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In the circadian system of cyanobacteria, the ldpA gene is a component of the input to the clock. We comparatively analyzed nucleotide polymorphism of this gene in populations of two closely related species of cyanobacteria (denoted as Synechococcus species S1 and S2, respectively) from extreme cold deserts in Antarctica, the Canadian Arctic, and Tibet. Although both species manifested similarly high haplotype diversities (0.990 and 0.809, respectively), the nucleotide diversity differed significantly (0.0091 in S1 and 0.0037 in S2). The populations of species S2 were more differentiated (F(ST) = 0.2242) compared to those of species S1 (F(ST) between 0.0296 and 0.1188). An analysis of positive selection with several tests yielded highly significant values (P < 0.01) for both species. On the other hand, these results may be somewhat compromised by fluctuating population sizes of the species. The apparent selection pressure coupled with the pronounced demographic factors, such as population expansion, small effective population size, and genetic drift, may thus result in the observed significant interpopulation differentiation and subsequent speciation of cyanobacteria.