Studies of the ecology and genetics of "Paxillus involutus" indicate that it may form a complex of multiple similar-looking species. In a field study near Uppsala, Sweden, conducted from 1981 to 1983, mycologist Nils Fries found that there were three populations of "P. involutus" unable to breed with each other. One was found under conifers and mixed woodlands, while the other two were found in parklands, associated with nearby birch trees. He found that the first group tended to produce single isolated fruit bodies which had a thinner stipe and cap which was less inrolled at the margins, while the fruit bodies of the other two populations tended to appear in groups, and have thicker stipes, and caps with more inrolled and sometimes undulating margins. There were only general tendencies and he was unable to detect any consistent macroscopic or microscopic features that firmly differentiate them. A molecular study comparing the DNA sequences of specimens of "Paxillus involutus" collected from various habitats in Bavaria found that those collected from parks and gardens showed a close relationship with the North American species "P. vernalis", while those from forests were allied with "P. filamentosus". The authors suggested the park populations may have been introduced from North America. A multi-gene analysis of European isolates showed that "P. involutus" "sensu lato" (in the loose sense) could be separated into four distinct, genetically isolated lineages corresponding to "P. obscurosporus", "P. involutus" "sensu stricto" (in the strict sense), "P. validus", and a fourth species that has not yet been identified. Changes in host range have occurred frequently and independently among strains within this species complex.