We refer to the chance loss of all individuals in the least-loaded class as "stochastic elimination." The second way the least-loaded class can be lost is through mutation pressure; if all progeny of the least-loaded individuals receive at least one additional mutation, the ratchet will click one notch.
With these modes of propagation, progeny typically inherit the full load of deleterious mutations carried by their parents (with only a slight deviation from this for recently arisen mutations in selfers) as well as acquire new mutations, and this mutation pressure combined with random genetic drift leads to an expected progressive decline in fitness (Muller 1964; Felsenstein 1974; Haigh 1978; Pamilo et al.
For Osawa, the primal force driving code evolution is "directional mutation pressure." In his earlier work, Osawa surveyed the genomic G+C content of a variety of bacteria and discover that it could be as low as 25% in species like Mycoplasma capricolum and as high as 75% in species like Micrococcus luteus.