AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 157:121–133 (2015)

Cultural Interaction and Biological Distance in Postclassic Period Mexico Corey S. Ragsdale* and Heather J.H. Edgar Department of Anthropology, University of New Mexico, Albuquerque, NM 87131 KEY WORDS

biological distance; population interaction; Postclassic period; Mexico

ABSTRACT Economic, political, and cultural relationships connected virtually every population throughout Mexico during Postclassic period (AD 900–1520). Much of what is known about population interaction in prehistoric Mexico is based on archaeological or ethnohistoric data. What is unclear, especially for the Postclassic period, is how these data correlate with biological population structure. We address this by assessing biological (phenotypic) distances among 28 samples based upon a comparison of dental morphology trait frequencies, which serve as a proxy for genetic variation, from 810 individuals. These distances were compared with models representing geographic and cultural relationships among the same groups. Results of Mantel and

partial Mantel matrix correlation tests show that shared migration and trade are correlated with biological distances, but geographic distance is not. Trade and political interaction are also correlated with biological distance when combined in a single matrix. These results indicate that trade and political relationships affected population structure among Postclassic Mexican populations. We suggest that trade likely played a major role in shaping patterns of interaction between populations. This study also shows that the biological distance data support the migration histories described in ethnohistoric sources. Am J Phys Anthropol 157:121–133, 2015. VC 2015 Wiley Periodicals, Inc.

Widespread trade networks, endemic warfare, imperial expansion, and rapid population growth are factors believed to have affected patterns of human variation in pre-European contact Mexico. Ethnohistoric records provide a wealth of information about migration history during the Postclassic period, and the archaeological record provides information for population interaction from as early as the first known civilization in Mexico, the Olmec (1500–500 BC). Migrations may involve a series of individuals acting upon the basis of common motives, or the movement of large social groups whose actions are coordinated by a central authority (Cabana and Clark, 2011). During the Postclassic period in Mexico (AD 900–1520), it is believed that migration varied in scale, from the large and long-distance migrations of the nomadic Chichimec populations in Northern Mexico (Smith, 1984; Bierhorst, 1992), to settlements of several families within the Aztec Empire from the homeland within the Valley of Mexico to the frontiers of the Gulf Coast (Berdan, 2004, 2014), to political leaders intentionally resettling merchants in high trade areas (Gasco and Berdan, 2003). It is clear that by the Postclassic period, economic, political, and cultural relationships connected virtually every population throughout Mexico (Smith and Berdan, 2003; Melgar, 2010; McGuire, 2012). At the time of Spanish contact in AD 1519, imperial powers such as the Aztec and Tarascan Empires dominated much of Mexico. What is not understood is how geographic distances and cultural relationships affected biological relationships and population structure in Mexico before Spanish contact. We investigate these phenomena by comparing a matrix of pair-wise biological distances among 28 Postclassic period samples based on dental morphology traits with matrices of distances derived from geographic distances, shared migration history, trade, and trade and political interaction combined

among these same groups. Biological similarity derived from morphological (phenotypic) data among groups is assumed to approximate genetic similarity (Scott and Turner, 1997; Hanihara, 2008; Ricaut et al., 2010), and are appropriate for comparing biological and cultural relationships. We test three nonmutually exclusive hypotheses about how biological variation relates to cultural variation among Postclassic Mexican groups:

Ó 2015 WILEY PERIODICALS, INC.

1. Geographic distances are correlated with biological distances 2. Shared migration histories are correlated with biological distances 3. Trade and political interaction are correlated with biological distances If population interaction was primarily influenced by geographic proximity, then the first hypothesis should be supported. If archaeological reconstructions and Additional Supporting Information may be found in the online version of this article. Grant sponsor: Student and Faculty Field Research Grants, Latin American and Iberian Institute, University of New Mexico; and the Research Collections Grant, American Museum of Natural History. *Correspondence to: Corey S Ragsdale, Department of Anthropology, MCO1-1040 1, University of New Mexico, Albuquerque, NM 87131, USA. E-mail: [email protected] Received 6 March 2014; revised 28 December 2014; accepted 1 January 2015 DOI: 10.1002/ajpa.22701 Published online 20 January 2015 in Wiley Online Library (wileyonlinelibrary.com).

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ethnohistoric accounts of migration are reflected in biological variation, then hypothesis two should be supported. Finally, if cultural relationships such as trade and political interaction had an effect on migration and genetic exchange among populations, then hypotheses three should be supported. A common approach to investigating political and economic interaction among Postclassic Mexican populations is to view Mesoamerica using a modified world systems approach (Blanton and Feinman, 1984; Blanton, 1994). Mesoamerica is a term that refers to the cultural area encompassing most of Mexico, Belize, and extending southward into Central America. A world system, a concept originally developed by Wallerstein (1974) to explain the origins of modern capitalism, is an economic zone composed of core and peripheral societies that extends beyond a single geographic region in which exchange benefits the core. The major criticism of applying this theory to Mesoamerica is that it is composed of too many competing cores, each distinguished by its own political or economic structure rather than being dominated by a single core (Smith and Berdan, 2000; Kepecs and Kohl, 2003). To account for this, the Mesoamerican world system extends cores and peripheries to interregional trade centers (exchange circuits), affluent production zones, resource extraction zones, and unspecialized peripheries (Smith and Berdan, 2003). Trade centers engage in inter-regional trade, have a high volume of exchange, and involve a high diversity of goods. Affluent production zones incorporate high production and exchange, but lack powerful polities and large cities, such as those found in the Valley of Mexico. Resource extraction zones are located along the periphery, and contain several mines or other extraction areas. Finally, unspecialized peripheries refer to areas that do not fall under any of the aforementioned categories and are often considered as lying along the periphery of outside Mesoamerica. For this study, trade networks include sites of varying socioeconomic structures categorized into these zones, each contributing to the greater Mesoamerican world system. A second criticism of the Mesoamerican world system is that such a perspective tends to underestimate the contributions of frontier provinces, or areas outside of the known core zones of Mesoamerica such as Northern Mexico, the Maya region, and parts of the Gulf Coast (Wells, 2006). To accommodate this latter reservation, we considered populations in these regions as part of the Mexican world system, at least as peripheral members, to test whether these populations contributed significantly to population interactions. A holistic approach to understanding population interaction and migration is optimal for gaining a better understanding of the effects of imperial expansion, intensified trade, and shared ideology on patterns of migration among prehistoric human populations. This study also evaluates how well migration histories from written sources and reconstructions of migration histories based upon material culture are reflected in population structure.

PREVIOUS BIOLOGICAL DISTANCE STUDIES Previous biological distance studies have focused on migration patterns among the Classic and Postclassic period populations in Northern, Western, and Central Mexico. Using craniofacial measurements, GonzalesAmerican Journal of Physical Anthropology

Jose et al. (2007) found Central Mexican samples to be similar to West Mexican samples, and different from earlier Central and Northern Mexican samples. Several other studies have utilized dental morphology data to address issues surrounding population relationships. Aubry (2009) found relatively close biological distances between Central Mexican and Maya groups during the Classic period (AD 300–900). These patterns were consistent with migration patterns identified on the basis of archaeological similarities, as well as inter-regional interaction through trade. Willermet et al. (2013) found similar results in a small-scale study of population structure in Mexico during the Classic and Postclassic periods, supporting a migration history from the Aztitlan region in West Mexico to other areas of West Mexico as well as Central Mexico. Gomez-Valdez (2008) investigated Classic period (AD 300–900) relationships among samples largely derived from West Mexico.” He found the Postclassic period sample from Tlatelolco in Central Mexico to be similar to the samples from West Mexico, as well as to other Central Mexico samples from the Classic period. Distances were especially close between Tlatelolco and the samples from the Aztitlan region of West Mexico. Finally, Ragsdale and Edgar (in press) found similarities between Central, West, and North Mexican Postclassic samples, thereby providing additional evidence of migrations from the North and West of Mexico into Central Mexico during this period. Their study also found shared migration and trade to be related to biological distances among Postclassic populations in Mexico and those located further north in the American Southwest. Other biological distance studies using dental morphology traits from these regions have focused on testing hypotheses about migration, language group distribution, and shared cultural group (Turner, 1993; LeBlanc et al., 2008). Turner’s (1993) study of prehistoric and modern American Southwest and Mexican samples indicated that biological distances were related to both geographic proximity and cultural group, but were not related to linguistic classification. LeBlanc et al. (2008) also compared samples from the American Southwest and Mexico to examine how culture and language correlate with biological distances. Their results revealed small biological distances between American Southwest and Northern Mexican, combined Central Mexican, and Coahuilan (Northeast Mexico) samples. Unlike LeBlanc et al. (2008), who combined Mexican samples to examine the spread of Uto-Aztecan speakers, the current study evaluates cultural and biological relationships among samples without combining them by region or by modern nation-state. Others have used ancient DNA as the basis for assessing the relationship between biology and culture. Kemp et al.’s (2005) analysis of mtDNA found the sample from Tlatelolco to be more similar to other samples from Central and Southern Mexico than to other Uto-Aztecan speaking groups in Northern Mexico and the American Southwest. In a second study, Mata-Miguez et al. (2012) used mtDNA to investigate the effect of Aztec political dominance on the population structure of the city-state capital of Xaltocan. Their results show that populations present before and after the Aztec expansion possessed different mtDNA haplotypes, suggesting a change in population structure corresponding to the Aztec conquest of Xaltocan.

BIOLOGICAL DISTANCE IN POSTCLASSIC PERIOD MEXICO

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Fig. 1. Location of sites used in the study.

MATERIALS AND METHODS The geographic locations of all samples included in the current study are provided in Figure 1. We observed dental morphology traits in the permanent dentition of 810 individuals whose skeletal remains are housed at various institutions in the United States and Mexico. Since the objective of this study is to investigate the effects of trade and political relationships on population movements during the Postclassic period, we selected samples to represent populations believed to have been involved in various forms of trade, military, or political interaction, and who to have been claimed to be characterized by several different migration histories and cultural affiliations. All samples date to the Middle to Late Postclassic period (AD 1200–1520), with some assemblages possibly encompassing a few individuals from the Late Classic (AD 600–900), Early Postclassic (AD 900– 1200), and Early Colonial (AD 1520–1600) periods. Twelve samples are from Central Mexico (n 5 428), six are from West Mexico (n 5 155), and ten are from the peripheral regions of Northern Mexico (n 5 110), the Gulf Coast (n 5 107), and the Maya region (n 5 110). A list of samples, their dates, cultural group, and world system zone classification is provided in Table 1. Biological distances among samples were obtained from dental morphology trait frequencies and were compared with model matrices to determine whether cultural, political and trade relationships correlate with biological distances. The distance matrix based on dental morphology serves as a phenetic proxy for the genetic distances between samples. Geographic and cultural distances were recorded as pair-wise relationships using matrices for each variable tested. These variables include geographic distance, shared migration history,

trade, and trade and political interaction combined. These models were created using information from archaeological and ethnohistoric sources. Mantel and partial Mantel tests were used to investigate correlations between geographic, cultural, and biological distances.

Biological distances Observations were made on 62 maxillary and mandibular dental morphology traits on all permanent teeth for which these traits could be assessed. Consequently, traits were not recorded in cases of damaged teeth, teeth affected by severe dental wear, or teeth suffering from large carious lesions. Trait expressions were scored on a standardized ordinal graded scale in accordance with the Arizona State University Dental Anthropology System (Turner et al., 1991) and dichotomized for statistical analysis. Traits were scored on both left and right antimeres. The highest score for each trait, representing the maximum expression, was used to control for asymmetry (Turner and Scott, 1977; Turner et al., 1991; Scott and Turner, 1997). Dichotomization breakpoints were largely drawn from Scott and Turner (1997), but in some cases breakpoints were adjusted based on variation among the samples in the study. These traits include shoveling of the maxillary incisors, shoveling of the mandibular central incisors, and the protostylid (lower first and second molars). Using the breakpoints described in Scott and Turner (1997) results in shoveling on the maxillary and mandibular incisors being scored as present at high frequencies (over 0.90) for all of the samples used here. Therefore, breakpoints for shoveling were adjusted to grade 4 and above for maxillary incisors and grade 3 or above among mandibular incisors to be considered American Journal of Physical Anthropology

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C.S. RAGSDALE AND H.J.H. EDGAR TABLE 1. Sites represented by samples

Archaeological site (n) Valley of Mexico Azcapotzalco (47) Cholula (58) Culhuac an (32) Huexotla (20) Tenayuca (20) Teopanzolco (26) Tlatelolco (70) Xochimilco (49) Toluca Valley Teotenango (27) Texcaltitl an (27) Oaxaca Valley Yagul (32) Zaachila (20) Northwest Mexico Guasave (20) Huejuquilla (18) West Mexico Autlan-Tuxcacuesco (15) Chalpa-Tecualilla (22) Tzintzuntzan (30) Zacap u (50) Gulf Coast of Mexico Tamtok (37) Vista Hermosa (25) Zapotal (45) Maya Region Chicoasen (20) Cozumel (40) Jaina Island (50) Northern Mexico Cuatro Cienegas (18) Candelaria-Paila (32) Paquime (35) Nararachic (25)

Date (AD)

Cultural group

Observer

World system zone

1200–1520 1300–1520 1220–1520 1200–1520 1200–1520 1200–1550 1340–1520 1200–1520

Aztec-Tepeneca Tolteca-Cholulan Tolteca-Chichimeca Aztec-Alcolhua Tepenec-Chichimeca Aztec-Tlahuica Aztec-Mexica Aztec-Xochimilca

CR CR/HE CR CR CR CR CR/HE CR

Core/Exchange Core/Exchange Core/Exchange Core/Exchange Core/Exchange Production/Exchange Core/Exchange Core/Exchange

1250–1450 1250–1550

Matlatzinca-Chichimeca Matlatzinca-Chichimeca

CR CR

Extraction/Exchange Extraction/Exchange

1200–1520 1200–1520

Zapotec Zapotec

CR CR

Production Production

900–1400 1000–1550

Aztitlan Aztitlan

CR/HE HE

Extraction Extraction

1100–1550 1100–1350 1200–1550 1200–1550

West Mexico West Mexico Tarascan-Patzcuaro Tarascan-Zacapu

CR CR CR CR

Extraction Production/Extraction Core/Exchange Production/Exchange

1200–1550 1200–1550 1000–1550

Huastec Huastec Totonac

CR CR HE

Production Production Production/Exchange

1200–1550 1200–1550 900–1200

Mixe-Zoque/Maya Yucatec Maya Yucatec Maya

CR CR HE

Production/Extraction Production/Exchange Production/Extraction

1300–1500 1300–1500 1180–1400 1300–1600

Coahuilan-Chichimeca Coahuilan-Chichimeca Casas Grandes Casas Grandes- Raramuri

CR HE CR CR

Periphery Periphery Periphery Periphery

Corey Ragsdale (CR) and Heather Edgar (HE).

present. For the protostylid, the foramen caecum molaris (grade 1) was not scored as present as use of this expression is controversial. Hence, the protostylid was only considered as present when scored at grade 2 and above. Intra- and interobserver error tests were conducted to evaluate repeatability for each observer and replicability between observers. Observations were made on 50 dentitions from a prehistoric skeletal collection. Samples were selected based on the highest number of possible observations. We calculated Cohen’s kappa, standard error, and percent agreement. Tetrachoric correlation matrices and pseudo-Mahalanobis’ D2 values were calculated using SAS Statistical Software v.9.1.3. We used PAST (Hammer et al., 2001) to create a three-dimensional scatterplot of sample scores for the first three principal components to show the relationships between site samples. Dental morphology traits tend not to be sexually dimorphic (Scott and Turner, 1997). To test the effects of sexual dimorphism on our observations, we calculated trait frequencies separately for males and females and compared the frequencies using a v2 test. v2 Values indicate no significant difference between males and females. We also conducted a Mantel test to evaluate the correlation between the male and female datasets. The patterning of trait frequencies between males and females was found to be highly correlated (r 5 0.967, American Journal of Physical Anthropology

P 5 0.0002), with differences between males and females ranging from 0 to 0.25 among the traits used in the final analysis. Trait frequencies and adjusted breakpoints for Central Mexican, West Mexican and peripheral samples are provided in Supporting Information Tables 1 to 3, respectively. The pseudo-Mahalanobis’ D2 distance statistic was used to create the biological distance matrix (Konigsberg, 1990; Irish, 2010). This statistic extends the squared Mahalanobis’ distance for use with frequency data by using a tetrachoric correlation matrix in the calculation (Konigsberg, 1990). The tetrachoric correlation matrix consists of correlation coefficients computed for two normally distributed dichotomized variables, and it serves to account for inter-trait correlations. Traits with similar frequencies among all samples were removed before calculating the tetrachoric correlation matrix since variation could not be detected. Additionally, traits with missing data for more than half of the individuals within any of the samples were removed before calculating the tetrachoric correlation matrix to avoid misrepresentation of trait frequencies. These criteria resulted in the elimination of 39 traits, so that the tetrachoric correlation matrix used to generate pseudo-Mahalanobis D2 distances was based upon a final selection of 23 of the 62 traits scored. Some biological distance studies based upon dental morphology trait frequencies use the Mean

BIOLOGICAL DISTANCE IN POSTCLASSIC PERIOD MEXICO Measure of Divergence (MMD) statistic, since it is useful in comparing groups with small sample sizes and high amounts of missing data (de Souza and Houghton, 1977). Previous comparisons of pseudo-Mahalanobis’ D2 and MMD have shown overall agreement between biological distance matrices generated using both methods (Edgar, 2004; Irish, 2010).

Geographic and cultural variables We used Mantel and partial Mantel tests (Mantel, 1967) to determine correlations between distances obtained from biological, geographic, and cultural variables. Mantel tests compare matrices in thousands of permutations, eliminating assumptions about the distribution of the data being used (Manly, 1986). These tests were used to calculate correlations (ZN) between the geographic and cultural variables. Partial Mantel tests, which allow two matrices to be compared while controlling for similarities with a third matrix (Legendre and Legendre, 1998), were used to account for correlations among the variables. These analyses were conducted using PAST (Hammer et al., 2001). The cultural variables tested include shared migration history, trade, and political interaction. We combined trade and political interaction into a single matrix to represent total economic-political interaction. Finally, we performed Mantel tests for groups by region to assess differences among geographic regional networks. Geographic distance. This study tests an isolation by distance model, which assumes that as geographic distances increase, so too do biological distances (Wright, 1943). Geographic distances were calculated using global positioning system (GPS) coordinates between any two sites following the shortest possible route given major geographic barriers such as mountains or large bodies of water (see Fig. 1 for GPS locations). GPS coordinates and geographic distances were obtained using Google Earth. The matrix of geographic distances is provided in Table 2. Shared migration history. A model matrix was used to determine the relationship between biological variance and shared migration history. The model matrix was based upon expected relationships drawn from available ethnohistoric and linguistic data for the Postclassic period (see Fig. 2a,b for an illustration of this model). Migration histories among the samples used here were estimated primarily from such ethnohistoric migration documents as the Boturini Codex, Codex Xolotl (Douglas, 2010), Codex Cozcatzin, and Codex Chimalpopoca (Bierhorst, 1992). These sources provide a wealth of information about migrations, primarily for Central and West Mexican populations. Further ethnographic sources for migration are provided in Smith (1984). Where ethnohistoric sources were unavailable, migration was inferred from archaeological data (Lekson and Cameron, 1995; Townsend, 2000; Benson et al., 2007; Beekman, 2010). For this study, we focus on largescale migrations, since these are the most evident in the ethnohistoric and archaeological records. These data were used to create a model matrix representing relationships among all of the samples, with each branch representing the amount of time since the most recent. Distances between samples used for this matrix were

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based on branch lengths from shared parent populations. These branches represent the number of generations since a split from a shared parent population based on 25-year generations. These distances account for the effects of genetic drift over time. To account for cases where two samples shared multiple migrant groups, we created a second matrix based on the number of parent populations shared between samples (see Fig. 2a). We then divided the values of the first matrix by the values of the second matrix to form the final shared migration history matrix used in the Mantel tests. For example, the Central Mexico samples from Culhuacan (CU) and Huexotla (HX) have a score of “5,” since they share 5 parental population groups (see Fig. 2b). This scaled the original matrix to account for multiple shared migrations between groups. For this model we assume all migrant populations are equal size, since it is not possible to discern otherwise from archaeological or ethnohistoric data. Since this model is based on populations founding new sites, we also assume equal size of populations for initial settlements. Trade. We understand trade as relationships determined by trade systems that crossed political boundaries as defined by Mesoamerican world systems theory (Smith and Berdan, 2000; Smith, 2001). Trade existed on a variety of scales among different regions throughout Mexico during the Postclassic period, ranging from local trade networks within a city or town, to intraregional exchange, to long-distance trade (Berdan, 1989; Blanton, 1996; Gasco and Berdan, 2003). Trade was facilitated through trade routes that stretched across political borders and through regional markets that were in continuous use throughout the year. Distances were recorded between samples according to trade relationships documented by archaeological and ethnohistoric data. We classified each sample in the study as a local, regional, or inter-regional market (Berdan, 1989; Blanton, 1996). Local markets were periodic events where local food and utility items were exchanged, and acted as venues for frequent intra-population trade. Regional markets were more frequent and often perennial events that incorporated luxury items as well as utility items (Berdan, 1989; Blanton, 1996), and were venues for much inter-population and intraregional interaction among populations. Lastly, inter-regional markets were year-round, large markets that featured the exchange of a wide variety of items from various regions within and adjacent to Mexico (Berdan, 1989; Blanton, 1996, Gasco and Berdan, 2003). Inter-regional markets were venues for inter-regional interaction, as well as a place of settlement for merchants such as the pochteca, who were professional Aztec merchants. Trade relationships were measured in distances between these various market exchange forms, reflecting population interaction through trade. Low scores were recorded for interactions between local markets and regional/inter-regional markets in the same region and between regional markets; high scores were recorded for interactions between local markets and regional markets from other regions, and between local markets in different regions. Since interregional markets served as regional exchange centers for numerous regions, interactions with these markets were scored as regional markets for multiple, adjacent regions. Trade distances were based on average, American Journal of Physical Anthropology

Azcapotzako Cholula Culhuacan Huexotla Teotenango Teopanzolco Texcaltitlan Tenayuca Tlatelolco Xochimilco Yagul Zaachila Tamtok Vista Hermosa Zapotal Chicoasen Cozumel Jaina Island Paquime Cienegas Candelaria Nararachic Tuxcacuesco Tecualilla Guasave Huejuquilla Tzinzuntzan Zacapu

5.43 34.81 31.21 10.42 18.52 24.17 29.66 16.18 13.43 13.09 16.40 22.67 10.63 15.01

9.27 25.69 27.01 14.06 16.26 25.42 25.09 18.53 12.14 15.47 30.85 41.43 7.83 11.80

CH

163 0.00 6.97 10.25 6.28 19.59 14.03 12.67 11.59 13.04 16.02 20.88 11.71 11.42

AZ

0.00 6.35 6.16 8.43 7.93 11.32 10.26 12.36 9.07 5.42 10.47 15.65 18.01 15.16

8.88 42.11 29.34 9.98 20.81 30.38 28.31 13.64 11.46 10.52 25.96 29.40 9.04 17.16

18 145 0.00 12.88 10.54 24.37 12.98 13.48 5.31 7.08 21.95 27.84 10.99 12.97

CU

15.50 25.51 27.73 17.18 14.67 34.90 43.89 21.85 9.96 9.97 22.67 32.63 8.66 17.28

31 125 30 0.00 10.64 12.24 6.17 9.63 9.31 11.78 7.84 6.69 10.25 7.82

HX

17.15 33.83 41.67 18.17 21.41 35.88 37.31 24.18 20.12 16.46 18.89 27.71 14.01 15.11

78 188 69 86 0.00 20.03 12.98 11.60 14.60 17.82 15.92 13.52 17.52 15.39

TG

25.75 13.81 23.19 34.50 20.38 29.06 39.26 40.86 20.11 11.13 44.42 58.94 14.07 10.26

8 137 23 30 66 0.00 11.32 13.19 22.75 11.35 7.80 11.55 31.43 21.18

TP

17.46 26.63 32.67 21.73 12.51 30.25 39.56 19.27 15.19 10.44 27.30 37.35 6.45 9.89

69 150 48 73 46 70 0.00 8.26 13.14 8.54 10.01 9.66 17.37 16.29

TX

14.39 37.80 29.55 15.77 14.48 32.24 35.43 19.94 19.14 16.74 17.70 23.78 12.30 13.56

128 206 110 125 40 144 75 0.00 10.18 13.60 18.04 14.41 12.10 11.25

TN

10.36 45.66 22.89 8.89 19.67 41.10 34.57 16.33 13.65 11.84 23.75 27.36 18.45 10.03

11 151 13 31 60 6 65 115 0.00 8.04 19.82 25.43 10.09 10.55

TO

15.02 26.37 17.36 21.45 16.80 25.13 27.25 20.42 9.52 13.22 36.64 44.80 15.60 11.87

20 130 10 37 53 35 40 95 29 0.00 12.06 21.85 17.35 17.60

XO

19.82 11.07 23.75 27.53 21.70 30.26 48.04 35.50 11.96 9.20 36.70 50.20 17.84 12.76

404 302 386 413 407 409 368 429 405 434 0.00 5.64 21.85 18.75

YG

25.97 19.75 40.32 29.42 20.53 40.55 56.15 36.78 21.47 16.53 31.48 46.93 12.57 11.87

385 287 363 390 384 386 343 404 382 411 31 0.00 25.80 21.60

ZA

12.66 47.36 26.03 12.73 25.32 36.62 43.73 18.00 10.06 18.61 13.85 13.23 10.19 12.11

300 327 304 274 346 345 363 275 286 309 626 684 0.00 5.03

TK

12.54 37.05 20.71 12.23 26.58 36.10 40.01 27.83 11.81 17.85 19.33 20.85 10.65 18.37

357 393 374 342 411 410 426 343 355 378 697 613 170 0.00

VH

0.00 36.36 27.93 3.17 23.72 25.64 25.22 20.28 15.73 17.96 17.91 21.44 13.61 16.19

482 382 466 452 512 468 539 497 502 486 189 224 592 763

ZP

230 0.00 20.81 28.06 49.57 41.69 26.15 48.63 67.49 21.31 24.01 28.95 23.40 29.04

703 598 681 672 725 682 756 712 712 704 356 392 820 993

CN

890 740 0.00 35.11 36.79 28.46 35.77 49.47 15.33 17.95 46.34 50.25 20.28 20.00

1303 1198 1281 1272 1325 1282 1356 1312 1312 1304 1077 1107 1475 1645

CZ

630 475 475 0.00 26.69 40.39 32.41 22.01 21.53 24.08 16.50 18.94 18.02 18.44

1603 1498 1581 1572 1625 1582 1656 1612 1612 1604 780 810 1190 1360

JA

1940 2100 2740 2560 0.00 26.91 26.19 12.07 22.00 17.27 19.76 33.88 15.39 12.39

1497 1587 1605 1635 1538 1655 1508 1598 1605 1625 1876 1871 1301 1123

PA

CP

NA

1227 1457 2500 2100 680 0.00 16.96 33.85 18.36 25.60 39.04 46.15 24.38 29.90

1105 1335 2357 1935 763 149 0.00 32.75 29.52 28.35 42.11 49.73 30.64 30.78

1657 1817 2457 2277 283 397 480 0.00 25.54 24.20 18.24 24.40 17.59 23.94

883 753 1229 961 833 1330 979 851 1348 1009 881 1378 948 809 1266 1029 901 1398 918 779 1236 972 844 1341 979 851 1348 999 871 1368 1286 1147 1604 1281 1142 1599 656 534 1018 466 371 840

CC

1023 742 1623 1923 1245 823 684 950 0.00 7.90 28.59 33.41 15.10 13.71

514 630 648 678 473 698 443 641 648 668 811 806 934 954

AT

TABLE 2. Pseudo-Mahalanobis’ D2 distance matrix (bottom diagonal) and geographic distances in kilometers (top diagonal) CT

1283 1002 1883 2183 884 597 483 591 367 0.00 24.51 25.36 8.20 11.21

765 890 908 938 764 958 734 901 908 928 1102 1097 1194 1214

GU

1685 1404 2285 2585 539 657 635 303 750 409 0.00 3.43 25.38 19.22

1169 1292 1310 1340 1181 1360 1151 1303 1310 1330 1519 1514 1596 1616

HU

963 682 1563 1863 1171 710 564 889 127 342 754 0.00 33.70 27.71

446 570 588 618 430 638 400 581 588 608 768 763 874 894

TZ

766 485 1366 1666 1354 821 666 1070 261 552 985 250 0.00 3.15

251 373 391 421 218 441 188 384 391 411 556 551 677 697

ZU

790 509 1390 1690 1321 788 633 1037 780 413 952 217 33 0.00

275 397 415 445 241 465 211 408 415 435 579 574 701 721

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Fig. 2. a. Model for shared migration history. Branch lengths representing time since split from parent population. Abbreviations: Autlan-Tuxcacuesco (AT); Azcapotzalco (AZ); Candelaria-Paila (CP); Chalpa-Tecualilla (CT); Chicoasen (CN); Cholula (CH); Cozumel (CZ); Culhuacan (CU); Cuatro Ci enegas (CC); Guasave (GU); Huejuquilla (HU); Huexotla (HX); Jaina Island (JA); Nararachic (NA); Paquim e (PA); Tamtok (TK); Tenayuca (TN); Teopanzolco (TP), Teotenango (TG); Texcaltitl an (TX); TlatelolcoTenochtitlan (TO); Tzintzuntzan (TZ); Vista Hermosa (VH); Xochimilco (XO); Zaachila (ZA); Yagul (YG); Zacapu (ZU); and Zapotal (ZP). Examples of scores are shown with arrows. b. Model for shared migration history. Shared migrant groups. Abbreviations are the same as in a. Examples of scores are shown with arrows.

generalized relationships among sample pairs to account for shifting trade relationships over time. For example, the samples from Tlatelolco (TO) and Tzintzuntzan (TZ) have a score of “2” because both sites were venues for regional markets, but Tlatelolco also served as an interregional market for both regions (see Fig. 3). Political interaction. We classified political interaction among samples as distances between groups based upon frequency of political contact. Political alliances, military conquest, and the control of resources led to dominant imperial polities such as the Aztec and Tarascan Empires. Particularly in Central Mexico, political relation-

ships manifested as tribute payments and military campaigns are well documented in ethnohistoric sources such as the Codex Mendoza (Berdan and Anawalt, 1997). We recorded all samples as either a city-state or a capital city-state. City-states are samples in the study drawn from populations that did not serve as a political capital, but are independent cities within a larger polity. These city-states may also be important trade or religious centers independent of, or in addition to, their status as political centers. City-state capitals are capital cities within a city-state cultural region or polity that controlled other city-states or towns with the same political affiliation. Political distances were measured for different political relationships among all city-states in the study. American Journal of Physical Anthropology

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Fig. 3. Model for economic exchange (trade). Abbreviations: Autlan-Tuxcacuesco (AT); Azcapotzalco (AZ); Candelaria-Paila (CP); Chalpa-Tecualilla (CT); Chicoasen (CN); Cholula (CH); Cozumel (CZ); Culhuacan (CU); Cuatro Ci enegas (CC); Guasave (GU); Huejuquilla (HU); Huexotla (HX); Jaina Island (JA); Nararachic (NA); Paquim e (PA); Tamtok (TK); Tenayuca (TN); Teopanzolco (TP), Teotenango (TG); Texcaltitl an (TX); Tlatelolco-Tenochtitlan (TO); Tzintzuntzan (TZ); Vista Hermosa (VH); Xochimilco (XO); Zaachila (ZA); Yagul (YG); Zacapu (ZU); and Zapotal (ZP). Examples of scores are shown with arrows.

Fig. 4. Model for political interaction. Abbreviations: Autlan-Tuxcacuesco (AT); Azcapotzalco (AZ); Candelaria-Paila (CP); Chalpa-Tecualilla (CT); Chicoasen (CN); Cholula (CH); Cozumel (CZ); Culhuacan (CU); Cuatro Ci enegas (CC); Guasave (GU); Huejuquilla (HU); Huexotla (HX); Jaina Island (JA); Nararachic (NA); Paquim e (PA); Tamtok (TK); Tenayuca (TN); Teopanzolco (TP), Teotenango (TG); Texcaltitl an (TX); Tlatelolco-Tenochtitlan (TO); Tzintzuntzan (TZ); Vista Hermosa (VH); Xochimilco (XO); Zaachila (ZA); Yagul (YG); Zacapu (ZU); and Zapotal (ZP). Examples of scores are shown with arrows.

Low scores were recorded for direct political relationships such as political allies and tributary provinces; high scores were recorded for relationships including frontier provinces, frequent hostile interactions between two samples, and groups that had only occasional interactions such as raiding. Frontier province relationships were scored lower than hostile interactions since these areas were subject to colonization (Berdan et al., 1996). Raiding was assigned the highest score since raids were sporadic, brief, and likely did not involve population interaction other than the taking of prisoners (LeBlanc, 1999; Kelly, 2000; Elliot, 2005). Political relationships American Journal of Physical Anthropology

exhibited considerable dynamism throughout the Postclassic period, so we average relationships in accordance with archaeological and ethnohistoric information. For example, the samples from Tlatelolco (TO) and Tzintzuntzan (TZ) have a score of “4” because there were frequent hostile incursions between these populations (see Fig. 4).

Trade and political interaction. There is missing data in the political relationship matrix, because not all groups interacted politically. For this reason, we

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BIOLOGICAL DISTANCE IN POSTCLASSIC PERIOD MEXICO TABLE 3. Results of Mantel and partial Mantel tests Variable (s)

Fig. 5. Three dimensional principal components analysis of pseudo-Mahalanobis’ D2 distances. Shapes represent geographic regions: square 5 Central Mexico; triangle 5 West Mexico; inverted triangle 5 Maya region; diamond 5 Northern Mexico; and cross5Gulf Coast. Abbreviations: Autlan-Tuxcacuesco (AT); Azcapotzalco (AZ); Candelaria-Paila (CP); Chalpa-Tecualilla (CT); Chicoasen (CN); Cholula (CH); Cozumel (CZ); Culhuacan (CU); Cuatro Cienegas (CC); Guasave (GU); Huejuquilla (HU); Huexotla (HX); Jaina Island (JA); Nararachic (NA); Paquim e (PA); Tamtok (TK); Tenayuca (TN); Teopanzolco (TP), Teotenango (TG); Texcaltitl an (TX); Tlatelolco-Tenochtitlan (TO); Tzintzuntzan (TZ); Vista Hermosa (VH); Xochimilco (XO); Zaachila (ZA); Yagul (YG); Zacapu (ZU); and Zapotal.

compared the biological distance matrix to a matrix representing combined trade and political interaction. The pairwise values of both matrices were summed together, as practiced by other researchers using matrices representing economic or ecological variables (Case, 1991; Beck et al., 2006; Deniz-Filho et al., 2013). Since neither trade nor political interaction can be accurately weighted between prehistoric groups, these matrices were not scaled when combined as suggested by Case (1991). Trade and political interaction were combined as they represent the population contact occurring during the Postclassic through cultural interaction. In other words, this combined matrix accounts for what is known about population interaction through trade, tribute, war, and political alliance based on archaeological and ethnohistoric records. Low scores represent groups with high frequencies of trade and political interaction; high scores represent groups with low frequencies of trade and political interaction, or groups with low frequencies of trade and no political interaction. For example, the trade/political distance between Tlatelolco (TO) and Tzintzuntzan (TZ) is “6” because the trade distance between them is “2” and the political distance is “4” (see Figs. 3 and 4).

RESULTS Results of the inter- and intraobserver error tests show high concordance between observations and observers. Kappa values for interobserver error range from 0.7 to 1.0, and percent agreement ranges from 86% to 100%. Both intraobserver tests showed a j value of 0.85 to 1.0, and a percent agreement of 91% to 100%. These results are also available in Ragsdale and Edgar (in press), and additional intraobserver error results are provided in Edgar (2002). The pseudo-Mahalanobis’ D2 distance matrix and the matrix of pairwise geographic distances are provided in

Mantel tests for geographic and cultural variables Geographic 3 migration Geographic 3 trade Geographic 3 trade/political Migration 3 trade Migration 3 trade/political Mantel and partial mantel tests for pseudo-Mahalanobis D2 distances Geographic distance Shared migration Trade Trade/political Geographic 1 migration Geographic 1 trade Geographic 1 trade/political Migration 1 geographic distance Migration 1 trade Migration 1 trade/political Trade 1 geographic distance Trade 1 migration Trade/political 1 geographic distance Trade/political 1 migration

Correlation (ZN)

P

0.581 0.173 0.334 0.219 0.366

0.001** 0.057 0.002** 0.011* 0.002**

0.054 0.348 0.344 0.374 0.073 0.072 0.101 0.258 0.272 0.218 0.369 0.362 0.382 0.342

0.794 0.002** 0.002** 0.002** 0.888 0.869 0.956 0.004* 0.005* 0.013* 0.001** 0.002** 0.001** 0.001**

P values are averaged over 10 tests. Italics indicate the variable that is controlled in the partial Mantel tests. *0.05; **0.005.

Table 2. Generally, there are low distances between sites with regional or inter-regional markets, and between capital city-states and other samples within their respective regions. Figure 5 is a scatterplot showing these relationships on the first three principal axes, which account for approximately 83% of the total variation. Results of the principal component analysis show that many of the samples from Central Mexico, West Mexico, and the Gulf Coast are clustered together on Components 1 and 2. The samples from Central Mexico are slightly separated on Component 1. Texcaltitl an (TX), Xochimilco (XO), Azcapotzalco (AZ), and Huexotla (HX) are more similar to some samples from West Mexico; Tenayuca (TN), Teotenango (TG), Cholula (CH), Culhuacan (CU), and Tlatelolco (TO) are more similar to the Gulf Coast samples from Tamtok (TK), Vista Hermosa (VH), and Zapotal (ZP). Other samples from each region, excluding the Gulf Coast, are located outside this cluster. Among the closest similarities are those between Azcapotzalco (AZ) and Xochimilco (XO), Cholula (CH) and Zapotal (ZP), Culhuacan (CU) and Tlatelolco (TO), Tamtok (TK) and Vista Hermosa (VH), Jaina Island (JA) and Zapotal (ZP), Huejuquilla (HU) and Guasave (GU), and Tzintzuntzan (TZ) and Zacap u (ZU). The matrices for shared migration history, trade, political interaction, and combined trade/political interaction are presented in Supporting Information Tables 4 to 7, respectively. Results of the Mantel and partial Mantel tests between all variables are provided in Table 3. Most of the distances are highly significant, which highlights the importance of the partial Mantel tests. The results of the Mantel tests comparing the pseudo-Mahalanobis’ D2 distances to the geographic and cultural variables show that shared migration (ZN 5 0.348, P 5 0.002), trade (ZN 5 0.344, P 5 0.002), and trade/political interaction (ZN 5 0.374, P 5 0.002) are significantly correlated with biological distances, while geographic distance (ZN 5 0.054, P 5 0.794) is not. When analyzed further American Journal of Physical Anthropology

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using partial Mantel tests, geographic distance remains uncorrelated. Mantel tests for analyses within regions are available in Supporting Information Table 8. In all regions, shared migration history is correlated with biological distances. Trade and trade/political interaction are correlated with biological distances among groups in Central Mexico and the Gulf Coast/Maya region, but trade and trade/political interaction are not correlated with biological distances among groups in Northern and West Mexico.

DISCUSSION The pseudo-Mahalanobis’ D2 distance matrix reveals strong phenetic similarities among most of the groups within Central Mexico, as well as between these Central Mexican samples and samples from parts of West Mexico and the Gulf Coast. It is not surprising that many of the groups in the Valley of Mexico are similar to each other, since they share migration histories (Lopez Austin and Lopez Lujan, 2001; Berdan, 2004; Douglas, 2010), were in frequent contact through economic processes such as regional and inter-regional trade (Smith and Berdan, 2003), and are geographically proximate to each other. However, some of the samples from the Valley of Mexico proved to be unexpectedly more similar to samples from other areas in Central Mexico and other regions. These include: Teopanzolco (TP) and the Oaxaca Valley samples; Cholula (CH) and the Gulf Coast sample of Zapotal (ZP); and the cluster of Chichimec samples from the Valley of Mexico (Tenayuca (TN) and Huexotla (HX)) and the Toluca Valley (Texcaltitl an (TX) and Teotenengo (TG)) to the West. The samples from the Huasteca region (Tamtok (TK) and Vista Hermosa (VH)) are also unexpectedly similar to samples from the Valley of Mexico (Tlatelolco-Tenochtitlan (TO) and Culhuac an (CU)), given the relative geographic isolation of this region. The similarities between some of the West Mexico samples and the samples from Central Mexico are also surprising. These regions were politically and geographically disparate, and interaction between the two regions is believed to have been primarily hostile (Hassig, 1988; Berdan et al., 1996; Pollard and Smith, 2003). None of the geographic or cultural variables are correlated with biological distances in West Mexico. This may be due to limited samples available for this region; more data from other groups in this region will help further elucidate these relationships. The results obtained concerning each of our hypotheses are discussed below.

Hypothesis 1: Geographic distances are correlated with biological distances This hypothesis is not supported by the Mantel or partial Mantel tests. Although many of the samples fit within clusters of other samples within their respective geographic region, the Mantel and partial Mantel tests indicate populations are not most similar to other populations to which they are geographically most proximate. This is particularly true in Central Mexico, where all of the samples are relatively similar to one another. Even when similarities with all other variables are controlled, geographic distance remains uncorrelated. The principal component scatterplot also shows significant overlap between the samples from West Mexico, Central Mexico, and the Gulf Coast. When viewed regionally, geographic distance remains uncorrelated in the Mantel tests, indiAmerican Journal of Physical Anthropology

cating that proximity within geographic regions is not related to phenetic similarity. We find it particularly surprising that geographic proximity shares no significant relationship with phenetic similarity. This result indicates that population interaction was not primarily a function of geographic proximity. Instead, population structure among Postclassic Mexican groups appears to have been more the product of a small geographic area and intense population interaction trough cultural processes over a relatively short period of time (see below). Populations frequently moved from one place to another throughout Mesoamerica, especially in regions such as Central Mexico. Smith (2014) points out that people, particularly peasants, move much more frequently than archaeologists have acknowledged, and that these movements were heavily influenced by economic and political factors. These movements include inter-regional and regional migration, as well as village nucleation, where peasants from adjacent hinterlands move to urban centers, due to both push factors (protection against invaders) and pull factors (economic opportunities) (Smith, 2014). Our results support the validity of these assertions throughout all regions in Mexico. This is further evidenced by the regional Mantel tests. Geographic distance is not significantly correlated in any region, though it comes close to significance in Northern Mexico (ZN 5 0.298, P 5 0.071), where seminomadic populations occupied much of the area and urban centers are limited to sites like Paquime. Overall, our results suggest that, regarding geographic distance, Postclassic Mexico is exceptional when compared to many other biological distance studies, because geographic distance is confounded by cultural interactions through trade and political processes.

Hypothesis 2: Shared migration history is correlated with biological distances This hypothesis is supported by the Mantel and partial Mantel tests. These results are concordant with migration histories developed using archaeological and ethnohistoric sources. Several Central and West Mexico samples are biologically similar to each other, as expected if those samples shared a parent population originating in West Mexico. There are also close similarities among the samples obtained from Chichimec or Matlatzinca settled sites. This agrees with the origin account of Northern Mexican migrations into West and Central Mexico (Lopez Austin and Lopez Lujan, 2001; Berdan, 2004; Douglas, 2010). Small biological distances among Central and the West Mexico samples also support these migration origin accounts. The Gulf Coast samples are also similar to the Central Mexican samples from Tenayuca, Teotenango, Cholula, Culhuac an, and Tlatelolco-Tenochtitlan. Such similarities may reflect the migration of Toltec populations after the demise of Tula in the twelfth century, particularly for Culhuac an and Cholula, remnant Toltec cities that survived the Toltec collapse (Berdan et al., 1996; Lopez Austin and Lopez Lujan, 2001; Smith and Berdan, 2003). The Mexica at Tlatelolco and Tenochtitlan claimed a strong heritage with the Toltecs in Central Mexico to legitimate their claims for power, and were mercenaries of Culhuac an before establishing their own city at Tenochtitlan (Berdan, 2004, 2014). Although migration origin accounts suggest migrants from Northern and West Mexico, migrations from adjacent cities in the Valley of Mexico

BIOLOGICAL DISTANCE IN POSTCLASSIC PERIOD MEXICO may have also contributed to population growth of the two cities. The sample from Tlatelolco-Tenochtitlan may include individuals from other remnant Toltec populations in the area, who were resident for economic or political reasons. As such, the similarities between the Gulf Coast and Central Mexico sites reflect the migrations of Toltec groups to the Gulf Coast (Lopez Austin and Lopez Lujan, 2001). The sample from Jaina Island is most similar to Gulf Coast and Central Mexico groups, and different from the other Maya samples. These results indicate a shared migration history between the Maya groups on the Gulf Coast of the Yucatan Peninsula, and groups from Central Mexico and the Gulf Coast around Veracruz. More data from the Maya region are necessary to evaluate these histories.

Hypothesis 3: Trade and political interaction are correlated with biological distances This hypothesis is supported by the Mantel and partial Mantel tests. Trade is correlated with biological distances, even when similarities with geographic distance, shared cultural group, and political interaction are held constant. Results suggest that groups in frequent contact through market exchange and trade routes share close biological affinities. These relationships are present in West and Central Mexico, where most of the samples are grouped together and small biological distances reflect gene flow facilitated by close trade relationships. The Gulf Coast samples, particularly Vista Hermosa and Tamtok, are similar to Central Mexico groups. In addition to a shared migration of Toltec populations, Central Mexico maintained strong trade relationships with the Gulf Coast (Diehl, 2000; Lopez Austin and Lopez Lujan, 2001; Pool, 2006). Cholula is also similar to samples from the Valley of Mexico and Gulf Coast. The large inter-regional market at Cholula was centrally located between the Gulf Coast and the Valley of Mexico, and likely served as a venue for interaction between these regions (McCafferty, 1996). The Central Mexico sample of Teopanzolco is similar to the samples from the nearby Oaxaca Valley (Yagul and Zaachila). Teopanzolco had an established regional market, and participated in political and economic activities independent of the Aztec Empire (Hodge and Smith, 1994; Smith and Berdan, 2003). It is possible these trade relationships with populations inhabiting the Oaxaca Valley to the south had an impact upon immigration between these areas. Additionally, the sample from Paquime is phenetically similar to many of the samples from Central and West Mexico. Paquime is located along a major inter-regional trade route connecting the American Southwest and Mesoamerica, and was likely a large center for trade between these regions (DiPeso, 1974; Kelley et al., 2012). Samples obtained from sites located away from major markets and trade routes, such as those from Northeast Mexico and the Maya region, are biologically most segregated from the other samples. The exception is the sample from Jaina Island, whose inhabitants, given the geographic location of the site, may have been involved in trade with other populations inhabiting sites along the Gulf Coast. The Maya site of Cozumel was located along a major trade route, and served as an important trade center (Gasco and Berdan, 2003), but its location is geographically isolated from the other samples in this study. Trade relationships remain correlated with biological distances

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when political interaction is held constant. Such results suggest that trade had an effect on population interaction independent of political relationships. This variable, trade, is the most significant variable in the Mantel and partial Mantel tests. Regionally, trade is correlated with biological distances in Central Mexico, Northern Mexico, and the Gulf Coast/Maya region, but not in West Mexico. The combination of trade and political interaction provides a more complete picture of the important cultural processes among Postclassic Mexican groups. Trade/ political interaction is highly correlated with biological distances, suggesting that political relationships, where they can be documented, contributed to population interaction. This is particularly true for Central Mexico and the Gulf Coast/Maya region, where trade/political interaction is highly correlated with biological distances (see Supporting Information Table 8). These results highlight the importance of considering political interaction regionally in future research.

CONCLUSION Migration during the Postclassic period in Mexico appears to have been frequent, and occurred at both regional and inter-regional scales. Much of what we know about migration history in prehistoric Mexico is based on account of origins, linguistic similarities, shared ideology, and similarities in material culture. Many of these migration patterns are debated among scholars of Mesoamerican population history, particularly with regard to those that involved the populations of Central and West Mexico. Our results provide biological support for several migration origins during the Classic (AD 300–900) and Early Postclassic (AD 900– 1200) periods, rather than a single migration from one particular region in Mexico. For Central Mexico, our results suggest migrants came from Northern, West, and Central Mexico. This supports the Aztec origin account of a migration to the Valley of Mexico from Aztlan, perhaps located in West or Northern Mexico (Lopez Austin and Lopez Lujan, 2001; Berdan, 2004). Migrations from Northern and West Mexico are also supported for the West Mexico samples. Biological similarities suggest shared migrations among the samples from both geographic regions. It is important to note that correlations for shared migration history are significant but relatively low, ranging from ZN 5 0.21 to ZN 5 0.27 when trade, trade/political interaction, and geographic distance are controlled in the partial mantel tests. These results indicate genetic drift has an effect on phenetic variation among samples, but does not fully explain the variation among samples. Trade networks throughout the period likely facilitated contact among groups all over Mexico and beyond, but imperial expansion did not occur until relatively late (after AD 1400) (Hassig, 1988; Berdan et al., 1996; Lopez Austin and Lopez Lujan, 2001; Berdan, 2004). The Spanish conquest of Mexico, beginning in AD 1520, put an abrupt halt to political expansion and control of the Aztec and Tarascan Empires. Before contact, the Aztecs had maintained a large empire, but the populations encompassed by this polity were loosely controlled through tributary and strategic provinces and shifting alliances. Unlike the Aztecs, the Tarascans maintained firm control within their political realm, but they were unable to expand their empire far beyond the Zacapu American Journal of Physical Anthropology

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and P atzcuaro basins (Lopez Austin and Lopez Lujan, 2001; Pollard, 2003; Beekman, 2010). Since many groups in our study shared no political relationships, this variable is not sufficient on its own for comparing with other matrices. When combined with trade to account for missing data, the combined trade/political interaction variable is highly correlated with biological distances. These results highlight the effects of cultural relationships on biological distances among populations in Postclassic period Mexico. Unlike political relationships, trade relationships were long-standing throughout the entire period. Many trade routes remained from the previous Classic period, and continued to permit population movement as political dominance shifted among groups. Archaeological findings and ethnohistoric records in Mexico indicate that trade existed independent of political relationships. Our results support these findings, and suggest that regional and inter-regional trade had an effect on migration and population structure among Middle to Late Postclassic Mexican groups. Migrations facilitated through trade were likely unrestricted by the boundaries of Mesoamerica, for groups considered peripheral to the core of Mesoamerican interaction sphere participated in interregional exchange, and this participation appears to have had an impact upon population structure. In this study, trade is the variable most significantly correlated with biological distances. We suggest overall trade relationships seen through market interactions had an important effect on population interaction in Mexico during the Postclassic period.

ACKNOWLEDGMENTS The authors are grateful to the individuals at the institutions from which data were collected for this study for their support and assistance: Jose Antonio Pompa y Padilla and David Volcanes, Instituto Nacional de Antropologia e Historia; David Hunt, National Museum of Natural History; Estela Martinez, Guillermo Cordova, and their staff at the Escuela Nacional de Antropologia e Historia; Emiliano Melgar and the staff of the Museo del Templo Mayor; Ian Tattersall and Gisselle Garcia, American Museum of Natural History; and the staff of the Maxwell Museum Laboratory of Human Osteology, Albuquerque, New Mexico. The authors also thank Frances Berdan for her valuable input on cultural interactions among the groups considered in this study. The authors are grateful to the editors and reviewers of the American Journal of Physical Anthropology for their insight and feedback.

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