How does natural selection affect allele frequency?

If an allele, through it's expresion, is of any benefit to the organism (in its chance of survival and/or in its chance of succesfull reproduction), it's frequency in the populasion should increase.

The offspring of an organism- that have advangate in the population due to presence of an allele, should be more numerous. The result will be an increase in the frequency of the said allele. Darwin tried to explain this phenomenon as Natural Selection.

On the other hand, if the allele decrease the chances of survival and reproducing, there should be less or no offspring from those individuals carrying it, relusting in a decrease in the frequency of the harmful allele.