Phylogeography of the widespread Caribbean spiny orb weaver Gasteracantha cancriformis

<p class="Normal1"><b>Background. </b>Modern molecular analyses are often inconsistent with pre-cladistic taxonomic hypotheses, frequently indicating higher richness than morphological taxonomy estimates. Among Caribbean spiders, widespread species are relatively few compared to the prevalence of single island endemics. The taxonomic hypothesis <i>Gasteracantha</i> <i>cancriformis</i> circumscribes a species with profuse variation in size, color, and body form. Distributed throughout the Neotropics, <i>G. cancriformis </i>is the only morphological species of <i>Gasteracantha</i> in the New World in this globally distributed genus.</p> <p class="Normal1"><b>Methods. </b>We inferred phylogenetic relationships across Neotropical populations of <i>Gasteracantha </i>using three target genes. Within the Caribbean, we estimated genetic diversity, population structure, and gene flow among island populations.</p> <p class="Normal1"><b>Results. </b>Our findings revealed a single widespread species of <i>Gasteracantha </i>throughout the Caribbean, <i>G. cancriformis,</i> while suggesting two recently divergent mainland populations that may represent separate species, diverging linages, or geographically isolated demes. The concatenated and <i>COI</i> (Cytochrome c oxidase subunit 1) phylogeny supported a Caribbean clade nested within the New World<i>.</i> Genetic variability was high between island populations for our <i>COI </i>dataset; however, gene flow was also high, especially between large, adjacent islands. We found structured genetic and morphological variation within <i>G. cancriformis</i> island populations; however, this variation does not reflect genealogical relationships. Rather, isolation by distance and local morphological adaptation may explain the observed variation.</p>