Just what is a genus? Comparing levels of postzygotic isolation to test alternative taxonomic hypotheses in Orchidaceae subtribe Orchidinae

Abstract

AbstractAn exceptionally wide range of systematic data has been accumulated on subtribe Orchidinae, which dominates the European orchid flora. Despite (or perhaps because of) this wealth of information, delimitation of genera within the subtribe continues to prompt extensive debate among taxonomists. Here, we use the controversial genus Orchis s.l. as a case‐study to explore the broader issues that surround the definition and circumscription of genera, and suggest a new approach to selecting among competing formal classifications that focuses on the biological cohesion of the resulting taxa. Levels of postzygotic reproductive isolation, assessed as differential embryo mortality through extensive artificial crosses between species, are compared with three taxonomic treatments: a pre‐cladistic morphological classification permitting polyphyly; a cladistic classification generated by applying monophyly to Internal Transcribed Spacer sequences; and a 'post‐cladistic' classification that aimed to compromise between morphological and molecular differences and permitted paraphyly. The monophyletic cladistic classification is by far the most consistent with levels of postzygotic isolation through embryo mortality. Reproductive isolation, sequence divergence and morphological similarity can all usefully be quantitatively compared with formal classifications, though floral morphology in orchids offers a high risk of rapid co‐evolutionary divergence with pollinator guilds, often generating spurious monospecific 'genera'. Best practice in taxonomy reflects reciprocal illumination between key biological datasets and formal classifications, pursued in a rigorous framework of precise definitions of taxa and well‐founded evolutionary‐phylogenetic concepts.