Though not directly cited on their site, the Mammal Diversity Database (MDD) follows Hope et al. (2016, cited below), who tested three hypotheses using mitochondrial DNA from red squirrels (Tamiasciurus sp.) found along the west coast. Their first hypothesis (H1) was that there are three species of red squirrel: T. hudsonicus, T. mearnsi, and T. douglasii. This was the standard taxonomy at the time followed by most authorities. Ultimately, T. douglasii was the only well-supported species in their analysis. Hope et al. (2016) found the differences between T. douglasii and T. mearnsi to be untenable. The mitochondrial lineage found in the geographically-isolated T. mearnsi (which occurs in Baja California) extends throughout the range of T. douglasii in Vancouver Island. Their divergence is "minimal". However, Mearns's Squirrel is a conservation-dependent population currently considered Endangered by the IUCN Red List (de Grammont & Cuarón, 2018). As employed by Hope et al. (2016), this taxonomic merge reassigns T. mearnsi as the subspecies T. douglasii mearnsi to preserve its conservation status.