VI.2 Molecular drive is the consequence of stochastic and deterministic processes responsible for the development and proliferation of repetitive DNA variants in the genome and in the gene pool of the population.
While some DNA segments occur in the haploid genome only in a single copy, other segments are present in the genome in more or less similar and more or less numerous variants. In dependence on the number of copies and partly on the basis of the degree of similarity of the individual copies, at least three classes of repetitive DNA can be distinguished in the genome of eukaryotic organisms. From a quantitative point of view, the most important class is apparently the medium-repetitive DNA, which is apparently mostly formed by various transposones and their inactivated products. The molecular processes affecting the behavior of segments of repetitive DNA differ from processes affecting the behavior of unique DNA segments. The evolution of copies of a single repetitive segment located at various loci is mutually interconnected, i.e. occurs primarily through gene conversion in the same way at all loci and, compared to the evolution of unique segments, is usually much faster. Consequently, repetitive DNA creates an unusually dynamic, rapidly developing component of the eukaryotic genome. The processes responsible for proliferation of the individual variants of repetitive DNA segments in the genome and, in sexually reproducing organisms, even in the entire gene pool are called molecular drive. Molecular drive is frequently considered to be one of the most important processes responsible for shifts in the frequency of the individual alleles in the gene pool of the population. The unique character of this process was first recognized in the 1970’s by G.A. Dover, who also described some of its specific mechanisms and impacts on the course of biological evolution. In contrast to selection and genetic drift, knowledge about molecular drive and especially its evolutionary importance so far tends to be limited.
Some authors consider that the most important consequence of the action of molecular drive is the fact the changes that it causes in the genome could simultaneously affect a great many individuals in the population. Consequently, the processes caused by molecular drive are termed synchronized, concerted or coincidental evolution (c.f. VI.2.4).