The origin and evolutionary dynamics of the spatial heterogeneity in genomic base composition have been debated since its discovery in the 1970s. With the recent availability of numerous genome sequences from a wide range of species it has been possible to address this question from a comparative perspective, and similarities and differences in base composition between groups of organisms are becoming evident. Ample evidence suggests that the contrasting dynamics of base composition are driven by GC‐biased gene conversion (gBGC), a process that is associated with meiotic recombination. In line with this hypothesis, base composition is associated with the rate of recombination and the evolutionary dynamics of the recombination landscape, therefore, governs base composition. In addition, and at first sight perhaps surprisingly, the relationship between demography and genomic base composition is in agreement with the gBGC hypothesis: organisms with larger populations have higher GC content than those with smaller populations.