Fig. 5.

Time course of typical compositional changes of a GC-rich region from a warm-blooded vertebrate in the conservative mode of evolution. In an early phase, the average GC level of the region, initially visualized at its compositional optimum (arbitrarily set here at 54% GC), is decreasing because of the mutational AT bias (the vertical blue bars crossing the black DNA line represent the “excess” GC→AT changes), but remains within a tolerated range (whose arbitrary thresholds are indicated by the thick horizontal broken lines). In a late phase, the average GC level trespasses the lower threshold (arbitrarily fixed here at 52% GC), because of the last changes, the critical changes. The chromatin (red boxes) then undergoes a structural change (broken blue box) that is deleterious for transcription and replication (see text). Until then, the changes may be neutral or, more probably, nearly neutral (modified from ref. 32).