evolvability

Cuevas JM, Moya A & Sanjuán 2009 A genetic background with low mutational robustness is associated with increased adaptability to a novel host in an RNA virus. J Evol Biol 22:2041-2048.

  • robustness can promote evolvability only in systems where large sets of genotypes are connected by effectively neutral mutations
  • in light of our results and previous evidence showing that the evolution of RNA viruses is highly constrained at the molecular level, we suggest that this condition might not be fulfilled in RNA viruses in general
  • in general, the relationship between robustness and evolvability might be time-dependent
  • if evolvability is defined as the ability to find adaptive solutions in the immediate mutational neighbourhood, i.e. adaptations involving one or few mutations, then by definition genotypic robustness hampers evolvability
  • robustness might allow populations to explore vaster regions of the genotypic space and thus, foster evolvability in the long-term
  • for an organism to be robust yet evolvable, networks of effectively neutral mutations need to span a large proportion of the total genotypic space
  • because otherwise, only a small region of the fitness landscape could be explored
  • in the case of RNA secondary structures, it has been suggested that the size of neutral networks might be too small to produce long-term correlations between robustness and evolvability
  • in the case of RNA viruses, indirect evidence suggests that neutral networks might be smaller than those of complex organisms
  • first, a large proportion of random mutations are highly deleterious or lethal
  • second, genome sequencing of different viral lineages adapted to a common novel environment has revealed that the number of mutations fixed during adaptation is often surprisingly small given the high mutation rates of RNA viruses
  • third, a striking number of parallel or convergent mutations have been reported
  • fourth, fitness trade-offs are particularly pronounced in RNA viruses
  • hence, although RNA viruses evolve rapidly in general, they can follow a limited number of alternative evolutionary pathways
  • in light of these data, it seems unlikely that the conditions needed for mutational robustness to promote evolvability are met in the case of RNA viruses