Table 1.

Definition of the terms used in this study

Term	Definition
Vector switch	Change of main arthropod vector by a virus acquiring the ability to circulate in natural populations of the new vector for a long time
Quantum shift	Drastic shift to the new adaptive zone with a rapid change in the genetic and phenotypic characteristics accompanied by the nonlinear accumulation of mutations
Quantum evolution	Step-like pattern of evolution comprising rapid genetic and phenotypic changes followed by long periods when the species evolves very little
Clusteron	A group of TBEV strains with identical amino acid sequences of the E glycoprotein fragment, as a rule phylogeographically close, and having a certain type of territorial distribution
Clusteron structure	Quantitative and qualitative composition of clusterons, visualized as a phylogenetic network
Clusteron-founder	The greatest (in number of strains) clusteron with maximum fitness
Clusteron-derivatives	Clusterons differing from the “clusteron-founder” by one or several amino acid substitutions
Transition point	An amino acid sequence containing deleterious mutations that are normally purged from the virus population