vol. 172, supplement the american naturalist july 2008 Genetics and Behavior of a Colonizing Species: The Invasive Argentine Ant A. V. Suarez, 1,* D. A. Holway, 2 and N. D. Tsutsui 3 1. Department of Animal Biology and Department of Entomology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801; 2. Division of Biological Sciences, University of California, San Diego, La Jolla, California 92093; 3. Department of Environmental Science, Policy and Management, University of California, Berkeley, California 94720 abstract: Baker and Stebbins’s 1965 book The Genetics of Colo- nizing Species aimed to draw together scientists from a variety of disciplines to provide a conceptual framework for the study of species introductions. A goal of their volume was to examine how studies on biological invasions could be used to provide insight into basic research questions as well as to develop practical strategies for control. In this article, we attempt to follow the goals of Baker and Stebbins by reviewing work on the genetics and behavior of a widespread colonizing species, the Argentine ant (Linepithema humile). Specif- ically, we examine the evolutionary changes that have taken place as a result of this species being introduced into new environments and synthesize recent research on Argentine ants from the perspective of population genetics, recognition systems, and the mechanisms that may underlie their ecological success. Keywords: biological invasions, colony structure, Linepithema humile, supercolony, unicoloniality. These transplantations of species have, in effect, been a series of experiments in evolution. As such, they are potentially much more informative than most laboratory experimental work, since they have faced the introduced species, not with some simple defined change in selective conditions, but with a whole new ecological system in which the species has to find a place for itself. (C. H. Waddington in Baker and Stebbins 1965; introduction to the symposium, p. 1) A goal of Baker and Stebbins’s (1965) edited volume on species introductions was to bring together research on * Corresponding author; e-mail: avsuarez@life.uiuc.edu. Am. Nat. 2008. Vol. 172, pp. S72–S84.  2008 by The University of Chicago. 0003-0147/2008/1720S1-50014$15.00. All rights reserved. DOI: 10.1086/588638 the “kinds of evolutionary change that take place when organisms are introduced into new territories” (preface, p. vii). The volume targeted scientists from a variety of backgrounds, from population geneticists to practitioners of purely applied branches of ecology, such as biological control. As the above Waddington quote suggests, studies on the ecology and evolution of introduced species are important not only from an applied perspective—research on species introductions also can provide insight into nat- ural processes that would ordinarily take prohibitively long periods of time to study and that occur over spatial scales that would be unfeasible or unethical to recreate using traditional channels of experimentation (Diamond and Case 1986). Understanding the ecological and evolutionary consequences of invasions has become increasingly im- portant because the economic and ecological fallout of species introductions continues at accelerating rates (Co- hen and Carlton 1998; Mack et al. 2000; Pimentel et al. 2000). Social insects rank among the most widespread and damaging of social organisms (Lowe et al. 2000). Well- known examples include the red imported fire ant (So- lenopsis invicta), the Argentine ant (Linepithema humile), Africanized “killer” bees (Apis mellifera scutellata), the German yellowjacket (Vespula germanica), and the For- mosan subterranean termite (Coptotermes formosanus). The cooperation of many individuals as a single eusocial superorganism is generally believed to underlie the eco- logical success of social insects (Wilson 1971; Ho ¨lldobler and Wilson 1990). Social insects possess complex and oc- casionally unusual modes of social organization that are strongly influenced by genetic relationships within and among groups. However, invasive social insects sometimes display even more extreme forms of sociality, including geographically expansive colonies with many reproductive queens and large worker forces that can dominate eco- systems in their introduced range (Chapman and Bourke 2001; Holway et al. 2002). Consequently, several widely introduced species have become emerging model systems for genetics, behavioral ecology, and invasion biology. In this article, we integrate previous work with new