XI GENE EVOLUTION
Most biologists assume that the traits of organisms are mostly determined genetically, i.e. follow from the properties of the genes that code them.The evolution of organisms must thus be accompanied (caused) by the evolution of the genes encoding their traits.
Since its establishment, the very concept of a gene has undergone quite dramatic development, frequently leading to attempts to completely abandon the concept of genes or to replace this by some other concepts that are more exactly defined.These, in many respects justified, attempts were never successful.However, in the recent past, molecular biologists were responsible for the most dramatic shift in the meaning of the concept of a gene.A gene is now understood to correspond to a continuous section of nucleic acids (DNA in most organisms) encoding some functional macromolecule (protein, rRNA, etc.).Thus, the concept of a gene actually overlaps the former concept of a cistron (see II.3.1).
It is only a matter of speculation whether this shift in meaning is definitive.However, it can be stated with certainty that, from a practical viewpoint, it was natural, but it was difficult to defend from a theoretical point of view.However, any discussion on this subject exceeds the framework of this text.
This chapter will be concerned with the evolution of genes as understood in the contemporary (molecular biological) conception.We will concentrate primarily on the anagenesis of genes, i.e. on the mechanisms of formation of new genes from old genes from a functional point of view.Then we will consider the evolutionary importance of the internal periodicity of genes and the presence of introns in the genes of organisms.Most of the information currently available in this area is applicable only to genes (cistrons) encoding the individual proteins.Consequently, although it is highly probable that the evolution of multicellular organisms tends to occur at the level of changes in the regulation area of genes, both in sections transcribed to RNA and in sections that are not transcribed, emphasis will also be placed in this chapter on the evolution of DNA sections encoding proteins.