Modelling the evolution of genomes with integrated external and internal functions

The genomes that organisms transmit between generations contain information about different kinds of functions. The genome with the "best" mix and number of genes for these functions is the one that natural selection favours. Here I introduce a new way to model simple organisms with genes for external and internal functions, and use it to study the evolution of genome size. The external functions are exemplified by resource use and the internal functions by mutation control (repair). It is shown that even with a suitable proportion of genes for mutation control, the genomes in the organisms do not forever incorporate genes that increase resource use. Instead they evolve towards an optimal genome of limited size. The optimal proportion of genes for mutation control is shown to have an upper limit given by the ease with which transmission accuracy is improved by adding extra genes for this purpose to the genome. The model illustrates how natural selection on genomes integrates systems for the transmission of genetic information with systems relating to the external adaptation of the organism. It also opens up for other, more detailed theoretical investigations of genome functions.