Psilocybin is present in varying concentrations in over 200 species of Basidiomycota mushrooms. In a 2000 review on the worldwide distribution of hallucinogenic mushrooms, Gastón Guzmán and colleagues considered these to be distributed amongst the following genera: "Psilocybe" (116 species), "Gymnopilus" (14), "Panaeolus" (13), "Copelandia" (12), "Hypholoma" (6), "Pluteus" (6), "Inocybe" (6), "Conocybe" (4), "Panaeolina" (4), "Gerronema" (2) and "Galerina" (1 species). Guzmán increased his estimate of the number of psilocybin-containing "Psilocybe" to 144 species in a 2005 review. The majority of these are found in Mexico (53 species), with the remainder distributed in the US and Canada (22), Europe (16), Asia (15), Africa (4), and Australia and associated islands (19). The diversity of psilocybian mushrooms is reported to have been increased by horizontal transfer of the psilocybin gene cluster between unrelated mushroom species. In general, psilocybin-containing species are dark-spored, gilled mushrooms that grow in meadows and woods of the subtropics and tropics, usually in soils rich in humus and plant debris. Psilocybin mushrooms occur on all continents, but the majority of species are found in subtropical humid forests. "Psilocybe" species commonly found in the tropics include "P. cubensis" and "P. subcubensis". "P. semilanceata" — considered by Guzmán to be the world's most widely distributed psilocybin mushroom — is found in Europe, North America, Asia, South America, Australia and New Zealand, but is entirely absent from Mexico. Although the presence or absence of psilocybin is not of much use as a chemotaxonomical marker at the familial level or higher, it is used to classify taxa of lower taxonomic groups.