What determines whether a phenotype occurs more frequent or not in certain population?

The selection pressure for or against that phenotype and whether the phenotype increases the reproductive fitness of an organism.

Explanation:

Selection pressures would include predation and ability to find food etc. Traits such as coat colour or agility which aid animals in these tasks would favour their survival and hence be selected for in the population. Through natural selection, these phenotypes will become more frequent in the population.

Traits that improve the reproductive success such as the ability to find mate would also increase the probability of genes being passed down to their offsprings which would ultimately affect the allele frequency in the gene pool. A trait which does not affect reproductive success in humans is the human lifespan hence it can be observed that there is no selection pressure to favour people with longer lifespan. (the increase in human lifespan over the years is due to the development of healthcare and technology)

Adaptation

The idea of "survival of the fittest" applies here. The phenotypes of the organisms the survive are the ones that get passed on to future generations while those of organisms who die off do not pass their phenotypes. Only the phenotypes that are better suited to the environment get passed on and, therefore, occurs more frequently in their populations.