Recent advances in studies of evolutionary relationships between proteins and nucleic acids.

Abstract

Evolution depends upon the occurrence of occasional changes, large or small, in hereditary characteristics. Molecular genetics gave rise to the new field of molecular evolution, which is currently exploring the changes that take place in proteins and nucleic acids over long periods of time. The following are some of the fundamental assumptions: (1) The phenotypic characteristics of organisms depend directly on proteins. (2) Proteins are synthesized in accordance with information carried in molecules of DNA as sequences of the four bases, adenine, guanine, cytosine, and thymine. The information is transcribed into molecules of messenger RNA and is translated into proteins by the intervention of the genetic code. (3) Changes in the composition of the base sequences in DNA can take place in living organisms, and these changes can affect the phenotypic characteristics of the next generation. (4) The process of natural selection favors the perpetuation of organisms which compete successfully in the struggle for existence. This process leads to the elimination of all but a small fraction of the astronomical number of possible protein molecules that could result from genetic translation of the possible variants of DNA. Furthermore, the number of protein molecules was originally much smaller than it is to-day, and it has increased by hereditary processes rather than by the chance appearance of entirely new proteins. (5) The DNA present in any single cell contains the complete information for all the hereditary characteristics of the organism. The amount of DNA per cell may increase during evolution and this increase has produced modern organisms that are ‘higher’, more specialized, and more complex, from carlicr and simpler forms. (6) Protein molecules are slowly and steadily differentiated during evolution if their genes are physically separated from each other, by allopatric speciation or even by duplication and translocation, whether or not the function of the proteins are changed. (7) Mutations, together with recombination, contribute to changes in the genetic pool which provide the variability within populations that is necessary for evolution of species.