A recent study on the evolutionary origin and spread of this species using genetic markers (amplified fragment length polymorphisms, DNA sequences and microsatellites) on a worldwide sample of specimens suggested the existence of two main lineages, a nonaggressive lineage found in North America, and an aggressive lineage found on all continents, both in natural environments and buildings.
A recent study using genetic markers (amplified fragment length polymorphisms, DNA sequences, and microsatellites) to analyze this species' evolutionary origins and spread found two main lineages: a nonaggressive one in North America and an aggressive one globally, including both natural habitats and buildings.