Evaluating Darwin’s Naturalization Hypothesis in Experimental Plant Assemblages: Phylogenetic Relationships Do Not Determine Colonization Success Sergio A. Castro 1 *, Victor M. Escobedo 1 , Jorge Aranda 1 , Gasto´ n O. Carvallo 2 1 Laboratorio de Ecologı ´a y Biodiversidad, Departamento de Biologı ´a, and Center for the Development of Nanoscience and Nanotechnology (CEDENNA), Universidad de Santiago de Chile, Santiago, Chile, 2 Instituto de Biologı ´a, Facultad de Ciencias, Pontificia Universidad Cato´ lica de Valparaı ´so, Valparaı ´so, Chile Abstract Darwin’s naturalization hypothesis (DNH) proposes that colonization is less likely when the colonizing species is related to members of the invaded community, because evolutionary closeness intensifies competition among species that share similar resources. Studies that have evaluated DNH from correlational evidence have yielded controversial results with respect to its occurrence and generality. In the present study we carried out a set of manipulative experiments in which we controlled the phylogenetic relatedness of one colonizing species (Lactuca sativa) with five assemblages of plants (the recipient communities), and evaluated the colonizing success using five indicators (germination, growth, flowering, survival, and recruitment). The evolutionary relatedness was calculated as the mean phylogenetic distance between Lactuca and the members of each assemblage (MPD) and by the mean phylogenetic distance to the nearest neighbor (MNND). The results showed that the colonization success of Lactuca was not affected by MPD or MNND values, findings that do not support DNH. These results disagree with experimental studies made with communities of microorganisms, which show an inverse relation between colonization success and phylogenetic distances. We suggest that these discrepancies may be due to the high phylogenetic distance used, since in our experiments the colonizing species (Lactuca) was a distant relative of the assemblage members, while in the other studies the colonizing taxa have been related at the congeneric and conspecific levels. We suggest that under field conditions the phylogenetic distance is a weak predictor of competition, and it has a limited role in determining colonization success, contrary to prediction of the DNH. More experimental studies are needed to establish the importance of phylogenetic distance between colonizing species and invaded community on colonization success. Citation: Castro SA, Escobedo VM, Aranda J, Carvallo GO (2014) Evaluating Darwin’s Naturalization Hypothesis in Experimental Plant Assemblages: Phylogenetic Relationships Do Not Determine Colonization Success. PLoS ONE 9(8): e105535. doi:10.1371/journal.pone.0105535 Editor: Diego Fontaneto, Consiglio Nazionale delle Ricerche (CNR), Italy Received March 20, 2014; Accepted July 22, 2014; Published August 20, 2014 Copyright: ß 2014 Castro 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 author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. Data are available from the supplementary material in the manuscript (see Table S1). Funding: This research has been financially supported by Fondecyt 11085013, Fondecyt PD 3130399, and Lı ´nea 6 of CEDENNA. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * Email: sergio.castro@usach.cl Introduction Biological invasions have attracted the attention of modern ecologists and biogeographers [1] because of their leading role as components of global change [2]. At present, organisms belonging to diverse taxonomic groups are being translocated from one region to another with which they do not share an evolutionary history [3,4]. Although it is estimated that most of the organisms that start this dispersion do not get to become established successfully, sometimes they can constitute a founding colony and become naturalized [5,6]. One of the central challenges in the study of biological invasions has been to understand what factors determine this naturalization process [3–4], [7]. Various hypotheses have been proposed to explain why some species are capable of colonizing successfully (i.e., become naturalized) while others are not [6], [8–9]. A particularly intriguing and controversial role is that played by Darwin’s naturalization hypothesis (DNH), which states that naturalization success depends on the phylogenetic relatedness between the colonizer and the members of the recipient community [10]. If this relatedness is close, then the colonization process will be inhibited as a result of the greater competitive intensity that there is – supposedly– between closely related species [11]. Conversely, if the phylogenetic relationship is distant, the establishment would be favored as a result of a lower competitive intensity. Underlying this relationship between phylogenetic distance and invasion success, it is assumed that closely related species shares similar resources and natural enemies [11]. Following the influential paper by Daehler [12], DNH has received renewed interest, generating controversy with respect to its explanatory value on the invasion process [10]. In fact, while some studies have supported the hypothesis [13–19], others have dismissed it [20–29]. Although most of the evidence relies on compositional pattern analysis at a regional scale [21], [30], recently some authors have implemented experimental approaches in communities of microorganisms, providing support to DNH [15], [17]. Even though the ecology of microorganisms is governed by processes equivalent to those that occur in multicellular PLOS ONE | www.plosone.org 1 August 2014 | Volume 9 | Issue 8 | e105535