Evolution, Molecular

Theories of molecular evolution try to explain the natural history of deoxyribonucleic acid (DNA), which is the material carrier of genetic information.

Evolutionarily relevant variations between organisms must be implemented in the biochemical structure of DNA sequences. Otherwise, those variations [Page 896]would not be genetically transmitted from an organism to its offspring, so they would disappear from nature after one generation. Molecular evolution is thus the foundation of evolution on all higher levels of biological organization, like the organism, the population, or the species.

Source: © iStockphoto/Andrei Tchernov.

After the famous discovery of the double-helical structure of DNA by James D. Watson and Francis H. Crick in 1953, it was known that genetic information is stored in sequences of four DNA building blocks, the nucleotides. That parts of the linear structure of DNA store genetic information means that those parts can instruct the biosynthesis of proteins, which are the most important macromolecules for cellular metabolism. More concretely, the specific succession of nucleotides can encode the primary structure of a protein, that is, the linear sequence of amino acids linked by peptide bonds. In the information bearing parts of DNA, three nucleotides (a codon) encode one amino acid. A gene is the informational unity the function of which is to encode the complete primary structure of a protein. The set of rules that relate each of the64 possible codons (one has to take the number of nucleotides, which is four, to the power of the number of nucleotides in a codon, which is three) to an amino acid residue constitutes the genetic code. Those rules cannot be deduced from the laws of biochemistry alone. The molecular evolutionist must also reconstruct the historical and contingent physico-chemical context in which the genetic code originated.

The biological disciplines that are involved in discovering the laws of molecular evolution and in reconstructing its course are, above all, biochemistry, molecular and population genetics, systematics, and general evolutionary theory.

Biochemistry and molecular genetics analyze the material structures and the physico-chemical mechanisms that realize the storage, replication, variation, transmission, and reading of genetic information. The success of this project is evident from the enormous technical progress that was made during the last 30 years in the development of genetic engineering.

Population genetics studies the evolutionary dynamics by which the relative frequency of genes changes with time in populations of organisms. Mathematical models were developed that describe different types of evolutionary dynamics in a more and more realistic way. These models can be tested against empirical data gathered by systematic analyses of wild-living populations.

Systematics classifies the rich variety of species, which lived and live on Earth, in order to reconstruct their evolutionary relationships in so-called “phylogenetic trees.” Such a tree shows graphically how some species, with which we are concerned, are related to each other by placing their common ancestor at its root and by illustrating the separation of a new species with a branching point. Finally, the species in question are to be found at the top of the resulting branches. Before the advent of automated DNA-sequencing techniques, morphological descriptions were the most important data on which phylogenetic trees were based. Nowadays, molecular data of DNA sequences are of equal importance for the reconstruction of natural history.

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