The hypothesis that genome size and complexity are inversely correlated with mutation rate has been challenged by a study of mitochondrial genomes in the Silene plant genus. Contrary to expectation, species that have experienced recently accelerated mutation rates have the largest genomes of all angiosperms, whereas the genomes of slower mutating species are smaller than average. The large genomes showed many structural alterations and changes in recombinational properties, suggesting that changes in mutation rate can drive rapid and unpredictable genome evolution.