Why is the gene pool of the summer insect population often different from the gene pool of the population

Why is the gene pool of the summer insect population often different from the gene pool of the population that existed a year ago?

A gene pool is the collection of all genes in a population.
Let’s say there were a lot of insects in the summer, so there was a varied gene pool. But not all of the insects hatched by this summer will be able to survive and give offspring. Only some of them are able to survive the winter, it is they who will lay eggs, from which new insects will appear. This new population will have a different gene pool than last year, because part of the alleles of various genes could be lost, and part of the genes could mutate. These processes occur constantly, so the gene pools are often different in different years.