Acclivation of Virtual Fitness Landscapes

Any part of a genome, considered separately from the rest of the genome, evolves against a "virtual fitness landscape" that results when the rest of the genome is held constant. We show how analyzing a genome in this way can explain one form of progressively increasing evolvability. When one part of a genome is a vector of numbers ("knobs") and the rest is a graph that determines the mapping from knobs to phenotype, the graph will respond to selective pressure to "acclivate" the virtual fitness function faced by the knobs-that is, to make it more hill-shaped. For as long as the knobs' virtual fitness function provides opportunity for distorting it to make knob-turning mutations improve fitness, the graph experiences pressure to evolve those distortions as a side-effect of responding to its own virtual fitness function. As the knobs' virtual fitness function grows more hill-shaped, the knobs track upward paths more easily and hence so does the genotype as a whole. A synergy develops between incremental exploration of phenotypes by knob-mutations and discontinuous exploration by graph-mutations. A favorable condition for this is a global fitness function that frequently varies, changing constants but leaving structural invariants unchanged. The graph then accumulates a memory of the invariants as revealed across many previous epochs, held in the form of bias limiting and directing future evolution.