中文摘要：

外群用于给树附根和推断祖先性状状态。通常，来自内群的姐妹群中的多个分类单元被共同选择作为外群。为了在经验上验证这一方法， 我们采用了3种外群选择策略： 姐妹群中的单一分类单元， 姐妹群中的多个分类单元和连续姐妹群中的多个分类单元。以隧蜂科（膜翅目： 蜜蜂总科）的系统发育重建为例， 我们评估了这3种策略对树拓扑结构的影响， 包括最大似然树、 最大简约树和贝叶斯树。初步结果表明： 相比其他两种策略， 采用姐妹群中的多个分类单元作为外群更有利于系统发育重建得到现已被广泛认可的隧蜂科系统发育关系； 相比最大似然法和贝叶斯法， 虽然隧蜂科系统发育关系没有被很好地解决， 但最大简约法在不同外群选择策略下得到了较为一致的拓扑结构.

英文摘要：

An outgroup roots a network to form a tree and/or to infer hypothetical ancestral character states. Usually, multiple taxa of a closely related sister group of the ingroup are selected. To empirically evaluate the choice of outgroup, we implemented three strategies of outgroup selection ： a single taxon from the sister group, multiple taxa within the sister group, and multiple taxa from successive sister groups. Subsequently, we evaluated their effects on tree topologies within the family Halictidae （Hymenoptera： Apoidea ） incorporating three tree reconstruction methods： maximum likelihood, maximum parsimony and Bayesian inference. The use of multiple taxa within the sister group produced more consistent results than the other two outgroup strategies. The tree topologies were generally consistent with the putative tree topology of Halictidae. Compared with the other two tree reconstruction methods, maximum parsimony produced more consistent results with different outgroup strategies, yet often obtained less resolution.