他认为，自然选择对于那些曾经是利他主义者的人是有利的，因为我们的祖先生活在小而密集的群体中，人们的利他主义可以指望得到相互间的利益互惠。
He argues that natural selection favored humans who were altruistic because in the small, close-knit groups in which our ancestors lived, altruists could expect reciprocity.
群体间一定程度的遗传分化可能是人为选择压力和基因流障碍引起的。
The genetic differentiation of among populations might be caused by human selective pressures and barriers to gene flow.
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