Dactylorhiza is a taxonomically complex genus in which closely related species have been combined into species aggregates (P. Vermeulen 1947; R. M. Bateman et al. 1997). Recognition of the aggregate taxa alone reduces the number of species by more than half. Recent research synthesizing morphometric and allozyme data to circumscribe species (R. M. Bateman and I. Denholm 1983, 1985, 1989; M. Hédren 1996), and DNA sequences and chromosome studies to determine the relationships of those species (R. M. Bateman et al. 1997; A. M. Pridgeon et al. 1997), is shedding much light on the evolution of the genus. The diploid lineage (2n = 40) appears to have evolved in Asia, migrating and speciating to both the west and northeast. Several alloploidy events (hybridization followed by chromosome doubling) occurred recently in Europe, apparently between the distinct diploids D. fuchsii and D. incarnata. That generated a highly complex suite of poorly distinguishable “prospecies” of 2n = 80, treated as a single species by some authorities and as many species by others (L. V. Averyanov 1990). Of the two North American species, D. aristata is a native diploid originating during the northeasterly migration, and D. majalis is an allotetraploid that originated in Europe and presumably is naturalized in North America (H. J. Clase and S. J. Meades 1996).

Despite an extensive literature, much taxonomic work still remains to be done.