外群，在用mega、treeview画树的拓扑结构的时候，可以指定个外群，这个时候，外群可以指定多个。
Thus the estimated divergence time of Fringillidae and Emberizidae is 8.0 million years, Muscicapidae and Turnidae 8.0 Myr, Aegithalidae and Sylviidae 9.3 Myr, Coccothraustinae and Carduelinae 7.3 Myr, Pallas's rosefinch and long-tailed rosefinch 5.0 Myr, two outgroups 12.4 Myr.
而两个外群的分歧时间在12.4百万年左右。
参考来源 - 21种雀形目鸟类细胞色素b基因序列分析及其系统发育研究·2,447,543篇论文数据，部分数据来源于NoteExpress
结果就取决于我们采用哪个外群了。
脑干动脉可分成前内群、前外群、外侧群和后群。
The arteries of the brain stem can be divided into four groups: the anteromedial, anterolateral, lateral and posterior arteries.
以往的研究一致认为这种积极评价是与外群比较的结果，并将这种现象命名为内群偏爱。
Past researches agreed that this kind of positive regard should be the outcome of a comparison with a contextually relevant out-group, which was called in-group favoritism.
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