­ Dunford 1980; Richerson and Boyd 1992; Runciman sion have been developed (Boyd and Richerson 1985; 2002; Shennan 1991,2000,2002). They both involve Richerson and Boyd 1992). The main ones are direct the transmission of phenotype-influencing informa­ bias, indirect bias, and frequency dependent bias. In di­ tion but operate via different mechanisms. The genetic rect bias, individuals evaluate alternative behaviors and system is based on reproduction, while the cultural choose among them. In indirect bias, individuals use one involves social learning. With this difference in some traits, such as those connoting health or prestige, mind, dual inheritance theorists hold that genetic evo­ to choose a cultural model, and then copy a range of lution and cultural evolution are similar in that they the model's behaviors. In frequency dependent bias, are both based on the process that Darwin referred an individual copies a behavior on the basis of its fre­ to as descent with modification, but they also accept quency in the population; the most common form is that the nature of social learning is such that cultural thought to be conformist bias, which involves copying evolution is influenced by forces that have no obvious the behavior that is most widespread in the population. equivalents in genetic evolution. Most notably, indi­ Significantly, guided variation and the various forms viduals can choose to copy practices from non-kin, of biased transmission can, in principle, lead cultural and they are also able to modify or discard practices evolution to be much faster than genetic evolution in the light of experience. The significance of these because they can affect the relative frequency ofdiffer­ processes is that cultural evolution cannot be assumed ent cultural traits within a generation. In addition, the to be always in step with genetic evolution. Sometimes various forms of biased transmission can potentially it will be, but frequently it will not. Our ability to learn lead to the "horizontal" or "oblique" movement ofcul­ from non-kin means that cultural patterns will often tural traits between individuals and populations in the not coincide with genetic patterns. Likewise, our abil­ absence of gene flow. The corollary of this is that dual ity to learn from other individuals and to pass on those inheritance theory anticipates that the distributions of behaviors to yet other individuals throughout our lives cultural traits and genes through time and space may means that cultural evolution will often be faster than differ in certain circumstances. genetic evolution. Dual inheritance theory even allows As the above quote from O'Brien and Lyman (2002) for the possibility that the transmission of some cul­ indicated, human behavioral ecology also treats cul­ tural traits might be maladaptive from a genetic point tural evolution as if it is primarily effected through of view (Boyd and Richerson 1985; Cavalli-Sforza and phenotypic plasticity. Linked to a substantial body of Feldman 1981; Shennan 2002). work in evolutionary biology (Krebs and Davies 1993), With respect to the mechanisms involved in cul­ human behavioral ecology seeks to explain human be­ tural evolution, dual inheritance theorists foreground havior in terms of its ongoing adaptive significance guided variation and biased transmission. Guided varia­ (Borgerhoff Mulder 1991; Cronk 1991; Smith 2000; tion is the name given to the process in which one individual copies the behavior of another individual, 100% modifies the copied behavior in the light of experi­ Adoption of new behavior ence, and then passes on the modified behavior to a 80% third individual. Biased transmission is the term used to refer to the process of model selection that dual c: 'tilo 60% :; inheritance theorists believe most individuals engage a. in when modifying their existing behaviors by copying ~ 40% '0 others rather than relying entirely on their own experi­ ~ o 20% ence. That is, dual inheritance theorists contend that individuals do not normally copy other individuals at random, but instead select models and behaviors on Time the basis of some criterion. Henrich (2001) has argued Figure 13.1. An S-curve, a theoretical plot of the shift of that this form oftransmission is probably the basis for one behavior to another through time. It begins with the first the S-curve that is characteristic of the diffusion of in­ instances of the new behavior, followed by an acceleration novations in human populations, in which adoption in the rate of change, gradual slowdown during the middle phases as the new behavior becomes the majority, and ends in is slow at first, rises to a peak, and then tails off as the the slowing ofthe conversion rate to zero as the new behavior innovation gradually reaches its ceiling in the popula- asymptotically approaches 100 percent of the group.

Evolutionary Biological Methods and Cultural Data 205 Smith and Winterhalder 1992; Smith et ai. 2001; Shen­ only processes involved. A good example highlighted nan 2002; Winterhalder and Smith 2000). That is, hu­ by Boone and Smith (1998) is the shift from snowshoes man behavioral ecology attempts to understand how to snowmobiles among the Boreal Forest Cree. Since a particular behavior contributes to an individual's this change is reported to have taken place in less than reproductive success and/or that ofhis or her relatives a generation (Winterhalder 1975), it simply cannot given the prevailing environmental (including social) be the result of differential reproductive success, and conditions. The key assumption of human behavioral natural selection and drift sensu stricto can therefore ecology is that humans are sufficiently flexible that the both be ruled out conclusively. Another good, and vast majority of behavioral differences among them perhaps even more archaeologically relevant, example are primarily the result of diversity in environmental that was also highlighted by Boone and Smith (1998) is conditions rather than differences in genes or cultural the adoption of the European horse by North Ameri­ inheritance (Shennan 2002; Smith 2000). Thus human can indigenous groups. Ramenofsky (1995) has argued behavioral ecologists concentrate on the relationship that the horse spread because "individuals who owned between behavioral strategies and ecological circum­ horses reproduced in greater numbers than others" (p. stances, and pay little attention to the mechanism(s) 139), which in turn was a consequence of the horse by which behavioral differences among individuals outcompeting the dog as a means of transportation can potentially arise (Smith et ai. 2001). One substan­ and also providing a significant advantage in hunting. tive consequence of this "black box" approach is that While the notion that the horse conferred advantages human behavioral ecology does not take into account with respect to transportation and hunting may be population-level phenomena such as drift. Another is correct, the idea that the spread of horses was the con­ that human behavioral ecology makes no allowance sequence of differential reproductive success almost for the possibility that some behavioral traits may not certainly is not. According to Ewers (1955), the horse be adaptively significant because they were acquired was adopted by groups distributed across much of as part of a package of traits. The first of these dis­ North America in less than two hundred years. Thus, tinguishes human behavioral ecology from both dual even if we had no other information, apriori it is likely inheritance theory and evolutionary archaeology. The that the population was too large and dispersed, and second further distinguishes human behavioral ecol­ the time frame too short, for natural selection sensu ogy from dual inheritance theory. stricto to be the mechanism involved. Fortunately we This is not the place for an extended discussion of do have other information available in the form of the pros and cons of the three schools of thought (see historical accounts, and it is clear from these that the Bentley et aI., chapter 8). Nevertheless, we believe it is primary processes involved in the spread of the horse important to make our theoretical preferences clear, among North American indigenous groups were trade, and to briefly explain why we think the way we do. exchange, and raiding (Ewers 1955). As mechanisms While the differences between dual inheritance theory, of horizontal or oblique cultural transmission, trade, evolutionary archaeology, and human behavioral ecol­ exchange, and raiding are aspects of phenotypic plas­ ogyare relatively small (Bettinger and Richerson 1996), ticity. As such, they do not fit well with evolutionary we believe a combination of dual inheritance theory archaeology. In contrast, the spread of the European and human behavioral ecology (e.g., Shennan 2002) horse among Native Americans and the adoption of provides a more useful framework for studying cul­ snowmobiles by the Boreal Forest Cree are easily ac­ tural evolution than evolutionary archaeology, at least commodated within a dual inheritance/human behav­ as it is practiced by the majority of its advocates. In ioral ecology framework, since, as noted earlier, guided our view, evolutionary archaeology's contention that variation and biased transmission can in principle ef­ culture change can be explained solely in terms of fect major cultural changes within a generation. natural selection and drift sensu stricto (i.e., where Evolutionary archaeology's contention that culture both processes involve differential reproductive suc­ change can be explained solely in terms of repro­ cess) is unconvincing. It may be the case that these duction-based processes is also unconvincing when processes were responsible for some of the changes viewed in the light of empirical work on cultural observable in the archaeological record. However, as transmission. Several studies suggest that copying others have noted (Boone and Smith 1998; Runciman from unrelated individuals can be an important pro­ 2002), there is overwhelming evidence that cultural cess in cultural evolution. For example, in a classic evolution often occurs too quickly for them to be the experiment the psychologist Solomon Asch (1955)

206 MARK COLLARD ET AL. •

1=1 1=2 1=3

Figure 13.2. A simple representation of the neutral-trait model. Shown are five individuals per time step t for three successive time steps. At each time step, we refresh the population with new individuals, and each is given a new copy of a variant (represented by numbers in­ side the circles). Each variant is assigned a new value by either (1) copying a randomly selected individual from the previous time step, with equal probability of choosing any individual; or (2) inventing a new variant (gray lightning bolts) with probability p, the mutation (innovation) rate per individual per time step t. After Bentley et al. (2007:fig. 1).

found that his subjects would agree with the conclu­ In a simple version focusing on cultural evolu­ sion of a group majority even when they could see tion, the neutral model assumes there are N individu­ clearly, with their own eyes, that the conclusion was als, each characterized by a behavioral variant (figure incorrect (the experiment involved matching lines 13.2). At each time step, each new individual copies a of different lengths drawn on paper). Asch's (1955) variant from an individual in the previous time step, study suggests that the urge to copy the majority such that any individual may be copied with equal view can be more powerful than one's independent probability. Innovation (analogous to genetic muta­ opinion. The importance of copying from unrelated tion) is added through the constant introduction of individuals in human affairs is underscored by work new, unique variants. The variable fj represents the reported by Simkin and Roychowdhury (2003) and innovation rate, in innovations per individual per Bentley (2006), which reveals clear patterns of such time step. These two parameters, the number of in­ copying in one ofthe most unlikely places-academic dividuals, N, and the mutation rate, fj, are the most publishing. This evidence too does not fit well with important in the neutral model (Gillespie 1998). In evolutionary archaeology but is easily accommodated the simple model, the number of individuals N per within a dual inheritance framework/human behav­ generation is kept constant and the innovation rate ioral ecology framework. fj represents a fixed probability for each individual acquiring a new unique variant in a given generation. CULTURAL EVOLUTION AND POPULATION As determined simply by the product Nfj, the neutral GENETICS-BASED MODELS model provides testable predictions concerning the In the 1960s, geneticists made the surprising discovery change over time in the number and relative frequen­ that much DNA does not actually code for anything, cies of different variants. and therefore evolves by drift rather than by natural se­ The neutral model can be easily simulated on a lection. This discovery formed the basis of the neutral computer, with the variant frequencies being recorded theory ofevolution (Kimura 1983; Kimura and Crow over sample intervals of multiple generations or time 1964). Although it was originally intended as a theory steps. A set of these simulations focusing on cultural for a particular form of genetic evolution, the neu­ evolution has suggested three general characteristics tral theory has been deployed in a variety of settings in terms of how the frequencies of variants change including the study of species distributions (Hubbell through time: 2001), birdsong (Lynch and Baker 1994; Slater and Ince 1979), and cultural evolution (Bentley and Shen­ 1. A histogram of the relative popularity levels (fre­ nan 2003; Bentley et al. 2004; Bentley et al. 2007; Bet­ quencies) of variants yields a distribution that fol­ tinger and Eerkens 1999; Brantingham 2003; Dunnell lows a mathematical function called a power law, 1978; Eerkens and Lipo 2005; Hahn and Bentley 2003; for small values ofthe innovation rate fj (Bentley et Herzog et al. 2004; Kohler et al. 2004; Lipo 2001; Lipo al. 2004; Hahn and Bentley 2003). et al. 1997; Meltzer 1981; Neiman 1995; Shennan and 2. Ifwe follow a set ofvariants introduced in the same Wilkinson 2001). generation, over time the average of their frequen-

Evolutionary Biological Methods and Cultural Data 207 Boys' IUUIleS

0.1

l;> 0.01 ~ J e 0.001 II. t. 1900-1909 0.0001 o 1950-1959 o 1980-1989 0.00001 -Theta= 4 model 0.000001 +------~------~------'f___, 0.0001 0.001 0.01 0.1 N_ frequeDCy

Figure 13.3. An example of a real-world data pattern that fits effect 1 described for the random-copying model (a histogram of the variant frequencies yields a power-law distribu­ tion, for small values of the innovation rate ~). The data are first names given to babies in the U.S. during the twentieth century. The plot shows frequencies of boys' names versus expected frequencies based on the random-copying model. After Hahn and Bentley 2003: fig. la.

cies stays the same, but the disparity (variance) are explained by drift (and hence neutral theory) was in their frequencies increases (Hahn and Bentley initially proposed by Robert Dunnell (1978). However, 2003). it was not until almost twenty years later that Nei­ 3. The set ofmost popular variants changes continu­ man (1995) reported the first study in which neutral ally, at a rate that (to a first-order approximation) theory was formally applied to archaeological data. depends on the innovation rate but not signifi­ Neiman (1995) used the neutral theory to develop a cantly on the population size. This can be mea­ series of predictions about the amount ofvariation to sured by the rate ofturnover on any list ofvariants be expected in the decoration of a pottery assemblage that are ranked according to popularity, as in "top if the motifs were neutral in terms of adaptation. He 40" or "top 100" lists (Bentley et al. 2007). analyzed the variation among rim decorations from seven successive phases of the Woodland period in As an example of effect 1, figure 13.3 shows the fit Illinois, and found that it matched the expectations of the random-copy model to the power law distribu­ of the neutral model. He concluded that the patterns tion of names for boys. Figure 13.4 shows how effect of variation depended on changing levels of inter­ 2 can be seen among the first names given to babies group contact, which started low, increased, and then in the 1900s in the United States, and following the declined again. The time of highest interaction was frequencies of those names through the twentieth also a time when exotic trade goods were widespread. century. In a case study of registered purebred dog Because the successful transmission of pottery-mak­ breeds in the United States, Herzog et al. (2004) dem­ ing traditions depends on long-lasting relationships onstrated how Dalmatians could be identified as an between teacher and learner, Neiman (1995) suggested exceptional case that cannot be explained by simple that the changing levels of intergroup contact related random copying. This was based on the fact that the to changes in the level of long-term residential move­ popularity of Dalmatians exceeded the envelope of ment ofpotters between groups. expected variations. Based on the rejection of the Since Neiman's (1995) study appeared, a number null hypothesis of simple random copying, Herzog of authors have reported work in which the neutral et al. (2004) hypothesized that the sudden popularity model has been applied to archaeological data. In increase of Dalmatians was due to the rerelease of the view of the constraints on space, we will highlight just Disney movie 101 Dalmatians. a few of these studies-Lipo et al.'s (1997) "Popula­ With 'regard to archaeological applications of the tion Structure, Cultural Transmission, and Frequency neutral theory, a formal distinction between artifact Seriation;' Shennan and Wilkinson's (2001) "Ceramic classes whose distributions are explained as a result Change and Neutral Evolution: A Case Study from of natural selection and those whose distributions Neolithic Europe," Kohler et al.'s (2004) "Vessels and

208 MARK COLLARD ET AL. O.(lOl

0.0008

•to 0.0006 ..• Iioo!' . 0.0004

0.0002

0 + + + 1910. 1920. 193.. 1940. 19581 1960. 19781 19181 19,..

Figure 13.4. An example of effect 2 described for the random-copying model (the dispar­ ity in variants of the same age will increase over time, even as the average frequency stays the same). The plot shows the mean frequency (filled circles) and standard deviation (lines) over time of 99 male names that were new to the top 1,000 in the 1910s. After Hahn and Bentley 2003:fig. 3.

Villages: Evidence for Conformist Transmission in the assemblages into the largest groups possible. They Early Village Aggregations on the Pajarito Plateau, then divided each group ofassemblages that displayed New Mexico," and Eerkens and Lipo's (2005) "Cultural a significant departure from unimodality into two Transmission, Copying Errors, and the Generation of groups. This process was repeated until all groups were Variation in Material Culture and the Archaeological unimodal, and therefore represented what Lipo et al. Record." (1997) assumed to be the archaeological signatures of The goal of Lipo et al:s (1997) article was to dem­ real communities. In order to test for the potentially onstrate the use ofneutral traits to map patterns of in­ confounding effects of sample size, Lipo et al. (1997) teraction in the archaeological record. To begin with, created new seriations by excluding assemblages that they employed simulations to investigate the effects were judged too small. The resulting spatial group­ on the evolution of neutral traits of the differential ings generally resembled the ones based on all of the interaction of individuals over space and time, and to assemblages, except in two cases where these new explore the way in which these effects impact seriation groupings overlapped and incorporated several of the analysis (see Webster, chapter 2). They found that the former ones, which suggested that sample size had geographic distribution ofa neutral trait through time in fact partially determined the spatial grouping of is determined by the distribution of individuals in an assemblages. Lastly, Lipo et al. (1997) examined how environment and their frequency of interaction. They estimates of interaction are affected by classification also found that when members ofa population are free level. They accomplished this by collapsing classes to to interact equally over an area it is possible to produce create more inclusive seriation groupings. Lipo et al. a single, nearly perfect seriation for the whole area, (1997) found increasingly larger groups patterned by whereas when the interactions among the members distance, which is suggestive of interacting communi­ of a population are restricted it is impossible to pro­ ties-in essence, that the degree of community inter­ duce such a seriation. With this in mind, they argued action indicated by archaeological evidence depends that seriation can be used as a tool to test hypotheses on the scale of classification of the artifacts. about the interaction ofpopulations. Specifically, they Shennan and Wilkinson (2001) applied Neiman's argued that the failure ofa set ofassemblages to seriate (1995) neutral model in a study of patterns of pot­ should be taken as indicative of a lack of interactions tery decoration from two settlements of the early among populations. Subsequently, Lipo et al. (1997) Neolithic Linear Pottery culture in the valley of the investigated the distribution of Mississippian-period Merzbach in western Germany. The Linear Pottery decorated ceramics from the Memphis and St. Francis culture is thought to represent the archaeological trace areas of the lower Mississippi valley with the aid of of an early agricultural population that spread across the large database of typed sherds published by Phil­ much of Central Europe between about 5700 and lips et al. (195l). Lipo et al. (1997) began by seriating 5400 B.C. It is especially well documented in the valley

Evolutionary Biological Methods and Cultural Data 209 of the Merzbach as a result of excavations carried out process-dependent activities such as innovation or in­ in advance of strip mining. Shennan and Wilkinson vention. They assumed that variation created through (2001) found that the neutral model could predict the copying error should be random and relatively small in amount ofvariation in pottery decorations only in the magnitude. With regard to the magnitude of copying early phases ofoccupation and not in the later phases. error, they specifically assumed that it should be below By using the neutral theory as their null hypothesis, the level at which people are aware there is a difference. Shennan and Wilkinson (2001) were able to identify They also assumed that variation created through cog­ the later Neolithic phases as an era ofdeliberate selec­ nitive mechanisms should be directional and ofgreater tion for novel decoration types, rather than the simple magnitude than copying error. The authors then mod­ drift that characterized the early phases. Shennan and eled assemblage variation related to several transmis­ Wilkinson (2001) proposed that the later-phase pot­ sion processes using Markov chain simulations where ters were using this novelty to establish their own local random processes are followed in successive gener­ identity with respect to neighboring groups. ations or time steps. They found that in unbiased Kohler et al. (2004) used the neutral model and transmission the amount of variation increases with previously compiled pottery data to examine cultural continued copying errors compounding variation evolutionary processes at Burnt Mesa Pueblo, a Late over time, but the average value for the trait stays the Coalition period village in New Mexico (Gray 1992). same. They also found that biased transmission such Kohler et al. (2004) subdivided the pottery into eighty­ as conformist and prestige-biased transmission acts to three distinct styles, based on general descriptions dampen the amount ofvariation expressed over time. of the surface designs (e.g., lines only, solid designs, Subsequently, Eerkens and Lipo (2005) used their sim­ hatched designs, then further data on line type, hatch ulation findings to interpret the results of analyses type, curved line). They then calculated estimates of of morphological variation in projectile points from the product Nfl (population size times the innova­ the Owens valley of California and Woodland period tion rate), which under the neutral model should be ceramics from Illinois. Using obsidian hydration ages directly proportional to the diversity of variants. By associated with the projectile points, they examined calculating Nfl and comparing it to the diversity of the distribution of coefficient of variation values for variants, Kohler et al. (2004) were able to test whether basal width and thickness over time. They found that the neutral model described stylistic change at Burnt variation associated with basal width decreases over Mesa Pueblo. The challenge is that for a typical archae­ time, indicating that some variation-reducing pro­ ological assemblage, the population size of the com­ cess acted to diminish variation. Variation in thickness munity is difficult to estimate, and innovation rate is tended to increase over time, fitting an unbiased trans­ practically impossible to measure directly. Fortunately, mission model. With regard to the Woodland period as Shennan and Wilkinson (2001) pointed out, N can ceramics, thickness and diameter measures were taken be estimated indirectly through the number of house­ from assemblages associated with dated features, and holds (assuming one potter per household) and/or coefficient of variation values calculated. It was found the number ofvessels in each period, and the innova­ that during some periods the variation in thickness tion rate fI in a chronological phase can be indirectly was consistent with the copying error model, while in estimated by the number of new decorations divided others there were increases in sherd thickness beyond by the total number ofvessels for the phase. Using this the simulated limits ofcopying error. Eerkens and Lipo method, Kohler et al. (2004) found less diversity in ce­ (2005) interpreted the latter as evidence for the opera­ ramic style than would be expected under the neutral tion of selection. The most noteworthy finding with model. Specifically, they found that the innovation respect to pot diameter is that it showed an increase rate at Burnt Mesa Pueblo declined steadily through in variation of nearly 16 percent during the final four­ the period of its occupation. Kohler et al. (2004) con­ hundred-year period of the sequence. This is much cluded from this that a conformist bias had developed greater than the simulated variation due to copying in the way pottery designs were made. error. Accordingly, Eerkens and Lipo (2005) suggested Eerkens and Lipo (2005) developed models to ac­ that a variation-increasing mechanism such as inven­ count for variation in continuously measured vari­ tion must have been operating. ables in archaeological assemblages such as projectile In recent years researchers have also begun to use point dimensions. They proposed that variation is selection-based genetic models to cast light on archae­ created either by simple copying error or by cognitive ological problems. For example, Shennan (2001) used

210 MARK COLLARD ET AL. population genetic modeling to investigate the impact hand, there will be variability from one person to the ofpopulation size on cultural evolution when innova­ next in attempts to imitate. Occasionally an individual tions affect fitness. In this study Shennan (2001) em­ may strike it lucky and, in a failed attempt to imitate, ployed two variations on a population genetics model produce an outcome that gives a better result than the developed by Peck et al. (1997) to assess the relative previous best. This then becomes the new gOjil for the benefits of sexual and asexual reproduction. In Peck rest ofthe population to aspire to. As a result, so long as et al.'s model, mutations can be either beneficial or the mean margin of failure does not increase, the level deleterious; there is a correlation between an allele's ofthe whole population will be improved. Ifthe mean fitness prior to mutation and its post-mutation fitness; margin of failure is high and the variation between and many mutations produce only very small changes individuals is small, then improvement is unlikely. in fitness. For the first version, Shennan (2001) al­ Thus the likelihood of cumulative cultural evolution tered Peck et al.'s (1997) model so that transmission is partly dependent on the ratio between mean margin was only possible from one "cultural parent" to one offailure and the amount ofinterindividual variation. "cultural offspring." To produce his second model, It is also partly dependent on population size, since Shennan (2001) modified Peck et al.'s (1997) model in large populations even improbable events-in this to allow transmission between individuals belonging case arriving at a behavior that gives a better result to different generations where the older individual is than the previous best-occur now and again, and not the biological parent of the younger individual. obviously the larger the population the more likely In simulation trials Shennan (2001) found a marked this is. Depending then on the ratio between the mean increase in the mean fitness ofthe population as effec­ margin of failure and the amount of interindividual tive population size increased. In the trials of the first variation, a larger or a smaller population size will model there was a 10,000-fold increase in the mean be required for cumulative cultural evolution to take fitness value of the population as effective popula­ place. It follows that, for a given ratio, if the size ofthe tion size increased from five to fifty. In trials of the interacting population changes for some external rea­ second model in which cultural traits were adopted son then this will affect the rate ofcumulative cultural from nonbiological parents 5 percent of the time, the evolution. Ifpopulation increases, then the probability population's mean fitness value increased a thousand­ of cumulative improvement increases. On the other fold as the effective population size increased from five hand, if it decreases then it is likely that a process of to twenty-five, and then increased by around five times cultural devolution will take place, because the proba­ as effective population size increased from twenty-five bility of someone improving on the existing situation, to seventy-five. Shennan's (2001) simulation studies or even equaling the current best, is small. Thus in the showed that larger populations have a major advan­ next generation the best individual to copy is likely to tage over smaller ones when it comes to cultural in­ be slightly worse than in the generation before, and novation due to the decreasing role ofsampling effects this process will be repeated through the generations, as populations get larger. When effective population until some equilibrium is reached. Henrich (2004) size is large, there is a far greater probability offitness­ suggested that this model explains the apparent loss enhancing cultural innovations being maintained and of cultural adaptations in Tasmania after it became deleterious ones being lost than when effective popu­ separated from the Australian mainland with rising lation size is small. In the latter situation, innovations sea levels at the end ofthe last Ice Age, since this isola­ that are maintained tend to be less beneficial in terms tion meant that the Tasmanians were no longer part of of reproduction and also less attractive for imitators. a larger interacting continental population. Recently Henrich (2004) has presented a rather In our view, the studies described above demon­ different model of the impact of population size on strate the considerable potential of population ge­ cultural evolution, and in particular the opportunities netics models, when suitably adjusted, to shed light it offers for cumulative change. His model assumes on cultural evolution. Crucially, such models provide that when learning cultural behaviors, in particular baselines of great epistemological value. Where cul­ complex skills, individuals will try to copy the best tural patterns agree with the patterns predicted by a practitioner ofthat skill within their population. Most given model, we can invoke the principle ofparsimony people will not do as well as the best, but the mean and discount processes that are more complicated margin of failure is likely to be variable-small for than the modeled process. In contrast, where the mod­ easy things, larger for more difficult ones. On the other eled and actual patterns disagree, we can legitimately

Evolutionary Biological Methods and Cultural Data 2 11 disregard the modeled process and seek a more com­ except for the one at the base, which is referred to as plex explanation. As such, the application of popu­ the root. The points of intersection between branches lation genetics-based cultural evolutionary models are called nodes. The last step of the process is to allows us to narrow the range ofpossible explanations compile an ensemble cladogram from the character for cultural patterns in a controlled manner. cladograms. Ideally, the distribution of the character States among the taxa will be such that all the character CLADISTIC ANALYSIS OF CULTURAL DATA cladograms imply relationships among the taxa that First outlined in the 1950s (Hennig 1950, 1965, 1966), are congruent with one another. Normally, however, cladistics is currently the dominant method of phylo­ a number of the character cladograms will suggest genetic reconstruction used in evolutionary biology relationships that are incompatible. This problem is and paleobiology (Kitching et al. 1998; Quicke 1993; overcome by generating an ensemble cladogram that Schuh 2000; Smith 1994; Wiley et al. 1991). Based on is consistent with the largest number of characters a null model in which new taxa arise from the bifur­ and therefore requires the smallest number of ad hoc cation of existing ones, cladistics defines phylogenetic hypotheses of character appearance or homoplasies to relationship in terms of relative recency of common account for the distribution of character states among ancestry. Two taxa are deemed to be more closely re­ the taxa. lated to one another than either is to a third taxon if Recently a number of researchers have begun to they share a common ancestor that is not also shared apply cladistics and related biological phylogenetic by the third taxon. Exclusive common ancestry is methods to cultural data to shed light on events in indicated by shared evolutionarily novel or derived antiquity (Atkinson and Gray 2005; Bryant et al. 2005; character states. Two taxa are inferred to share a com­ Buchanan and Collard, in press; Cochrane 2004; Dar­ mon ancestor to the exclusion of a third taxon if they went and O'Brien 2005; Eerkens et al. 2005; Foley exhibit shared derived character states that are not also 1987; Foley and Lahr 1997, 2003; Forster and Toth exhibited by the third taxon. 2003; Gray and Atkinson 2003; Gray and Jordan 2000; In its simplest form, cladistic analysis proceeds via Greenhill and Gray 2005; Harmon et al. 2005; Holden four steps. First, a character state data matrix is gener­ 2002, 2005; Holden et al. 2005; O'Brien and Lyman ated. This shows the states of the characters exhibited 2003b; O'Brien et al. 2001, 2002; Rexova et al. 2003; by each taxon. Next, the direction of evolutionary Robson-Brown 1996). This approach acknowledges change among the states of each character is estab­ that reconstructing human population history from lished. Several methods have been developed to fa­ linguistic and archaeological data shares the same cilitate this, including communality (Eldredge and problems as reconstructing the evolutionary history Cracaft 1980), ontogenetic analysis (Nelson 1978), of a group of species (Foley 1987; Kirch and Green and stratigraphic sequence analysis (Nelson and Plat­ 1987). In both cases, the key challenge is to distinguish nick 1981). Currently the favored method is outgroup the similarities resulting from shared ancestry (ho­ analysis (Arnold 1981), which entails examining a mologies) from those due to mechanisms other than close relative ofthe study group. When a character oc­ shared ancestry (homoplasies). While the processes curs in two states among the study group, but only one that generate biological and cultural homologies and of the states is found in the outgroup, the principle of homoplasies are not the same (e.g., gene transfer ver­ parsimony is invoked, and the state found only in the sus cultural transmission), the epistemology and on­ study group is deemed to be evolutionarily novel with tology ofestablishing ancestral relationships is general respect to the outgroup state. Having determined the enough to warrant the application of phylogenetic probable direction of change for the character states, methods to cultural data. Most significantly, in both the next step in a cladistic analysis is to construct a cases a model is sought that explains the distribution branching diagram ofrelationships for each character. of resemblances among a group oftaxa in the absence This is done by joining the two most derived taxa by of prior knowledge of how those resemblances arose. two intersecting lines, and then successively connect­ The bifurcating tree model is used because it is the ing each of the other taxa according to how derived simplest, and therefore the most defensible, way of they are (fig4re 13.5). Each group of taxa defined by linking taxa together. Once a tree model has been a set of intersecting lines corresponds to a clade, and generated for a group of taxa, is it possible to classify the diagram is referred to as a cladogram or tree. The the similarities among them as homologous or homo­ lines that form a cladogram are usually called branches plastic. Homologous similarities support relationships

212 MARK COLLARD ET AL. A

B

IHomoplasy~ C

...,. .. N.. ~.. .. on.. \C......

Figure 13.5. An example of a tree of evolutionary relationships generated via cladistics. together with the character state data matrix from which it was derived. Trees of evolu­ tionary relationships generated with cladistics are usually referred to as "c1adograms." They are read from the tips to the root. Thus. the c1adogram shown indicates that taxa Band C form a monophyletic group to the exclusion of taxon A based on the shared possession of derived character states for characters 3 and 4. It also suggests that taxa A, B, and C form a monophyletic group based on the shared possession of derived charac­ ter states for characters 1 and 2. Taxon C is the most derived taxon, having the derived states for characters 5, 6, and 7 in addition to the other derived characters. Character 7 is homoplastic as it is in a derived state in taxa A and C, even though taxa A and C are not directly related through one common ancestor. Two equally parsimonious solutions are available to resolve this character data matrix; in the c1adogram shown above, taxa Band C are shown more closely related and forming a clade to the exclusion of taxon A. In the second solution taxa A and C form a clade to the exclusion of taxon B. From Buchanan and Collard (2007). copyright Elsevier (2007). reprinted with permission. that are compatible with the tree model, whereas ho­ and Lyman 2003b; O'Brien et al. 2001, 2002) and moplastic ones suggest relationships that conflict with Buchanan and Collard (in press). Gray and Jordan the tree model. (2000) employed cladistic methods to assess the two This approach is illustrated by the historical lin­ main competing models regarding prehistory in the guistic studies of Gray and Jordan (2000), Holden Pacific, the express train model and the entangled (2002), and Gray and Atkinson (2003), and the ar­ bank model (see Bellwood, chapter 14). The former chaeological work ofO'Brien and colleagues (O'Brien suggests a rapid dispersal of Austronesian speakers

Evolutionary Biological Methods and Cultural Data 213 from a homeland in Taiwan around 6,000 years ago time Bantu-speaking communities have not moved to through Island Melanesia and into the Polynesian any great extent. islands of the remote Pacific, whereas the latter con­ Gray and Atkinson (2003) employed phylogenetic tends that the Polynesian colonizers derived from a analysis of Indo-European languages to test the two population in Island Melanesia that had been there main competing hypotheses for Indo-European ori­ for tens of thousands of years. In the entangled bank gins, one that Indo-European languages spread with model, the cultural and linguistic patterns among Kurgan pastoralists beginning around 6000 B.P. (Gim­ Polynesians are the complex result of not just their butas 1973), and the other that the language family colonization, but also founder's effects associated expanded with the spread ofagriculture from Anatolia with original colonization, and the continued cul­ around 9500-8000 B.P. (Renfrew 1987; see Bellwood, tural contact between different islands during the chapter 14). Working with a restricted list of essential subsequent millennia, with genetic, linguistic, and vocabulary words across Indo-European languages cultural traits transmitted at varying intensities be­ both past and present, Gray and Atkinson (2003) re­ tween populations. In the entangled bank model lin­ constructed a set of likely phylogenetic trees of their guistic patterns largely reflect human interaction and evolution, and then used statistical methods to esti­ continued cultural transmission rather than the phy­ mate their divergence times. Since the overwhelming logenetic history of the language speakers described majority of these trees produced estimated dates be­ by the express train model. To test these conflicting tween 7800 and 9800 B.P. for Indo-European origins, models, Gray and Jordan (2000) used a cladistics consistent with the estimates Renfrew (1987) obtained computer program to produce a phylogeny of Pacific from the archaeology, Gray and Atkinson's (2003) phy­ languages, onto which they then mapped the pre­ logenetic analysis strongly supports Renfrew's (1987) historic events suggested by the express train model. Anatolian farming hypothesis. They found a close fit between the chronological O'Brien et al.'s (2001, 2002; O'Brien and Lyman stages of the express train model and the branching 2003b) archaeological application ofcladistics focused pattern of their language phylogeny. In that lan­ on the long-standing problem ofthe evolution ofpro­ guages that were closely related in the phylogeny were jectile point form in the southeastern United States not necessarily close geographically. Gray and Jordan during the Paleoindian period (ca. 11,500 to 10,000 (2000) concluded that these linguistic patterns result 14C yr B.P.). O'Brien et al. began by recording three predominantly from colonizing migrations of the qualitative and five quantitative characters on a sam­ language speakers rather than cultural contact since ple of621 specimens representing a range ofprojectile the time of initial colonization. point types, including Clovis, Dalton, and Cumber­ Holden (2002) conducted a comparable analy­ land. They then subjected the specimens to paradig­ sis in which she used maximum parsimony analysis matic classification in order to cluster them into taxa to reconstruct the relationships among seventy-five with unique combinations of character states. In the Bantu and BantoidAfrican languages from ninety-two next part of the study, O'Brien et al. carried out a items of basic vocabulary. As in the Polynesian case, cladistic analysis of the seventeen taxa that contained some researchers contend that the Bantu languages at least four specimens. One of the seventeen taxa evolved rapidly during the Neolithic and Iron Age was selected as the outgroup on the basis of least-step with the colonization of farmers into sub-Saharan occurrence seriations and chronological consider­ Africa, while others hold that the evolution is more ations; the remainder were treated as the ingroup. The the result of diffusion of Bantu words among neigh­ cladistic analysis yielded a single most parsimonious boring speech communities. Holden's (2002) analysis cladogram. O'Brien et al. evaluated the fit between returned a relatively small set of possible phylogenetic the cladogram and the data set with a goodness-of-fit trees in support of the tree model of Bantu language index called the Consistency Index (CI). This index history, and found these trees to be consistent with the ranges between 1.0 and 0.0, with values close to 1 model for the spread offarming in sub-Saharan Africa indicating a good fit between the cladogram and the constructed by archaeologists through chronological data set and values close to 0 indicating a poor fit. The analysis of pottery. Holden (2002) concluded, there­ cladogram obtained by O'Brien et al. had a CI of0.59, fore, that the dispersal and diversification ofthe Bantu which suggests that it is a reasonable depiction of the languages was linked to the expansion of farming relationships among the taxa. In the final part oftheir during the Neolithic and Iron Age, and that since that study, O'Brien et al. used the cladogram to investigate

214 MARK COLLARD ET AL. the character state changes that occurred in the course level (Collard and Shennan 2000; Collard et al. 2005; of the evolution of Paleoindian projectile point form. Jordan and Shennan 2003, 2005; Moylan et al. 2005; Buchanan and Collard's (in press) study also fo­ Shennan and Collard 2005; Tehrani and Collard 2002). cused on the archaeology ofthe Paleoindians. Specifi­ So far, the debate in question has concentrated on two cally, they applied cladistic techniques to qualitative competing hypotheses, which have been termed the and quantitative data from a North America-wide branching hypothesis (also known as the demic dif­ sample of projectile points in order to test competing fusion or phylogenesis hypothesis) and the blending models of Early Paleoindian colonization and adapta­ hypothesis (also known as the cultural diffusion or tion. The archaeological cultures of the Early Paleo­ ethnogenesis hypothesis) (Bellwood 1996; Collard and indian period, which include the well-known Clovis Shennan 2000; Kirch and Green 1987; Moore 1994, culture, represent the first well-documented indica­ 2001; Tehrani and Collard 2002). According to the tions of human occupation in North America. Bu­ former, the cultural similarities and differences among chanan and Collard (in press) employed three sets of human populations are primarily the result ofcultural tests to determine whether or not a phylogenetic sig­ assemblages dividing as the communities that produce nal was present in the size-corrected projectile point them repeatedly grow and split. The branching hy­ data. Results from a permutation tail probability test pothesis predicts that the similarities and differences (p =0.0001), goodness-of-fit statistics (CI = 0.56; RI among cultures can be represented by a cladogram, = 0.67), and the phylogenetic bootstrap (producing and that there will be a strong association between cul­ clade support of 52-84 percent for branches) indi­ tural variation and linguistic, morphological, and ge­ cated that a significant phylogenetic signal exists in netic patterns (Ammerman and Cavalli-Sforza 1984; the data. Subsequently, the cladograms derived from Renfrew 1987). In contrast, supporters ofthe blending the maximum parsimony and bootstrap analyses were hypothesis (Dewar 1995; Moore 1994, 2001; Terrell compared with cladograms constructed to represent 1988,2001; Terrell et al. 1997,2001) contend that it is different colonization routes, cultural diffusion, and unrealistic "to think that history is patterned like the environmental adaptation hypotheses. The coloniza­ nodes and branches of a comparative, phylogenetic, tion models analyzed include the ice-free corridor or cladistic tree" (Terrell et al. 1997:184). Instead, they model, the Northwest Coast model, and Stanford and argue that the biological, linguistic, and cultural evo­ Bradley's (2002; see also Bradley and Stanford 2004) lution of our species is best characterized by "a con­ Solutrean model. The strength of fit between the ob­ stant flow of people, and hence their genes, language, served and hypothetical cladograms was assessed us­ and culture, across the fuzzy boundaries of tribes and ing the Kishino-Hasegawa (K-H) test (Kishino and nations" (Moore 2001:51). The blending hypothesis Hasegawa 1989). In this test, a p-value is calculated for predicts that a reticulated graph can best represent the the length difference between a hypothetical clado­ similarities and differences among cultures (Terrell gram and an observed cladogram by comparing it to 2001), and that there will be a close relationship be­ a distribution of length differences obtained from a tween cultural patterns and the frequency and inten­ randomly generated sample of cladograms. Buchanan sity ofcontact among populations. Other models have and Collard (in press) found that the four most-par­ been proposed (e.g., Boyd et al. 1997), but to date these simonious cladograms and the bootstrap cladogram have received little attention in the literature. were all significantly different from the hypothetical The researchers who have applied cladistics to cul­ cladograms. Therefore, none of the hypotheses was as tural data in the context of the phylogenesis/ethno­ well supported by the data set as the most parsimo­ genesis debate have done so on the grounds that it nious cladogram. Comparison of cladogram lengths enables a quantitative estimate of the relative contri­ then was used to assess the fit of the hypothetical bution of the two processes to any given data set to cladograms to the observed cladograms (Jordan and be obtained (Collard and Shennan 2000; Collard et Shennan 2003). Using this approach, the ice-free cor­ al. 2005; Jordan and Mace 2005; Jordan and Shennan ridor entry model was found to best account for the 2003, 2005; Shennan and Collard 2005; Tehrani and structure in the data. Collard 2002). They have argued that in the absence Another group of researchers has applied cladistics of prior knowledge of how the resemblances among and related phylogenetic methods to cultural data in a group of cultural assemblages arose, a reasonable an effort to resolve an ongoing debate about the pro­ course of action is to fit the bifurcating tree model cesses involved in cultural evolution at the population to characters derived from the assemblages, and then

Evolutionary Biological Methods and Cultural Data 215 determine how many character states fit the tree and throughout the whole of the ten-phase period. They how many do not. Those that fit the tree model are conjectured that, ifthe phylogenesis hypothesis is cor­ assumed to have been transmitted vertically (in the rect, analyses ofthe assemblages should divide the set­ sense oftraits being passed from an ancestral group to tlements into the same groups in consecutive phases. its descendants, rather than necessarily from parents On the other hand, if the ethnogenesis hypothesis is to children) and are therefore the result of phylogen­ accurate, the analyses should place the settlements esis, while those that do not are assumed to have been into different groups in consecutive phases. The four transmitted horizontally and are therefore the result settlements were divided into the same groups in six of ethnogenesis. As was the case with using cladistics of the instances in which consecutive phases could to investigate events in prehistory, this course of ac­ be compared. In the remaining three instances, the tion is justified on the grounds that the bifurcating settlements were divided into different groups in con­ tree model is the simplest, and therefore the most secutive phases. These results are not wholly compat­ defensible, way of linking a group oftaxa together. An ible with either hypothesis. Rather, they indicate that additional reason for employing cladistics to measure phylogenesis and ethnogenesis were both involved in the relative contribution of vertical and horizontal the generation ofthe pottery assemblages. Collard and transmission to cultural data sets as opposed to the Shennan's (2000) second set of analyses focused on regression-based approaches that conventionally have three instances in which a new pottery assemblage ap­ been used to investigate to cultural evolutionary pro­ pears. They reasoned that, if the phylogenesis hypoth­ cesses (Guglielmino et al. 1995; Moore and Romney esis is correct, then the newly founded assemblages 1994,1996; Roberts et al. 1995; Welsch 1996; Welsch et should have a single parent assemblage in the preced­ al. 1992) is that the latter may inflate the significance ing phase. Conversely, ifthe ethnogenesis hypothesis is of ethnogenesis. As noted earlier, geographic proxim­ accurate, then the newly founded assemblages should ity is usually employed as the proxy for ethnogenesis. have multiple parent assemblages. The second set of However, phylogenesis can also be expected to cor­ analyses supported the phylogenesis hypothesis rather relate with geographic proximity, since sister groups than the ethnogenesis hypothesis. A newly founded are likely to be nearest neighbors. Thus the use ofgeo­ assemblage was strongly linked with a single parent graphic proximity solely as a proxy for ethnogenesis assemblage in two of the analyses. The results of a will overstate the latter's importance and understate third analysis were more ambiguous, but the simplest the importance ofphylogenesis. interpretation of them also supported the notion that Again, in view of the constraints on space, we will the newly founded assemblages had a single parental highlight just a few studies in which cladistics has assemblage in the preceding phase. Overall, therefore, been applied to cultural data with a view to shedding Collard and Shennan's (2000) analyses of the Merz­ light on the phylogenesis/ethnogenesis debate-Col­ bach valley Early Neolithic pottery supported the lard and Shennan's (2000) "Ethnogenesis Versus Phy­ phylogenesis hypothesis more strongly than the eth­ logenesis in Prehistoric Culture Change: A Case-Study nogenesis hypothesis. New settlements seem to arise as Using European Neolithic Pottery and Biological Phy­ a result ofthe fissioning ofa single existing settlement logenetic Techniques;' Tehrani and Collard's (2002) rather than from the amalgamation of people from "Investigating Cultural Evolution through Biologi­ several existing ones. While they do show evidence of cal Phylogenetic Analyses of Turkmen Textiles," and influence from adjacent settlements in their pottery, Jordan and Shennan's (2003) "Cultural Transmission, there tends to be a microcultural continuity. Language, and Basketry Traditions amongst the Cali­ Tehrani and Collard's (2002) study examined deco­ fornian Indians." rated textiles produced by Turkmen groups between Collard and Shennan's (2000) study examined dec­ the eighteenth and twentieth centuries. Two sets of orated pottery from seven multiphase early Neolithic cladistic analyses were carried out. The first focused Linear Pottery culture settlements in the valley of on the period before the Turkmen were incorporated the Merzbach stream on the Aldenhovener Platte of into the Russian empire. These analyses indicated that western Germany. Their aim was to establish whether in the precolonial period the evolution of Turkmen within this small area there was evidence for pro­ textile designs was dominated by branching. A ran­ cesses of cultural branching or blending. Their first domization procedure (the permutation tail probabil­ set of analyses focused on the assemblages from the ity test) suggested that the data contain a phylogenetic four settlements that have evidence for occupation signal, and parsimony analysis indicated that the data

216 MARK COLLARD ET AL. fit the bifurcating tree model associated with cultural analyses, Jordan and Shennan (2003) used the K-H branching reasonably well. The fit between the model test to assess the fit between the data sets and trees and data was not perfect, indicating that blending reflecting linguistic relationships, geographic distance, played a role in the evolution of Turkmen culture. ecological similarity, and adjacency. This test enabled However, goodness-of-fit statistics (CI, Retention In­ them to distinguish between two different potential dex) and a second randomization procedure (boot­ sources of homoplasy-independent invention and strapping) suggested that blending was markedly less blending. In an analysis ofthe complete sample ofbas­ important than branching. According to the good­ kets, the fit between the data set and the adjacency tree ness-of-fit statistics, about 70 percent of the simi­ was considerably better than the fit between the data larities among the assemblages are homologous, and set and the other trees. This suggests that blending approximately 30 percent are homoplastic. This is had a bigger impact on the distribution of similarities compatible with the borrowing of designs and mo­ and differences among the basketry assemblages than tifs being responsible for a third of interassemblage branching or adaptation to local environments. In an resemblances, although the possibility of indepen­ analysis of just the coiled baskets, blending was also dent invention as a source of homoplastic similari­ found to playa more significant role than branching ties cannot be completely discounted. Tehrani and or adaptation to local environments. The analysis of Collard's second set of analyses dealt with weavings the twined baskets contrasted with the preceding anal­ produced after the defeat of the Turkmen by the Rus­ yses in that the language tree fitted the data set better sian military. These analyses suggested that the social than the other trees. This suggests that branching was and economic changes experienced by the Turkmen more important in generating the twined baskets than after their incorporation into the Russian empire led blending or adaptation to local environments. Jordan to a greater role for blending in Turkmen cultural and Shennan concluded on the basis of these results, evolution. Branching remained the dominant cultural and the results ofa range ofmultivariate analyses, that evolutionary process, but the importance of blend­ the evolution of Californian Indian baskets is best ex­ ing increased. The goodness-of-fit statistics indicated plained by ethnogenesis. that ~60 percent of the interassemblage resemblances In our view, the studies discussed in this section are homologous, and ~40 percent are homoplastic. suggest that cladistics and related phylogenetic meth­ Thus there is a 10 percent increase in the number of ods can be a useful tool for tackling certain problems homoplastic resemblances among the woven assem­ concerning cultural evolution. It offers a well-char­ blages from the period of Russian domination. This is acterized model that can be used to analyze material consistent with more intertribal borrowing of designs culture and linguistic data sets. Where the fit between and motifs, but again independent invention cannot a cultural data set and the tree model is close, we can be entirely discounted as a source of the homoplasies. invoke the principle of parsimony and legitimately Tehrani and Collard concluded that the two sets of conclude that the similarities and differences among analyses supported the branching hypothesis more the cultural units are best explained by vertical trans­ strongly than the blending hypothesis. mission. On the other hand, where there are numerous Jordan and Shennan (2003) reached a contrasting homoplasies and the fit between a cultural data set and conclusion. These researchers used cladistics to exam­ the tree model is consequently poor, we can infer that ine variation in Californian Indian basketry in relation horizontal transmission and/or convergent evolution to linguistic affinity and geographic proximity. They played a more important role in generating the simi­ carried out three sets of cladistic analyses. In the first, larities and differences among the cultural units than they used the permutation tail probability test to de­ branching. The instances of homoplasy can then be termine whether or not their basketry data sets (coiled investigated with other methods that are not based baskets, twined baskets, all baskets) contain a phyloge­ on the bifurcating tree model, such as spectral analy­ netic signal. These analyses suggested that a significant sis (Hendy and Penny 1992) or split decomposition phylogenetic signal is present in all three data sets. In (Bandelt and Dress 1992; Dopazo et al. 1993; Dress et the second set ofanalyses, Jordan and Shennan (2003) al. 1996; Huson 1998). used the CI to assess the fit between the data sets and Regarding further applications of phylogenetic the bifurcating tree model. These analyses suggested methods to problems in archaeology, there are some that the phylogenetic signal detected by the permu­ cases where we can predict quite confidently that the tation tail probability test is weak. In the third set of bifurcating tree model is likely to be relevant and where

Evolutionary Biological Methods and Cultural Data 217 the data patterns seem to point in this direction even transmission within the groups that produced the though formal analyses remains to be done. Thus, as assemblages was dominated by horizontal and/or we mentioned earlier, the spread of farming into Cen­ oblique transmission. tral Europe seems to have involved a fast initial colo­ nization process followed by more local population CONCLUSION expansion, mirrored by the increasing regionalization Applying evolutionary biological methods to cultural ofpotterystyles. A similar pattern ofinitial uniformity data with a view to shedding light on cultural evolu­ followed by increasing regionalization seems apparent tion is a relatively new approach. Most work on cul­ in the case of Bell Beakers, but it is by no means clear tural evolution involves identifying patterns in data that the mechanisms are the same, since the pattern sets and then trying to determine which processes are seems to be restricted to a limited part of the Bell likely to have produced those patterns. Evolutionary Beaker cultural inventory, especially the Beaker vessels biological methods are advantageous in this regard be­ themselves. Here, and more certainly in subsequent cause they tend to be based on well-characterized pro­ periods that seem to have had large sedentary popula­ cess models. Ifthe method indicates a good fit between tions, it seems likely that cultural innovations relat­ the data and the model, we can invoke the principle ing to different aspects of social life and originating of parsimony and discount processes that are more in different places would have spread through those complicated. Conversely, if the method suggests that populations more or less independently, resulting in the fit between the data and the model is poor, we can multiple cultural lineages rather than a single cultural legitimately disregard the modeled process and seek a "core" (e.g., Boyd et al. 1997). more complex process to explain the data. Evolution­ Lastly, we think it is worth reiterating that the ary biological methods, therefore, allow us to select phylogenesis/ethnogenesis debate is concerned with process explanations for cultural patterns in a rigorous cultural evolution at the group level as opposed manner. A further important benefit of applying evo­ to cultural transmission within groups. There is lutionary biological methods to cultural data is that it undoubtedly a link between within-group cultural allows archaeologists, anthropologists, and historical transmission and among-group cultural evolution, linguists to provide information about patterns and but it cannot be assumed to be a straightforward processes of cultural evolution that can be linked with one. In principle, cultural transmission among in­ contemporary genetic data and with the increasingly dividuals within a group can be vertical, oblique, available evidence for past human mobility from stud­ and/or horizontal and still be compatible with both ies ofstable isotopes and ancient DNA. We believe this phylogenesis and ethnogenesis. For example, even combination has the potential to give us unparalleled if cultural transmission within each group in a insights into the interrelations among populations sample is predominantly horizontal, cultural evolu­ and the genetic, linguistic, and nonlinguistic cultural tion at the population level may still be dominated attributes associated with them. by phylogenesis, providing the amount of trade, exchange, copying, and/or intermarriage among ACKNOWLEDGMENTS groups is limited. Conversely, even if within-group MC, SJS, and RAB thank the Arts and Humanities Re­ cultural transmission is predominantly vertical, search Council for supporting their work on cultural cultural evolution at the population level may still evolution. BB is grateful for the support provided to be dominated by ethnogenesis, providing there is him by the National Science Foundation through its a large amount of trade, exchange, copying, and/or postdoctoral fellowship (# 0502293). 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