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Indeed, one can be tempted to interpret the discussed examples of the animals who have lost some functional ability living in an unnatural environment as examples of genetic adaptation. One can be also tempted to draw the analogy between such animals and the blind species living in the dark, who lost the ability to see when adapting to new environmental conditions in the course of evolution, with species-parasites who lost their breathing organs that proved to be unnecessary within the body of the host, etc. Another temptation is to interpret the facts either as decay of the genetic program of a species or as genetic adaptation and consider these two categories as definitions arbitrarily chosen by the researcher according to his aesthetic preferences. However, this is not so. Which of these two fundamental categories is realised, can be empirically tested.
States of adaptation of different species and different decay phenotypes that appear in the course of decay of genetic information of a single species are of a principally different nature. The full set of states of adaptation inherent to a given species enables the latter to exist sustainably during several million years (on average). On the contrary, the decay phenotypes that appear as a result of genetic decay of a population under unnatural conditions, are unable to exist sustainably. This is because the corresponding genotypes have lost some information necessary for long-term sustainable existence of the species.
A vivid example may be given by the well-known case of the genetic defect of sickle-cell anaemia. In homozygous state, this defect causes a severe disease with probable lethal outcome. On the other hand, individuals who are heterozygous for this defect, are known to resist malaria better. This makes it possible to discuss the 'adaptive potential' of this defect, which became a textbook example.
However, for this 'adaptive potential' to get realised, the advantage of genotypes carrying the defect must be manifested. This means that the population should be kept under condition of malaria epidemic. Obviously, such an environment may not be considered as natural. If this state of the environment persists, this will finally lead to extinction of the whole population. The fact that sickle-cell anaemia heterozygotes will be the last to perish, does not mean that they have adapted to the new environmental conditions. We underline once again: the difference between states of adaptation in different species, on the one hand, and various decay states within a single species, on the other, consists in the fact that the former contain information ensuring stable existence of species, while the latter do not.
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