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Matings and RAPD-PCR analysis were used to differentiate two closely related basidiomycetes fungi, Pleurotus pulmonarius and P. ostreatus, which are widespread in Russian forest biocenoses with moderate continental climate. Monokaryon-monokaryon (mon-mon) and dikaryon-monokaryon (di-mon) matings demonstrated complete reproductive isolation of the two species, which have partly overlapping morphological traits. The prevalence of a particular species in nature was shown to depend to a great extent on the natural conditions, namely, the day and night temperature fluctuations. The clustering of natural Pleurotus strains displayed two trends: one was associated with natural reproductive isolation of the two species ( D = 0.61) and the other, with the geographical factor ( D = 0.39). A relatively recent origin is suggested for the divergence of the two species and the reproductive barrier between them. Adaptation to natural conditions was considered to be the main factor causing the divergence of natural Pleurotus populations and, eventually, allopatric speciation. [ABSTRACT FROM AUTHOR]