IX.1 At the DNA level, selectively neutral mutations seem to represent a large portion of evolutionary changes
From the perspective of the effect on the fitness of their bearers, mutations can be divided into positive (advantageous, useful) mutations, negative (disadvantageous, detrimental) mutations and selectively neutral mutations (see Chap. III).Mutations affecting classical morphological and behavioral traits are usually selectively important and thus fall in the categories of positive or negative mutations.In contrast, most evolutionary changes observable at the DNA level and probably a major part of the changes observable at the level of proteins originate from selectively neutral mutations and the random processes of genetic drift and genetic draft rather than the semi-deterministic processes of selection play a greater role in their spreading and potential fixation in the gene pool of the population.
Types of neutral mutations were described in Chapter III (Mutations).Neutral mutations occur repeatedly in the individual DNA sections, i.e. as a consequence of mutation pressure.Most of them subsequently disappear from the gene pool of the population through the action of genetic drift or genetic draft (see below).On the other hand, part of the mutations is evolutionarily fixed, i.e. spread? to all the members of the population.This fixation can occur both by rapid evolutionary drive and by slow evolutionary drift.Neutral mutations can also be relatively rapidly fixed by genetic draft also termed evolutionary hitchhiking, i.e. they spread in the population because they are located on a chromosome close to a positive mutation that spreads in the population through selection (see IX.5.2).In organisms in which genetic recombination does not occur, neutral mutations can spread through genetic draft without regard to their position with respect to positive mutations, while, in sexually reproducing organisms, the effectiveness of genetic draft is inversely proportional to the probability of recombination (crossing over) in the section between the positive and neutral mutations, i.e. directly proportional to the strength of genetic linkage between the relevant loci.