Il Presidente della Pontificia Accademia delle Scienze spiega perché l’evoluzione è un fatto e perché la sua spiegazione è neodarwiniana

Il Presidente della Pontificia Accademia delle Scienze, professor Werner Arber, biologo Premio Nobel per la Medicina e la Fisiologia nel 1978, ha tenuto il 12 ottobre scorso una relazione sui rapporti tra scienza e fede, presentata al Pontefice e ai membri del Sinodo dei Vescovi, nella quale ha illustrato con estrema chiarezza le basi della spiegazione evoluzionistica contemporanea. Riportiamo di


Il Presidente della Pontificia Accademia delle Scienze, professor Werner Arber, biologo Premio Nobel per la Medicina e la Fisiologia nel 1978, ha tenuto il 12 ottobre scorso una relazione sui rapporti tra scienza e fede, presentata al Pontefice e ai membri del Sinodo dei Vescovi, nella quale ha illustrato con estrema chiarezza le basi della spiegazione evoluzionistica contemporanea. Riportiamo di seguito il link al testo della relazione presente sul sito della Pontifica Accademia delle Scienze e il passaggio nel quale Arber spiega che variazioni genetiche spontanee e selezione naturale costituiscono la forza motrice dell’evoluzione biologica. Facendo riferimento alle scoperte e agli importanti aggiornamenti teorici più recenti, Arber elenca anche i molteplici meccanismi di variazione genetica contingente che alimentano il processo selettivo e che – insieme ad altri fattori come la deriva genetica, la migrazione e gli schemi ecologici su larga scala – rendono conto dell’accumulo di quei cambiamenti microevolutivi che costituiscono il presupposto per l’evoluzione della biodiversità. Pikaia saluta con soddisfazione questo evento, che costituisce una buona premessa per superare molti fraintendimenti ancora presenti in alcuni settori conservatori del mondo cattolico. Speriamo inoltre che una presa di posizione ufficiale così autorevole, in un tale contesto, possa mettere definitivamente la parola fine alle campagne negazioniste prive di qualsiasi riscontro scientifico che trovano ancora spazio nella pubblicistica online dell’integralismo religioso.
 

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Spontaneously occurring genetic variation as the driving force of biological evolution
For roughly 60 years we have known that life processes are dependent on the genetic information that is encoded in very long stands of the nucleic acid DNA. The specific linear sequences of only four different building blocs (nucleotides) encode for all life processes and control their expression at the required times and sites within the organism. If we compare the sequences of nucleotides with the sequences of letters in our texts, the genetic information of a single-celled bacterium corresponds to the content of a book. For example, the widely studied E. coli bacterium corresponds to the information content of the Bible. By contrast, the genetic information of multicellular plants and animals often corresponds to an encyclopedia of 100 up to 1000 volumes of the size of the Bible. The human genome corresponds to about 700 such volumes.
Genetic information is passed from generation to generation. Occasionally an alteration of the parental nucleotide sequences occurs in this process. Some of these alterations cause a change in a phenotypic trait of the concerned organism. Such alterations are more often known to affect life activities negatively rather than provide a functional advantage to the concerned organism. Moreover, a considerable part of spontaneously occurring sequence alterations has no immediate effect on life functions.
According to the theory of biological evolution based on Charles Darwin’s postulate of natural selection acting on phenotypic variants, the spontaneous generation of genetic variants is the driving force behind biological evolution. Scientific research in recent decades has made it clear that a multitude of different specific mechanisms can contribute to the generation of novel genetic variants. These so far known molecular mechanisms can be assigned to contribute to one – and in some cases to two – general mutagenic strategies found in living organisms. One of these natural strategies of genetic variation implies a local nucleotide sequence change, such as a nucleotide substitution, the deletion of one or a few adjacent nucleotides, the insertion of one or a few additional nucleotides, or finally a scrambling of a few adjacent nucleotides. This can happen by means of the replication of DNA molecules or the impact of a mutagen. A second natural strategy of genetic variation produces a segmental rearrangement of the available genetic information of an organism. This can result in a duplication, in a translocation or in a deletion, normally of a small part of the genetic information of the concerned organism. The third natural strategy of genetic variation consists in the acquisition of a relatively small segment of genetic information from another kind of organism by so-called horizontal gene transfer.
It is natural selection that will sort out and maintain those rare variants that provide a functional advantage to the organism. We can further note that each of the three natural strategies of genetic variation contributes with a different quality to biological evolution. Local DNA sequence changes can contribute to a stepwise improvement of a particular function. DNA rearrangements of segments of available genetic information can lead to novel fusions of functional domains or the fusion of an existing gene with an alternative element for the control of gene expression. Finally, the strategy of DNA acquisition is seen as a participation in the functional success of different kind of living organism.
 

The natural potential to evolve and its impact on biodiversity
Particular gene products and some non-genetic elements are both generally involved in the natural generation of genetic variants. The products of so-called evolution genes thereby act as variation generators and/or as modulators of the rates of genetic variation. Non-genetic elements may be caused by chemical or physical mutagens, random encounter and structural flexibilities such as isomeric forms of biological molecules. One can assume that in the long-term past evolution, evolution genes had become fine-tuned to exert their evolutionary functions consisting in the occasional generation of novel genetic variants. These processes are largely contingent as regards the site of the DNA sequence alteration and also with regard to the time of mutagenesis. The rates of any kind of genetic variation are naturally kept quite low. This ensures sufficient stability of the genetic information of living organisms, which is a prerequisite for sustainable living in populations. In conclusion, the living world actively takes care of biological evolution thanks to its natural potential to undergo biological evolution. In other words, biological evolution is a steadily ongoing natural process of permanent, step-by-step creativity.
We are aware that the natural potential to evolve is the source of biodiversity and that ongoing biological evolution also guarantees a steady, although very slowly progressing, replenishment of biodiversity. However, in view of the largely contingent generation of genetic variants, one cannot expect lost biodiversity to be restored precisely in the future process of evolution. Replenished biodiversity can rather be expected to represent mainly novel kinds of mutant organisms. …