Features of evolutionary-genetic differentiation of bird taxa at the level of transitions and transversions of the CytB gene

Nucleotide substitution analysis was performed for nucleotide substitutions during evolutionary divergence across major taxonomic levels within the class Aves. The frequencies of transitions (homotypic A↔G, T↔C substitutions) and transversions (heterotypic A↔T, A↔C, G↔T, G↔C substitutions) of the cytochrome b (Cytb) gene, as well as their numerical ratio (transition-transversion shift) vary widely. A pronounced predominance of transitions over transversions is observed at intraspecific and generic levels, while a slight predominance of transversions characterizes the formation of orders. This variability in transition-transversion bias is driven by an accelerated accumulation of transition at the genus level and lower levels of divergence against the background of relative stability of transversion frequencies within the evolutionary row of the class of birds. This means that at the late stages of phylogenesis (the formation of taxa of the order level and higher), the taxon formation is driven by a spontaneous mutation process, whereas at the early stages, evolutionary changes are realized through gene regulations. The variability in the nature of nucleotide substitutions is also evident across the same taxonomic levels within different avian orders. It manifests itself as an increase of the transversions frequency and a decrease in the magnitude of the transition-transversion bias within families of small-sized evolutionarily young groups. The reason for the increase in transversion frequencies is an increase in mutation rate within small-sized evolutionarily progressive groups of birds with intensive metabolism compared to families of more ancient large-sized order groups. Intensification of metabolism within birds is not only a factor of diversification and evolutionary progress, but also leads to an increase in the mutational load. There are two possible explanations for this phenomenon. First, point mutations may represent neutral events, the accumulation of which does not necessarily increase the genetic load. Second, the progressive intensification of metabolism inevitably becomes the "gravedigger" of the most evolutionarily progressive groups of birds.