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 utilizing genetic markers (amplified fragment length polymorphisms, DNA sequences, and microsatellites) on specimens from around the world suggested that this species has two main lineages: a nonaggressive one found in North America and an aggressive one present globally, including both natural environments and buildings.