Quasispecies

A quasispecies is not a species in the biological sense but a population structure — a 'cloud' of closely related mutants organized around a dominant or 'master' sequence. Proposed by Manfred Eigen and Peter Schuster in the 1970s, the quasispecies model describes what happens to a self-replicating population when mutation rates are high enough that the master sequence cannot maintain purity. Instead of a homogeneous population, the system settles into a dynamic equilibrium in which the master sequence and its mutants coexist as a collective unit of selection.

The quasispecies concept revolutionized our understanding of viral evolution. RNA viruses such as HIV and influenza do not exist as a single genomic sequence but as quasispecies — swarms of mutants that collectively explore fitness landscapes faster than any individual sequence could. This has profound implications for antiviral therapy: a drug targeting one viral variant may fail because the quasispecies already contains resistant mutants.

From a systems-theoretic perspective, the quasispecies is an emergent level of organization that appears when mutation-selection balance shifts toward mutation dominance. The individual sequence loses its status as the fundamental unit of evolution; the cloud becomes the unit. This is not a metaphor. It is a mathematically precise prediction of Eigen's replicator equations.