markovian
Lynch M 2012 The evolution of multimeric protein assemblages. Mol Biol Evol 29:1353-1366.
- for more complex adaptations requiring multiple mutations, it is often argued that adaptive evolution essentially never occurs via intermediate deleterious states
- evolutionary routes to adaptation involving deleterious early steps impose a bottleneck in mean population fitness
- it is now clear that the rescue effect (stochastic tunneling) by secondary mutations provides a powerful mechanism for vaulting an adaptive valley while avoiding negative repercussions at the population level
- with sufficiently low recombination rates, adaptive progress can be made without a population ever experiencing a decline in mean fitness
- deleterious intermediate-state alleles never rise to high frequencies
- the existence of such evolutionary pathways to adaptive exploitation raises significant challenges for laboratory studies that strive to reconstruct the historical order of events leading to the establishment of complex adaptations under the assumption that all intermediate steps must have been nondeleterious and procured in a stepwise fashion
- via the rescue effect, mutations that are individually deleterious not only can become fixed but do so simultaneously with the rescuing mutations, leading to episodic evolutionary change
- a number of recent studies suggest that this sort of evolution is common with respect to the evolution of gene function
- variation in the power of random genetic drift among phylogenetic lineages has contributed significantly to the emergence of many of the complex (and often arcane) features of eukaryotic cells
