Download Disruptive selection, also called diversifying selection, is a

Disruptive selection, also called diversifying selection, is a descriptive term
used to describe changes in population genetics that simultaneously favor
individuals at both extremes of the distribution. When disruptive selection
operates, individuals at the extremes contribute more offspring than those
in the center, producing two peaks in the distribution of a particular trait.
Disruptive selection is of particular significance in the history of
evolutionary study, as it is involved in one of evolution's "cardinal cases",
namely the finch populations observed by Darwin in the Galápagos.
Directional selection occurs when natural selection favors a single phenotype
and therefore allele frequency continuously shifts in one direction. Under
directional selection, the advantageous allele will increase in frequency
independently of its dominance relative to other alleles (i.e. even if the
advantageous allele is recessive, it will eventually become fixed).
Well-known instances are the many cases of insect resistance to pesticides,
which are synthetic substances not present in the natural environment.
When a new insecticide is first applied to control a pest, the results are
encouraging because a small amount of the insecticide is sufficient to bring
the pest organism under control. As time passes, however, the amount
required to achieve a certain level of control must be increased again and
again until finally it becomes ineffective or economically impractical. This
occurs because organisms become resistant to the pesticide through
directional selection.