What's the difference in meaning between "variation" and "alteration"?
By alteration, they mean a change in the DNA of a single organism. An alteration to one organism makes its DNA different from other organisms in the same population. Variation is the range of differences in a population. If all the organisms in a population are very similar, variation is low. If some organisms in a population are quite different from others, variation is high. Over time, mutations increase variation. Natural selection reduces variation.