Natural selection is based on molecular copying, more precisely on the indefinite repetition of a process of molecular copying in a world of limited resources, and this means that natural selection would be the sole mechanism of evolution if molecular copying were the sole basic mechanism of life.
As a matter of fact, this could have happened. If living systems could have been made entirely of RNA enzymes and RNA genes, only the copying of RNA molecules would have been necessary, and natural selection could indeed have been the sole mechanism of evolution. But that is not what happened. Long before the origin of the first cells, proteins were being made on the primitive Earth, and proteins, unlike genes, could not be made by copying. The manufacture of proteins required codemakers, not copymakers. It required two independent recognition processes at each step, not one, and above all it required the rules of a code. In an RNA world, in short, molecular copying - and therefore natural selection - could have been enough, but in a world where proteins exist there must necessarily be natural conventions, and these cannot be reduced to natural selection because coding cannot be reduced to copying.
There is, however, another scenario where we could say that natural selection has virtually been the sole mechanism of evolution. If no other organic code had appeared on Earth after the genetic code, we would have to conclude that copying has been the sole mechanism of change for almost 4 billion years, and natural selection could legitimately be regarded as the sole mechanism of evolution for almost the entire history of life. In this case, the origin of the genetic code, at the beginning of that history, and the origin of the cultural codes, at the end of it, could be regarded as two extraordinary exceptions, and natural selection would remain in practice the sole mechanism of evolutionary change.
But the genetic code is not the only code of life. There are many other organic codes in Nature, and this means that they came into being in the course of evolution. This in turn means that copying and coding operated throughout the whole history of life, and gave different contributions to it, which makes us realize that there have been two distinct mechanisms of evolution. It also makes us realize that natural selection and natural conventions had complementary roles: natural conventions account for the discontinuities of the history of life whereas natural selection explains the gradual transformations that took place in between.
Evolution, in short, was not produced only by natural selection but by natural selection and by natural conventions (Barbieri, 1985, 2003), which in no way is a belittlement of natural selection. It is only an extension of it.
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