为什么鸡没有丁丁，而鸭子有丁丁？
【写在最后】
有性繁殖是如何起源的？为什么会有性？这个曾经令达尔文十分困惑的问题，到了150年后的今天，也没有足够权威的答案。一般认为，有性繁殖优于无性繁殖，但为什么有性繁殖就优于无性繁殖，生物学家的理解却莫衷一是，主要有以下几种观点：
参考资料
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