A Hypothesis for Localized Community Patterns on Rapa Nui (Easter Island, Chile)
Total Page:16
File Type:pdf, Size:1020Kb
Binghamton University The Open Repository @ Binghamton (The ORB) Anthropology Faculty Scholarship Anthropology Spring 5-7-2021 Population structure drives cultural diversity in finite populations: A hypothesis for localized community patterns on Rapa Nui (Easter Island, Chile) Carl P. Lipo Binghamton University--SUNY, [email protected] Robert J. DiNapoli Binghamton University--SUNY, [email protected] Mark E. Madsen Binghamton University--SUNY, [email protected] Terry L. Hunt University of Arizona, [email protected] Follow this and additional works at: https://orb.binghamton.edu/anthropology_fac Part of the Archaeological Anthropology Commons Recommended Citation Lipo CP, DiNapoli RJ, Madsen ME, Hunt TL (2021) Population structure drives cultural diversity in finite populations: A hypothesis for localized community patterns on Rapa Nui (Easter Island, Chile). PLOS ONE 16(5): e0250690. https://doi.org/10.1371/journal.pone.0250690 This Article is brought to you for free and open access by the Anthropology at The Open Repository @ Binghamton (The ORB). It has been accepted for inclusion in Anthropology Faculty Scholarship by an authorized administrator of The Open Repository @ Binghamton (The ORB). For more information, please contact [email protected]. PLOS ONE RESEARCH ARTICLE Population structure drives cultural diversity in finite populations: A hypothesis for localized community patterns on Rapa Nui (Easter Island, Chile) 1 1 2 3 Carl P. Lipo , Robert J. DiNapoliID *, Mark E. MadsenID , Terry L. Hunt 1 Department of Anthropology, Environmental Studies Program, Harpur College of Arts and Sciences, a1111111111 Binghamton University, Binghamton, NY, United States of America, 2 Department of Anthropology, University of Washington, Seattle, WA, United States of America, 3 The Honors College and School of a1111111111 Anthropology, University of Arizona, Tucson, AZ, United States of America a1111111111 a1111111111 * [email protected] a1111111111 Abstract Understanding how and why cultural diversity changes in human populations remains a cen- OPEN ACCESS tral topic of debate in cultural evolutionary studies. Due to the effects of drift, small and iso- Citation: Lipo CP, DiNapoli RJ, Madsen ME, Hunt lated populations face evolutionary challenges in the retention of richness and diversity of TL (2021) Population structure drives cultural diversity in finite populations: A hypothesis for cultural information. Such variation, however, can have significant fitness consequences, localized community patterns on Rapa Nui (Easter particularly when environmental conditions change unpredictably, such that knowledge Island, Chile). PLoS ONE 16(5): e0250690. https:// about past environments may be key to long-term persistence. Factors that can shape the doi.org/10.1371/journal.pone.0250690 outcomes of drift within a population include the semantics of the traits as well as spatially Editor: Borja Esteve-Altava, Institute of structured social networks. Here, we use cultural transmission simulations to explore how Evolutionary Biology, Pompeu Fabra University, social network structure and interaction affect the rate of trait retention and extinction. Using SPAIN Rapa Nui (Easter Island, Chile) as an example, we develop a model-based hypothesis for Received: September 28, 2020 how the structural constraints of communities living in small, isolated populations had dra- Accepted: April 13, 2021 matic effects and likely led to preventing the loss of cultural information in both community Published: May 12, 2021 patterning and technology. Copyright: © 2021 Lipo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original Introduction author and source are credited. Rapa Nui is a small (164 km2) and remote island in the easternmost South Pacific (Fig 1) that Data Availability Statement: All Python code for was colonized by Polynesian voyagers in the 12th-13th Century AD [1±4]. The island is per- the simulation, lists of parameters used in each of haps best known for the hundreds of multi-ton stone statues (moai) that the islanders con- the analyses, and R code for producing the paper's structed and transported over volcanic terrain to every part of the island and placed atop visualizations can be found at https://github.com/ clipo/network-drift. All of the paper's analyses and massive stone platforms (ahu) [5±7]. The magnitude of Rapa Nui's monumental architecture results can be reproduced using the Docker file is often seen as paradoxical in contrast with the island's size. At just 23 km in its longest available at https://hub.docker.com/repository/ dimension, the island can be easily traversed at a walking pace in under a day. Curiously, for docker/mmadsen/network-drift. an island so small archaeological and ethnohistoric evidence indicates that pre-contact com- Funding: Parts of this research were supported by munities did not interact in an island-wide, panmictic fashion, but rather were organized into National Science Foundation award #1841420 to a series of distinctive subgroups. Traditionally, at least eight clan groups are known to have PLOS ONE | https://doi.org/10.1371/journal.pone.0250690 May 12, 2021 1 / 25 PLOS ONE Population structure drives cultural diversity in finite populations RJD and TLH. The funders had no role in study spatial territories in the late 19th and early 20th century [8,9]. Detailed analyses of the archaeo- design, data collection and analysis, decision to logical record, however, show that many aspects of community interaction likely occurred on publish, or preparation of the manuscript. There an even more localized basis with communities consisting of multiple small, dispersed, and rel- was no additional external funding received for this study. atively autonomous groups [e.g., 10,11]. Analyses of spatial variability in biological and cul- tural traits suggest an island population structured into many relatively independent Competing interests: The authors have declared communities organized around ahu locations and their immediate local resources [e.g., 12,13]. that no competing interests exist. This evidence points to a significant degree of diversity over small geographic areas on the island in cultural and biological traits. While this diversity and degree of localized community structure is unexpected for such a small island, no model-based hypotheses have been pre- sented to account for these patterns. Models of random drift offer a productive source of hypotheses for the patterns and distri- bution of diversity. Drift, whether among inherited genetic or cultural variants, is the change produced by the underlying mechanisms through which variation is sampled and passed on through time. Understanding how random drift can shape cultural diversity has been a signifi- cant focus of investigations for cultural evolutionary studies [e.g., 14±19]. Using this concept, researchers have explored how one can detect drift from other factors that shape changes in trait frequencies through time and space [e.g., 20±27]. As has been demonstrated, the effects of Fig 1. Rapa Nui. The location of Rapa Nui in the easternmost corner of Polynesia. https://doi.org/10.1371/journal.pone.0250690.g001 PLOS ONE | https://doi.org/10.1371/journal.pone.0250690 May 12, 2021 2 / 25 PLOS ONE Population structure drives cultural diversity in finite populations drift depend on the size of the population. In large populations, drift tends to cause relatively small changes in trait frequency. In small populations, however, drift can lead to rapid changes in trait frequencies, potentially resulting in fixation or loss of variants. In these situations, drift can produce dramatic differences in historic outcomes: two populations drawn from a single population may begin with an identical frequency of traits, but can rapidly diverge in composi- tion. As we explore in this paper, the effect of drift in small populations often leads to rapid loss of richness (i.e., the number of variants within a population) and diversity (i.e., the distri- bution of variants within a population) [28]. The factors that drive the effects of driftÐpopulation size in particularÐhave received a great deal of attention as a possible explanation for cultural change. On one hand, some have argued that demography and population sizes strongly shaped cultural history. For example, Henrich [29] argues that the loss of technology among Tasmanian communities throughout the Holocene was a consequence of drift acting on the reduced size of isolated populations. In an opposite scenario, Powell et al. [30] have argued that increases in population sizes were part of the process leading to the explosion of technology and cultural variants in the Late Pleistocene [see also 31±33]. Yet, these kinds of explanations have been strongly criticized [e.g., 34±41]. While debate continues about whether changes in population size explain particular cases of cultural variability, it is well-established that drift contributes to cultural change among small and isolated populations [e.g., 31,42±47]. Population size, however, is not the only signif- icant factor that can change the degree to which drift leads to changes in cultural variability. Of particular note is the work of Premo [24,25,48,49], who has demonstrated how population structure, in particular, plays a key role in shaping diversity in populations. Derex and col- leagues