Linearized ML tree deduced from concatenated nucleotide sequences of the 4 mitochondrial loci. A time interval of 30–35 Mya for the obscura-melanogaster divergence  was used as calibration. Shaded areas (gray) indicate the time span of temperate forest developing in the Northern Hemisphere . The numbers in the circle indicate the calibration points, and those in the panes indicate the estimated intervals of divergence times in light of the calibration points. For some very recent divergences, the estimates are not shown. Abbreviations: Eo. = Eocene; Oligo. = Oligocene; Mio = Miocene; Plio. = Pliocene; Q. = Quaternary. Symbols are same as in Figure 2.