Selective breeding, also known as artificial selection, affects DNA in a fundamental way: it alters the frequency of specific alleles within a population's gene pool.

Here's how:

1. Choosing Desirable Traits: Breeders select individuals with traits they want to enhance or preserve. This could be anything from increased milk production in cows to brighter flower colors in roses.

2. Focusing on Specific Genes: These desirable traits are often linked to specific genes, which are units of heredity containing the instructions for building and maintaining an organism.

3. Increased Frequency of Favorable Alleles: By repeatedly breeding individuals with these desirable traits, breeders increase the frequency of the corresponding alleles in subsequent generations.

4. Shifting Genetic Makeup: Over time, the genetic makeup of the population changes. The alleles associated with the desirable traits become more common, while those linked to undesirable traits become less common.

This doesn't mean new genes are created, but rather the existing genetic variation is shifted in a specific direction.

Here are some key points to remember:

* The process is gradual: Selective breeding takes multiple generations to produce noticeable changes.

* It can lead to both benefits and drawbacks: While it helps produce desirable traits, it can also decrease genetic diversity, making the population more susceptible to diseases or environmental changes.

* It's not always intentional: Sometimes, selective breeding occurs unintentionally due to environmental factors or human activities, impacting the evolution of species.

In summary, selective breeding alters the genetic composition of populations by increasing the frequency of desirable alleles and decreasing the frequency of undesirable alleles, impacting the overall DNA of the species.