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Genetics and environmental factors often prevent organisms from achieving evolutionary perfection. A. Darwin first challenged the idea of evolutionary progress by observing that more than 99.9 percent of all evolutionary lines that once existed on Earth have become extinct. B. A single adaptation to an organism's environment may determine the way in which the organism's subsequent ancestors are able to evolve. C. The structure of the genotype itself restricts natural selection, since genes must line up like pearls on a necklace and cannot be moved out of their proper order. D. The development of vertebrates and arthropods from a single ancestor demonstrates the power of the environment to limit natural selection. E. A single structure or gene in an organism cannot respond to the forces of natural selection without affecting the functioning of other structures or genes. F. Both environmental and genetic chance may prevent even the fittest organisms from surviving and reproducing.