Abstract
Cyanobacteria have high adaptive potential and occur in the most extreme habitats. The available literature data indicate that the versatility of cyanobacteria is related to their higher polymorphism under stress. The studies of a filamentous cyanobacterium, Nostoc linckia, from the ecological microsite models known as "Evolution Canyons" showed that, among the evolutionary forces maintaining the higher polymorphism and genome diversity under permanent natural stress, the various types of natural selection play a key role.