Here's why:

* Mutation is a change in the DNA sequence of an organism. These changes can be beneficial, harmful, or neutral.

* Beneficial mutations can introduce new traits that increase an organism's fitness, allowing them to survive and reproduce more successfully.

* When these beneficial mutations are passed on to offspring, they become part of the gene pool, introducing new genetic variations.

Let's break down why other options are not the primary source of new genes:

* Gene flow is the movement of genes between populations. It can introduce existing genes from one population to another, but it doesn't create new genes.

* Genetic drift is the random change in the frequency of alleles in a population. It doesn't introduce new genes, but can eliminate existing ones.

* Natural selection acts on existing variations within a population, favoring traits that increase fitness. It doesn't create new genes, but it can influence which genes are passed on.

In summary: While gene flow, genetic drift, and natural selection all play important roles in shaping the gene pool, only mutation can introduce entirely new genes into a population.