• allozymes ;
  • genotypic diversity ;
  • Orchidaceae ;
  • small populations

We examined the genetic diversity of two orchid species, the nectar-rewarding Platanthera chlorantha and the nectarless Cephalanthera rubra, in north-eastern Poland. We found lower differences in genetic diversity between the species than we expected. The level of genetic variation at species level was lower in C. rubra (PPOL = 14%, A = 1.14 and HE = 0.060) than in P. chlorantha (PPOL = 25.5%, A = 1.35 and HE = 0.078). In the majority of populations of both species a high proportion of unique genotypes was noted. The overall FST values for all populations were moderate and similar for both species (P. chlorantha: 0.251, P < 0.001; C. rubra: 0.267, P < 0.001). No relationship was found between genetic and geographical distances in either species (P > 0.05, Mantel test). We discuss the breeding systems, small population size and population subdivision as the most important factors affecting the genetic diversity of this species. We suggest that conservation programmes should be initiated to maintain or even increase the fitness and genetic variation of populations of both species. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 171, 751763.