There are different types of natural selection: directional, stabilizing and disruptive selection. Each of these types selects for a different phenotype.
Natural selection can occur in different ways, being classified into three types: directional, stabilizing or disruptive selection.
→ Directional selection
Directional selection is the type of natural selection in which an extreme phenotype is favored and has its frequency increased in the population. It is the type of selection that causes changes more quickly than the others, being the type most used in artificial selections.
An example of directional selection can be seen in salmon that live in the Pacific Northwest. They have shown a reduction in their size in recent years because of the increase in net fishing in the region. This type of fishing ends up selecting larger individuals, which made the smaller ones have an advantage over the others, since they were not captured. Over time, a reduction in the average size of these fish was observed.
→ Stabilizing selection
An example of stabilizing selection occurs in some African regions, where the presence of a recessive allele(s) is observed that determines sickle cell anemia , a relatively serious disease. Despite being severe, many people present the gene, however, in heterozygosity (Ss). The permanence of this gene is important because it confers protection against malaria , a disease common in certain African regions. In this case, therefore, natural selection favored the permanence of the gene in heterozygosity, decreasing the frequency of the gene in homozygosity (SS and ss)
→ Disruptive selection
Disruptive selection is one in which extremes are favored and intermediate organisms are eliminated. An example of this type of selection can be seen in some African finches that are adapted to eating seeds. They vary in size (large, medium and small), but intermediate forms are rarely observed. This is because there are seeds that vary in hardness. Larger finches can feed on hard seeds, and smaller finches feed on soft seeds.