Here's why:

* Evolution is defined as a change in the genetic makeup of a population over time.

* Alleles are different versions of a gene.

* Allele frequencies refer to how common each allele is in a population.

So, if the frequencies of different alleles change within a population, that means evolution has occurred.

Important Note: While a change in allele frequencies *is* a hallmark of evolution, it doesn't necessarily mean the population has become *more complex* or *better adapted*. Evolution can occur through various mechanisms, including genetic drift, natural selection, and gene flow, and these can lead to changes in allele frequencies in different ways.