What is the primary basis of the natural selection process?

The primary basis of the natural selection process is phenotype, which refers to the observable physical and behavioral traits of an organism. Natural selection operates on phenotypes because these traits affect an organism's ability to survive and reproduce in its environment. For example, certain physical characteristics might provide advantages in finding food, evading predators, or attracting mates, which would lead to a greater likelihood of passing those advantageous traits to the next generation.

While genotype, which is the genetic makeup of an organism, plays a crucial role in determining the phenotype, natural selection itself acts on the phenotypic variations that are expressed and visible. This means that it is the phenotypic traits that contribute to an organism's fitness and selection in the natural environment. Therefore, understanding natural selection primarily focuses on how these traits are suited to particular environments and how they contribute to reproductive success.