Zoology 10th Edition by Stephen Miller,John Harley

Exercise 10

Evolution occurs within populations. The sum of all alleles within a population is called the gene pool and represents the set of genetic resources available for a population. The variety within individuals of populations results from cellular processes occurring in meiotic cell division, random fertilization, and meiosis.
Based on what you know about natural selection, why does evolution occur in groups of individuals of the same species over many generations (i.e., within populations).