often treated as a separate species, Parus bokharensis, the Turkestan tit. This form was once thought to form a ring species around the Tibetan Plateau, with gene flow throughout the subspecies, but this theory was abandoned when sequences of mitochondrial DNA were examined, finding that the four groups were distinct (monophyletic) and that the hybridisation zones between the groups were the result of secondary contact after a temporary period of isolation. A study published in 2005 confirmed that the major group was distinct from the cinereus and minor groups and that along with P. m. bokharensis it diverged from these two groups around 1.5 million years ago. The divergence between the bokharensis and major groups was estimated to have been about half a million years ago. The study also examined hybrids between representatives of the major and minor groups in the Amur Valley where the two meet. Hybrids were rare, suggesting that there were some reproductive barriers between the two groups. The study recommended that the two eastern groups be split out as new species, the cinereous tit (Parus cinereus), and the Japanese tit (Parus minor), but that the Turkestan tit be lumped in with the great tit. This taxonomy has been followed by some authorities, for example the IOC World Bird List. The Handbook of the Birds of the World volume treating the Parus species went for the more traditional classification, treating the Turkestan tit as a separate species but retaining the Japanese and cinereous tits with the great tit, a move that has not been without criticism. The nominate subspecies of the great tit is the most widespread, its range stretching from the Iberian Peninsula to the Amur Valley and from Scandinavia to the Middle East. The other subspecies have much more restricted distributions, four being restricted to islands and the remainder of the P. m. major subspecies representing former glacial refuge populations. The dominance of a single, morphologically uniform subspecies over such a large area suggests that the nominate race rapidly recolonised a large area after the last glacial epoch. This hypothesis is supported by genetic studies which suggest a geologically recent genetic bottleneck followed by a rapid population expansion. The genus Parus once held most of the species of tit in the family Paridae, but morphological and genetic studies led to the splitting of that large genus in 1998. The great tit was retained in Parus,