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Under certain circumstances , natural selection can lead to an entirely new species. In this process , called speciation , two species arise from one. For sexually reproducing species , a new species is fromed when some members of a population have evolved to the point where they can no longer breed with other members to produce fertile offspring.

在某些環境下，天擇會導致新種產生。兩個物種從一個物種產生而來，這個過程稱為物種形成。
對於用性別來繁殖的物種，當物種繁衍時，一個新的物種的產生會依照需求，把不好的基因淘汰，逐漸改變，以便繁衍出基因優良的後代。