The number of animal and vegetable species present on Earth is astronomical. Moreover, the majority of the species are very complex : it is enough to contemplate our organization to realize it. And yet, all the species result from simple maritime bacteria, at least according to the actual more credible theory. That is explained thanks to the evolution, a concept which is very related to the natural selection. This phenomenon is observed on long scale, since one finds many fossils of disappeared animals, and on short scale, as one sees it with the mosquitos : some frightening insecticides one year become ineffective the next summer, because the mosquitos adapt. Then how does the evolution occur ? To study it, we will keep the example of the mosquitos.
The evolution is allowed by the genetic changes. Whereas many carries out to diseases, some can be beneficial to the individual. However, these changes are entirely random and never concern a whole population but isolated individuals. It is one of the principal difficulties which have of facing the researchers to understand.
First of all, it is important to fight a very widespread but false idea. It is often said that « the mosquitos acquire a resistance to insecticides because they adapt ». Even if that approaches the truth, this is entirely false : the changes are random, the evolution does not have any goal, no direction and in no case an individual cannot transfer according to his environment.
The true mechanism is however very close. Let us look at our mosquitos. Thanks to genetic mixing, they are very different the ones from the others. Some have a better resistance to insecticides than others. In other words, some are naturally favoured, others are not. Then they are subjected to an insecticide. The natural selection enters then in action : those which are handicapped will die before being able to procreate. In parallel, the favoured mosquitos will not suffer any damage and they will reproduce. Thus, the following generation will include more immunized against insecticide mosquitos. That's why the insecticide loses its effectiveness : it makes devastations on a generation but the reverse is that the following generation will be formed individuals having the capacity to resist (if not, they could not have been born). Rising generation thus is equipped better.
One can also take the example of English butterflies. The industrialization of Great Britain entirely polluted many areas like Oxford. To a point such as the birches passed from the white color to the black color. Before industrialization, the white butterflies could be camouflaged on the trees and thus escape from their predatory whereas the black butterflies were quickly made have. There were thus more white butterflies than black. Then after industrialization, that did occur ? The black butterflies, very few, could now escape from the predatory ones. Their population thus increased considerably thanks to the reproduction. Opposite, the white butterflies did not reproduce any more and their number fell. One thus attended an inversion of the proportions.
Unfortunately, this theory has evil to develop because of some religions, owing to the fact that it touches with the concepts of goal and direction of the evolution. Whereas it has already more than one century (even if it since were improved), certain American states prohibit to their professors to teach it...