They can even be completely eliminated. The number of advantageous alleles, on the other hand, increases, hence increasing the probability of their inheritance by progeny. Natural selection changes the composition of the gene pool in the direction consistent with the environmental requirements. Frequency of rare alleles, hence of extreme traits, does not increase. Directional selection increases the number of one type of alleles at the sacrifice of the others, thus shifting the probability distributions in a given direction.
Disruptive selection is a type of the directional one, which acts in several directions.
A population is divided into subgroups that are still connected. They can still reproduce, but the flow of genes is reduced. A mechanism of natural selection is almost always the same. They are not able to grow up to an age of reproduction, they cannot find a partner or they have a small number of progeny while the individuals with a profitable phenotype have always a very large number of progeny. Such a situation we can observe also in our tanks, where more and more frequently we can find a new, albinotic variant of a very popular Ancistrus sp.