The morphology of Papaver dubium and P. lecoqii are contrasted. The colour of latex, leaf dissection, petal shape and height of stamens are considered good characters for distinguishing between them. The shape of the capsule and stigmatic disc are considered unreliable taxonomic characters.
Experimental cross-pollinations show that the species can produce hybrids readily, that the hybrid seed germinates with little or no dormancy, to give plants showing some degree of hybrid vigour but morphological characters intermediate between the parents. The F1 hybrids show very severely reduced fertility, but this is rapidly restored in many of the F2 and backcross progeny.
Naturally occurring hybrids have not been found, and there are problems in the way of recognizing them and proving their hybrid nature if they do occur.
The continued coexistence of these two species without merging of their specific characters is made possible by barriers to gene flow occurring at three levels of organization: (a) ability of the parents to set seed by selfing, (b) ill-adapted hybrids and (c) infertility of the F1 generation.