Natural selection ensures only the fittest survive to pass their genes on to the next generation.自然选择确保只有最适应环境的物种得以存活下来，把基因传给下一代。Natural selection is a key element of Darwin's theory of evolution.自然选择是达尔文进化论的关键部分。The theory of natural selection remains the key to our understanding of the natural world.自然选择的理论仍然是我们理解自然界的关键。