Why is phenotypic variation necessary for natural selection and sexual selection?

Natural selection only works when there are genetic variations between the individuals (within the population). It is because only those individuals who are best adapted to live in an environment will be favored by natural selection. Within a given population, there exists a normal amount of genetic variation that may or may not make an individual more adapted to the environment (changes in the environment). Conditions in environmental determine the success of certain individuals because they possess a trait that allows them an advantage in that environment (being faster, more colorful, bigger, or smarter than others in their species or have higher reproductive rates). Natural selection will favor the most adapted individuals.

The similar thing is with sexual selection where some individuals have better reproductive success than others (variation exist) either from being more attractive or preferring more attractive partners to produce offspring.