translation.en: They've been shaped by two powerful evolutionary forces: natural selection, the struggle for survival, and sexual selection, the struggle for reproductive opportunity.
translation.zh: 它们是受两种强大的进化力量 影响形成的： 自然的选择是为了生存而奋斗； 而性的选择， 是为了争取繁衍的机会而努力；