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 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.
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 the 64 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 physicochemical 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.
General evolutionary theory tries to synthesize the insights of the before-mentioned biological disciplines in an all-embracing picture of molecular evolution. This integration is to be accomplished by discovering the causal mechanisms that can explain the facts of natural history.
What mechanisms are responsible for molecular evolution? Of course, the answer that first comes to mind is natural selection, the evolutionary mechanism that stands in the center of Charles Darwin’s research on the origin of species. His theory can be applied to the molecular level, if we remember that the differences between individual organisms and therefore also between species are caused by differences in their genetic material, which are, in turn, caused by mutations in the nucleotide sequences of their DNA.
Since those mutations are inherited by offspring, fitness as the measure for the evolutionary success of an organism can be redefined on the molecular level as the replication rate of its genes in the gene pool of its population. The struggle for life described by Darwin is thus the struggle of genes to raise the number of their copies in the next generation. The more copies of a DNA sequence that are successfully transmitted to offspring, the fitter it is.
But there is another theory that claims also to contribute important insights to the explanation of the course of molecular evolution. This claim is laid by the neutral theory of molecular evolution, which the Japanese population geneticist Motoo Kimura has developed since the 1960s. He postulates that most genetically transmitted mutations are selectively neutral; they do not have any positive or negative consequence on the fitness of the organism in which they occur. The spread of neutral mutations in the gene pool of a population, called “genetic drift,” has then nothing to do with natural selection. It is a process that follows, from the perspective of natural selection, a completely random course.
Today, even staunch Darwinians accept that genetic drift constitutes an important mechanism that is effective in natural history. On the other hand, it seems to be also clear that randomness cannot explain everything in evolution. Kimura’s neutral theory does not completely replace Darwin’s selection theory, but it describes an explanatory scheme that supplements traditional Darwinian reasoning. In concrete cases of explaining a molecular evolutionary process that happened between kindred species, it is often very difficult—and perhaps impossible—to decide which of the two theories proves to be right.
How difficult it is to develop a general evolutionary theory on the molecular level will be clear after one has realized that it has to address not only the evolution of DNA. Molecular evolution also encompasses the evolution of precursors of DNA and, last but not least, the origin of the first material carrier of genetic information. Since the existence of genetic information presupposes the existence of a genetic code, the origin of the latter belongs to the main problems of research into molecular evolution, too.
References:
- Graur, D., & Wen-hsiung, L. (2000). Fundamentals of molecular evolution (2nd ed.). Sunderland, MA: Sinauer.
- Kimura, M. (1983). The neutral theory of molecular evolution. Cambridge: Cambridge University Press.
- Küppers, B.-O. (1985). Molecular theory of evolution: Outline of a physicochemical theory of the origin of life (2nd ed.). Berlin: Springer.
