The extent and evolutionary significance of interspecific introgression have long been subjects of considerable discussion. Many researchers have postulated that introgression increases genetic variability through the production of recombinant genotypes, which provides populations a means of coping with environmental change or evolving novel adaptations. However, there are few empirical demonstrations of this process in natural plant populations. We analyzed 44 morphological characters and 21 allozyme loci from 17 populations of yellow ladyslipper orchid (Cypripedium pubescens), nine populations of small white ladyslipper (C. candidum), and five sites that appeared to contain hybrid swarms, to assess the direction and magnitude of interspecific exchange of genes. The data demonstrate the existence of bidirectional gene flow wherever the two species are sympatric. Hybrid populations consist largely of individuals that are later-generation backcrosses or recombinants, with few "pure" parents or F1 hybrids noted. Some individuals that appear to be of one morphological species contain marker alleles from the other species. In Iowa, C. pubescens is generally a woodland species but has been previously noted to occasionally occur on prairies. Allozyme and morphological data and ecological considerations suggest the prairie ecotype of C. pubescens may have arisen as a direct consequence of its acquisition of genetic information from C. candidum. Implications for conservation of C. candidum are discussed.
