Natural selection acts on phenotypes, which are the observable characteristics of an organism. These phenotypes are determined by the interaction of genes and the environment. While natural selection does not directly act on genes or alleles, it does influence their frequency in a population over time.

Here's a breakdown:

* Genes: Units of heredity that are responsible for a particular trait. They contain the instructions for building proteins, which contribute to the organism's characteristics.

* Alleles: Different versions of a gene. For example, the gene for eye color might have an allele for blue eyes and another for brown eyes.

* Phenotypes: The physical expression of the genotype. This includes observable characteristics like eye color, height, and behavior.

How Natural Selection Affects Alleles:

1. Variation: Individuals within a population have different alleles, leading to variation in their phenotypes.

2. Differential Survival and Reproduction: Individuals with phenotypes better suited to their environment are more likely to survive and reproduce. This means they pass on their alleles to the next generation.

3. Allele Frequency Change: Over time, the alleles that lead to favorable phenotypes become more common in the population, while those linked to unfavorable phenotypes become less common.

In summary:

* Natural selection doesn't directly act on genes or alleles.

* It acts on the phenotypes that are expressed by the genes.

* Through differential survival and reproduction, natural selection influences the frequency of alleles in a population.

This process of allele frequency change is what drives evolution.