Fig. 2 Changes in mitochondrial substitution rate among close relatives within the genus Silene. (a) Average pairwise sequence divergence in 23 shared
mitochondrial genes between S. conica and S. noctiflora is greatly accelerated relative to other species in the genus, including S. latifolia and S. vulgaris. By
contrast, average pairwise divergence for nuclear loci (140 cytosolic ribosomal proteins) is essentially the same for the two species pairs. Gray bars, nuclear;
brown bars, mitochondrial. Error bars, + 1 SE. (b) The extreme difference in rates of mitochondrial sequence evolutionamongclosely related species allows for a
full set of comparative analyses to test for evidence of compensatory cytonuclear co-evolution. In this case, strong positive selection on nuclear genes would be
expected only in cases that involve species with high mitochondrial substitution rates and nuclear genes that are targeted to the mitochondria