There are basically three types of natural selection, but before getting into those, we must first have a better understanding of what the term ‘natural selection’ actually refers to. Let’s take the example of a giraffe population. Not all giraffes share exactly the share biological characteristics; for example some have bigger ears than others, or more spots on their fur than others. Natural selection is the process by which nature favors a specific biological characteristic over another. In the case of giraffes, you can understand how natural selection works if you consider the length of their neck. There can be giraffes that have long necks and giraffes with shorter necks. It makes sense, however, that those that have a longer neck will be able to reach higher up the trees in order to eat their fruits, while those with a shorter neck will have trouble reaching for fruits. In others words, slowly but surely the number of short-necked giraffes will start decreasing, since they will start dying from starvation and they won’t be able to reproduce (obviously). So natural selection in this case will favor long-necked giraffes. As Charles Darwin put it, natural selection is essentially the survival of the fittest. The three types of natural selection are: directional selection, stabilizing selection and disruptive or diversifying selection.
The example of giraffes presented above corresponds to the first of those three types of natural selection. There were two extreme variations: the one extreme had long necks, while the other had short necks. Directional selection occurs when one extreme is favored over the other, as it was the case with the population described.
Natural selection, however, could favor an intermediate variation, instead of extremes. The example here will be hypothetical but the point is still valid. So, in a population of giraffes with short necks, long necks, and intermediate necks, it could be that those with intermediate necks are the fittest to survive. Let’s assume that long-necked giraffes are the first to be spotted by predators or hunters, and thus the first to be killed; short-necked giraffes have the same problem of reaching their food; as a result intermediate-necked giraffes are will probably be the fittest to survive, since their neck is both long enough to reach tree fruits, but not that long so as to be spotted from distance. When natural selection favors the intermediate variation, it is called stabilizing selection.
As for the third of the three types of natural selection, this occurs when both extreme variations are favored equally and intermediate variations are lost. Again, we shall hypothetically apply the concept of disruptive or diversifying section to the giraffe example in order to get a better grasp of it. It could be that both long-necked and intermediate-necked giraffes could be spotted by predators, but since long-necked giraffes could always reach higher for fruits they still had a better chance of survival compared to intermediate necked. Similarly, although short-necked giraffes could not have the same access to food resources as the other two variations do, the fact that they are rarely spotted and killed still gives a good chance of survival. Therefore, in this case of disruptive selection both extreme variations will be favored at the expense of the intermediate variation.
Keep in mind, though, two things: the one is that you cannot really predict which of the three types of natural selection will occur for a certain population. There are simply too many variables like weather conditions and the number of species competing for the same food resources, for example. The other all three types of natural selection always depend on sexual selection, which basically refers to the ability of an organism to attract a mate of the opposite sex and its ability to reproduce.