Mutations at the level of the entire DNA section
Mutations at the level of the entire DNA section can be classified into several types (Fig. III.2). In deletions, insertions andduplications, a certain DNA section is lost or, to the contrary, duplicated. The duplified section can either be immediately next to the original section (tandem duplication) or can be in an entirely different part of the genome. The sections that are, themselves, tandem duplicated can duplicate most readily. At the sites of tandem duplication, incorrect pairing and then nonreciprocal recombination can occur between two homologous chromosomes, as a consequence of which deletion of a certain DNA section occurs on one chromosome, with insertion at another chromosome.
Translocation entails relocation of a certain DNA section to a different site in the genome. If this is reciprocal translocation, then two DNA sections exchange places on the chromosomes; in transposition, only one DNA section is relocated.
In inversion, a certain DNA section is cut out of the chromosome and inserted in the same place with the opposite orientation. In mammals, the vast majority of chromosome restructuring entails translocations; in contrast, inversions are involved in drosophila (Spradling & Rubin 1981). Drosophila are apparently pre-adapted to tolerate inversion in that recombination does not occur during meiosis in the nuclei of male gametes and thus deletion also does not occur in the inversion sections in heterozygote males.
Translocations and inversions can be of great importance in speciation as one of the mechanisms of formation of interspecies barriers. If two individuals differing in the presence of a greater number of chromosomal mutations breed together, they have greatly reduced fertility. In recombination including the restructured DNA sections, frequent deletions occur of entire chromosome sections, so that many of these recombinants are not viable (Fig. III.3). For example, if two individuals differ in the presence of paracentric inversion, i.e. inversion not including the centromere region, then recombination is prevented over entire long sections of the relevant chromosome or recombination leads to chromosomes without centromeres or with two centromeres, i.e. structures that reliably prevent nuclear division.
An increased frequency of inversion in some taxons could be the cause of an increased rate of speciation of these species and, as a consequence of the action of species selection (IV.8.4), also the cause of their evolutionary success (Trickett & Butlin 1994).