Historical Collections Help Trace the Origin of Introduced and Native Populations of the Endangered Butterfly, Troides magellanus sonani (Lepidoptera: Papilionidae)

Island endemics are highly vulnerable due to restricted ranges, small population size, and genetic erosion. The Magellan birdwing on Lanyu Island (Troides magellanus sonani) exemplifies this vulnerability. Despite decades of legal protection, whether it constitutes an independent evolutionarily significant unit (ESU) has remained uncertain. Recent reports of introductions into northern Taiwan highlight the need for a genetic assessment. Here, we applied genome skimming to analyze 46 specimens of the Magellan birdwing, mostly from museum collections. Conservation genetic and phylogenetic analyses revealed that the Lanyu population is genetically distinct, derived from a south-to-north expansion originating in the southern Philippines and colonizing Lanyu around 210,000 years ago. Haplotypes from Lanyu, even spanning a century, remained nearly identical, and those from northern Taiwan were identical to those from Lanyu, confirming human-mediated introductions. These findings establish the Lanyu population as an ESU and emphasize the urgency of targeted conservation actions. This study demonstrates how integrating next-generation sequencing (NGS) with museum specimens can provide critical insights into the evolution and conservation of endangered insular taxa while minimizing impacts on wild populations.