Different Mechanisms at Different Levels

The idea that natural selection can work at different levels of organization (genes, organisms, species) has been at the center of countless debates in evolutionary biology. Less attention has been given to the alternative possibility that at different levels of organization there may be at work different mechanisms of evolution. There is, however, at least one case that gives us a clear example of this alternative. It is the origin of mitochondria in primitive eukaryotic cells.

For a long time it has been assumed that mitochondria were the result of a gradual evolution from within the cell, but then it was found that they originated by the incorporation of whole cells into other cells by endosymbiosis. Those two types of cell had been in existence for millions of years before the symbiosis event, and all their components had been copied at each generation, and had been subject to evolution by natural selection. Their coming together in symbiosis, however, was a process that took place at the cellular level. It was the cells acting as whole systems that gave origin to endosymbiosis. Their components had to be "compatible" with endosymbiosis, but in no way had been selected for that purpose. Endosymbiosis, in short, is a mechanism that exists only at the cellular level, not at the molecular level, and represents therefore a distinct mechanism of evolution.

In the case of the organic codes, the situation is somewhat intermediate between the molecular and the cellular level. The genetic code, for example, is at the same time a supramolecular system and a subcellular one. All its molecular components must be inherited and copied individually, and yet a code is necessarily a collective entity. The important point is that coding, like endosymbiosis, simply does not exist at the molecular level. It is a mechanism that belongs to the supramolecular level just as endosymbiosis belongs to the cellular level. There is no doubt that copying is absolutely necessary for coding, but the important point is that it is not sufficient for it. Coding cannot be reduced to copying because they are mechanisms that belong to different levels of organization.

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