Survival of the steepest: hypersensitivity to mutations as an adaptation to soft selection

Darwinian evolution favours genotypes with high fitness ('survival of the fittest'). Models of quasi-species evolution, however, suggest that in some cases selection may favour genotypes that are more robust against the impact of mutations ('survival of the flattest') even if these genotypes have lower fitness. I show that the opposite effect will be observed if competition occurs during development (e.g. among embryos or ovules) or before the adult phase (e.g. among the progeny of an individual). If viability is not affected by selection at these initial stages (soft selection), the genotypes that are more sensitive to the effects of mutations may increase in frequency because they get rid more easily of deleterious mutations. In a simple theoretical model of mutation and selection, genotypes located in steeper regions of the fitness surface are favoured ('survival of the steepest') even if they do not have higher viability, and even if they have slightly deleterious effects. Hypersensitive genes are potentially harmful for the individual, but with soft selection during the juvenile phase they persist in the genome because they reduce competition with their mutants. Soft selection occurs in practically all vascular plants and in many animals, therefore antirobustness may be a very common feature of the genome of multicellular organisms.