Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Point mutationsmost frequently consist in the replacement of one nucleotide by another; these are replacement mutations, substitutions.If a nucleotide with a certain type of base, e.g. pyrimidine (C, T), is replaced by a nucleotide with a different type of base – purine (A, G), then this corresponds to transversion; if it is replaced by a nucleotide with a base of the same type, then this is a transition.Point mutations also include deletion and insertion, in which the number of nucleotides is changed at a certain place in the DNA, usually by one; however dinucleotide insertions anddeletions are also quite frequent.The frequencies of the individual types of mutations differ considerably and depend not only on the type of organism (bacteria, eukaryote) and on the genome, in which the mutation occurs (nucleus, mitochondria, plastid), but also on the nucleotides occurring close to the given position.For example, it was found for chloroplast DNA that, if a certain nucleotide is adjoined from both sides A and T, then the probability of transversion is 2.2 times greater at this site than the probability of transition.However, if at least one of the neighbouring nucleotides is G or C, then transitions are 1.5 times more probable at the given site than transversions (Morton & Clegg 1995).However, these ratios differ somewhat for various genes, so that it is apparent that the probability of the individual types of exchanges is not affected only by the immediately neighbouring nucleotides (Morton 1997).It was found in primates (Saitou & Ueda 1994)that the frequency of insertion and deletion in nuclear noncoding DNA is approximately 0.2/kb per million years; this frequency is 10x times higher in mitochondrial DNA.In both cases, the frequency of deletion is approximately twice as high as the frequency of insertion, and the frequency of substitution is approximately 10x higher in the nucleus and mitochondria.The frequency of transitions mostly differs very substantially in the individual sites of the sequence, while far rarer transversions are distributed much more evenly in the DNA.For example, study of human mitochondrial DNA has demonstrated (Wills 1995)that ¾ of polymorphous sites exhibit a frequency of transitions that is approx. 4 times greater than the frequency of transversions.However, the frequency of transitions in the remaining quarter of the polymorphic sites is about 160 times greater than the usual frequency of transversions.Simultaneously, the frequency of transversions is approximately the same at all sites.When assessing the importance of similar data, it is, however, necessary to take into account that the experimental data mostly reflect both the frequency of the actual mutations, i.e. the frequency of their formation, as well as the frequency of their fixation or partial fixation, i.e. the frequency or the probability of their complete predominance in the population or their preservation as a part of polymorphism of the given population or species s.