An information-theoretic approach to complexity in molecular evolution reveals three distinct pressures in the evolution of genomes. Two of these directly affect the size of the genome: the compression of sequence length allows for faster transmission in the replicative process (both through having fewer base pairs to be transfered and due to a lesser requirement for resources), while information acquisition entails the expansion of genome size whenever the information will sufficiently augment the gathering of resources or otherwise hasten the reproductive process. Beyond these two intuitive factors, genomes are also under a third pressure to evolve genetic neutrality, contributing to the evolution of evolvability. We formulate the three types of selective pressures within standard molecular evolution theory and demonstrate their relevance in experiments with digital organisms.