Sphagnum is arguably the most controversial genus of mosses with regard to species delineation. The four large sections of Sphagnum are readily distinguishable, but within sections there are complexes of closely related taxa that some bryologists recognize as a single polymorphic species while others distinguish multiple, more narrowly defined species within these complexes. Phylogenetic architecture within sections of Sphagnum was investigated using DNA sequences from the chloroplast and nuclear genomes. Although it is still too early to make taxonomic decisions on the basis of molecular data, evidence available to date suggests that such groups as the S. capillifolium complex represent monophyletic groups but that the "little" species within such complexes are often para- or polyphyletic. These observations suggest that either the "little" species within complexes are unnatural and are based on convergent morphological features, or that these species are so recently derived that gene coalescence predates speciation. On the other hand, some Sphagnum species, including S. molle, S. fuscum, and S. tenerum, are clearly distinct in terms of DNA sequences. Across the genus, phylogenetic structure at the population and species level is very heterogeneous. Populations in the section Rigida, for example, are relatively distinct, as are the two species of Rigida (S. compactum and S. strictum). Molecular approaches to species delineation have not reached maturity yet, but appear very promising.

Key words: molecular systematics, peatmosses, speciation, species concepts, Sphagnum