We describe the evolution of macromolecules as an information transmissionprocess and apply tools from Shannon information theory to it. This allows usto isolate three independent, competing selective pressures that we termcompression, transmission, and neutrality selection. The first two affectgenome length: the pressure to conserve resources by compressing the code, andthe pressure to acquire additional information that improves the channel,increasing the rate of information transmission into each offspring. Noisytransmission channels (replication with mutations) gives rise to a thirdpressure that acts on the actual encoding of information; it maximizes thefraction of mutations that are neutral with respect to the phenotype. Thisneutrality selection has important implications for the evolution ofevolvability. We demonstrate each selective pressure in experiments withdigital organisms.