Here's why:

* Gene frequency: This refers to how common a particular gene variant (allele) is within a population.

* Evolution: Evolution is the process of change in the heritable characteristics of biological populations over successive generations.

The link: Evolutionary change occurs when gene frequencies in a population shift over time. This can happen due to several mechanisms, including:

* Natural selection: Individuals with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous genes.

* Genetic drift: Random fluctuations in gene frequencies, especially pronounced in small populations.

* Gene flow: The movement of genes between populations, introducing new alleles or altering frequencies.

* Mutation: Changes in the DNA sequence, which can introduce new alleles.

Therefore, a change in gene frequency is the bedrock of biological evolution. It's the raw material that drives the gradual transformation of species over generations.