Discussions that have accompanied biological species definitions often imply correspondence between breeding groups and morphological and ecological units. It is often assumed that the cause of such correspondences is gene flow or the lack of it. This has generated confusion because breeding, morphology, and ecology can be, and often are, uncoupled, and the effects of gene flow may be limited. Another source of confusion has been the failure to distinguish clearly between species themselves (taxa) and concepts of the species category. The biological concept fails to provide unambiguous criteria for grouping organisms (or for assigning species rank), and as a consequence, biological species may not be monophyletic. A phylogenetic species concept, which emphasizes monophyly, is more appropriate for purposes of phylogeny reconstruction, cladistic classification, and the study of evolutionary processes.