Zoology 10th Edition by Stephen Miller,John Harley

Exercise 13

Population genetics is the study of genetic events in gene pools. When frequencies of alleles change in populations, evolution is occurring. The Hardy-Weinberg theorem describes four very restrictive conditions that must all be present if evolution is not occurring in a population. The restrictive nature of these conditions means that most populations are evolving.
Why is it accurate to say that evolution does not have to be occurring in all populations but most populations are evolving?