Cryptic species in the genus Phylloscopus (Old World leaf warblers)

In almost all ecological and evolutionary research, it is important to assess the number of species under study. Cryptic species, which are morphologically similar, present a special problem because they need to be identified through studies of behavioural or genetic variation. Here we review the important factors in the recent discovery of three species and elevation of nine previously known taxa to species status within the genus Phylloscopus (Old World leaf-warblers] and we examine the case of three morphologically similar taxa (inornatus, humei and mandellii) that until recently were considered to be members of the single species Phylloscopus inornatus. We have identified several locations at which humei and inornatus coexist at high density, and the results of playback experiments, observations of interactions, and spectrogram analysis of vocalizations at these sites all indicate that there are significant behavioural differences and a lack of recognition between humei and inornatus. Estimated relationships within the species complex based on mitochondrial control region DNA sequences show that there is little variation within the geographic ranges of either humei or inornatus compared with the deep split between the two taxa. These observations support the division of humei and inornatus into separate species. The third taxon, mandellii, which is geographically separated from both humei and inornatus, is behaviourally and genetically more similar to humei. Under the biological species concept, it is difficult to determine the species status of allopatric taxa. Alternative species concepts, such as the phylogenetic and recognition concepts, are more easily applied to allopatric taxa but lead to differing conclusions over whether mandellii and humei are separate species. This confusion arises from the fact that the two taxa are in an early stage of allopatric divergence. The review of other recently designated species reveals that song divergence is of primary importance in their designation and that song variation, playback experiments and genetic analysis lead to similar conclusions regarding species status.