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Confessions oor kerkwees

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Jaco Strydom

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“Such then is the nature of quasispecies : the density of the sequence cloud at any point in sequence space is determined by the relative fitness of the sequence; regions of the cloud representing sequences of lesser fitness will be less densely populated and those with higher fitness, most populated. Here lies the most powerful quality of viral quasispecies: the density distribution of fitness variants dictates that sequences are represented at frequencies in relation to their relative fitness. Genomes with lower fitness will replicate poorly, or not at all, and the fittest genomes will replicate most efficiently. It therefore follows that there is a large bias toward the production of well-adapted genotypes: there are more of them, and they undergo most replicative cycles. This can permit viruses to experience evolutionary adaptation at rates that are orders of magnitude higher than those that could be achieved by truly random unbiased mutation. Sequences rapidly condense around the fittest area of the sequence space. Should the environment change, and, therefore, selective pressures change, a quasispecies can opportunistically exploits its inherent adaptive potential. Genotypes rapidly and ever-faster gravitate toward the cloud's new notational center of gravity. Changes in the fitness landscape of the sequence space that is occupied by a quasispecies are the natural consequence of altered selective pressures operating on the virus population. Such alterations may be the consequence of changed immunologic pressures exerted by the host, the application of antiviral drug therapy, or even cross-species transmission requiring the virus to adapt to a new host. Genotypes that once occupied the 'central' space, reserved for the fittest genotypes, are reduced in frequency and now occupy the more sparsely populated fringes of the fitness landscape; the very edge of the sequence cloud if you will. Here too lies an advantage for a quasispecies: it has a memory. The once best-adapted genotypes, now at a fitness disadvantage, can persist in the quasispecies as minor sequence variants. Under circumstances of fluctuating selective pressures, the ability of the population to recall an 'old' genome variant is a great asset. The quasispecies can rapidly respond and adapt by plucking out a preexisting variant and quickly coalescing around it to recreate an optimal fitness landscape.”

LandscapeFitnessGenomesFitness LandscapeQuasispecies
Book:Viruses: Agents of Evolutionary Invention
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