I probably should not be the one writing an answer to this because I am not sure I agree with all the arguments for keeping things together, but I can retell what I read in a recent paper on thrushes: There was arguments in a DNA analysis for several rather deep clades within the thrushes currently in genus
Turdus. Basal to them all was one species, which happened to be the type species for Turdus. What that meant was, that if the genus was split, the large genus of
Turdus would then become a one-member genus, with e.g., European Blackbird belonging to
Merula if I remember correctly; this was seen as a too radical change by the authors (again if I remember correctly). Additionally, several of the groupings within
Turdus was very close to each others, with much larger interspecies distances in other groups: the former groups were therefore seen as superspecies, because if you looked at those as genera, it would necessitate a splitting of the clades into several genera each. I am personally not sure that the mere fact that
Turdus would be reduced from a lot of species to only one would be a strong enough argument for status quo; the latter argument against necessarily allowing superspecies to become a genus seems a much stronger one.
I hope I did not hopelessly mangle this up, my copy of the
Turdus paper is in my office, not here at home.
One observation that can be made in all of this is that the average number of species per genus is about three in the hummingbirds, while the genus
Turdus contains sixty-odd species (about 66). This raise the suspicion that not all orders have been treated equally so far (and probably never will be
).
Cheers
Niels
