User:J. C. Martinez-Garcia/Notebook/HMS Activities/2008/10/13
|Again on Mutations||Main project page|
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The Jablonka exposition
Today I continue with the study on mutations. I will repeat in what follows the expositions given in the Jablonka book:
Synthetic Darwinism suppose that natural selection drives evolution, i.e. a new species comes from a existing one when the individuals belonging to this existing species -all of them essentially the same but with very small variations due to blind mutations- reacts to environmental changes in slightly different ways because of the underlying difference at the genotype level. The individual which have the right behavior for the present environment are selected and then its difference becomes a characteristic of theirs descendants. With the time the mechanisms produce new species, and the way the new characteristics are preserved by the individuals is through DNA-based heredity. Synthetic Darwinism does not accept that the traits acquired by the phenotype will be inherited by the descendants, as a consequence of the evolutionary adaptation.
When considering the genetic mechanisms, where the supposed source of genotipic variability is due to mutations, there is no place for like lamarckism mechanism to have a a role in the game (for tenants of the Synthetic Darwinism approach).
Accepting that only mutations are the source of genotipic variability allowing adaptation to environmental changes, the only way to accept that the interaction with the environment directly modifies the genome is suggesting that the interaction drives modifications in mutation rates, and that mutation rates are not the same for all the genome, but that there exists a variability in the mutation rates in function of the changes required for the adaptation. This supposed mechanism can be completely justified in terms of natural selection.
Jablonka and Lamb gave the following table in their book:
These mechanisms exist in nature and are validated by experimental evidence. The spectrum of mutations go from completely blind (are no associated to adaptiveness of the type of change) to completely induced in the developmental case and associated to adaptiveness of the type of change.
My own ideas
We identify two main systems constituting an organism:
The genotype essentially corresponds to the collection of genes and the regulatory devices coded by the DNA. The Phenotype corresponds to the individual traits resulting from the interaction between the expressed genotype and the environment. Both the genotype and the phenotype present variability in a given species, i.e. two individuals are not identical. What are the sources of variability:
Variability of the genotype influences fitness, but variability in the phenotype also.
Evidently the phenotype and the genotype are not isolated, since the phenotype is something like the reader of the information recorded in the genotype. The basic dogma in Synthetic Darwinism is that the lecture of the information does not modify the recipient of it, i.e. the phenotype does not change the genotype, and it only changes by accident.
The history of the phenotype of a individual is lost when the it dies? Synthetic Darwinism says that the answer to this question is essentially positive. But what if natural selection helps differences in the individuals -at the genotype level-, which guarantee that certain differences in the phenotype are inherited -the ones improving fitness-?
What are the possible mechanism insuring hereditability of acquired traits?
A natural question is necessary here:
The goal oriented modification of the genotype can be driven by the phenotype? Which is to say, the genotype encodes a mechanisms supporting this startegy of adaptation?