The Darwinian concept of evolution through natural selection has been revised and put on a solid physical basis, in a form which applies to self‐replicable macromolecules. Two new concepts are introduced: sequence space and quasi‐species. Evolutionary change in the DNA‐ or RNA‐sequence of a gene can be mapped as a trajectory in a sequence space of dimension ν, where ν corresponds to the number of changeable positions in the genomic sequence. Emphasis, however, is shifted from the single surviving wildtype, a single point in the sequence space, to the complex structure of the mutant distribution that constitutes the quasi‐species. Selection is equivalent to an establishment of the quasi‐species in a localized region of sequence space, subject to threshold conditions for the error rate and sequence length. Arrival of a new mutant may violate the local threshold condition and thereby lead to a displacement of the quasi‐species into a different region of sequence space. This transformation is similar to a pha...