Genetic variability

If the process of DNA replication always occurred without errors there would be no species evolution and there would have been no branching of life into the enormous biodiversity that is before our eyes. The first simple organism that appeared on the earth billions of years ago would have continued to replicate without any change. The genome instead must be on the one hand very stable - after all, one of the main characteristics of the organisms is that they can make remarkably similar copies of themselves - but on the other hand it must be rather flexible, or capable of development, evolution and branching, in other words diversification. The driver of genome variability is the phenomenon of genetic mutation: during mitosis and meiosis there can occur several errors in DNA replication (see Fig. 14) which give rise to mutated cells or to mutant individuals (Fig. 15).

Figure 14: The various types of genetic mutation.

However, if mutations, basically due to randomness, were the only cause of diversification, we would not see organized and coherent structures in the terrestrial biosphere. Instead, the organization of life is shaped by the process of natural selection, understood and analysed for the first time by Charles Darwin (1859). In fact, in the first place, because of the genome stability, mutations are anyway very rare (a typical rate of mutation of a pair of DNA bases per generation is in the order of $10^{-6}$); also, the vast majority of mutations are deleterious. Among those not detrimental to the survival of organisms only a few manage to pass through the sieve of natural selection, which favours those mutants that have a demographic advantage. Selection is very strict, but the sieve operates continuously on myriads of organisms. It is therefore very effective in the long run and produces organisms adapted to their environment.

Figure 15: Example of bithorax mutant in the fruit fly Drosophila melanogaster.

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In very many cases, as pointed out in more recent times by Kimura and Crow (1964), may nevertheless appear and remain in a population even so-called neutral mutations - i.e. neither advantageous nor deleterious. This makes it possible for the same species and more in particular the same population to be generally characterized by high genetic diversity. Keeping rare genes within a population can not confer an immediate advantage but may be nevertheless useful in the future for the conservation of the species when new environmental conditions might change the selective pressures. The loss of genetic variability can limit the ability of a population to respond to external changes in the long term such as those due to pollution, climate change, new emerging diseases. Unfortunately, in small populations, which is typically the case of populations threatened by extinction, purely random processes can lead to the disappearance of genes thus further increasing the risk of extinction in the long run. In the following sections we will analyse these phenomena in greater detail.