Genomic Insights Into the Origins and Demographic History of the Invasive Green Swordtail ( <scp> <i>Xiphophorus hellerii</i> </scp> )

ABSTRACT Understanding the factors driving invasion success is critical for managing introduced species. Xiphophorus hellerii (Green swordtail), an ornamental fish popular in the aquarium trade, has established populations worldwide, yet the mechanisms underlying their spread remain unclear. Using nuclear single nucleotide polymorphisms (SNPs) from the X. hellerii native range in Mexico and Central America and the introduced range in the continental United States and Hawaii, we tested the hypothesis that introduced X. hellerii are hybrids with two widely introduced congeners, X. maculatus and X. variatus . To determine whether a single introduction occurred or multiple introductions from different sources, we investigated the geographic sources of introduced X. hellerii . We found no evidence of hybridization with congeners, indicating that hybridization has not significantly contributed to invasion success. We suggest that the source of introduced X. hellerii includes multiple admixed populations from the center of the native range in Mexico, rather than from peripheral populations. Introduced populations form a monophyletic clade, consistent with establishment from a limited number of founders, followed by isolation and genetic drift that homogenized genotypes. Within the native range, peripheral populations show restricted gene flow, while central basins exhibit greater connectivity, consistent with dendritic river systems where mainstem connections and Pleistocene drainage changes enhance genetic exchange. These findings highlight the roles of founder effects and demographic history in facilitating invasion. Resolving these processes is essential for guiding management strategies, predicting potential range expansions, and informing future studies on the colonization of novel environments.