Evolutionary biology does not require that organisms be able to accumulate unlimited mutations. Organisms can and do accumulate large numbers of mutations. Explore Evolution is deeply misleading about the role of mutations and adaptations.
Explore Evolution claims that
The cell cannot endure an unlimited number of mutation-induced changes at these critical active sites. At some point, the cell's information processing system will be damaged so badly that it stops functioning altogether. For this reason, multiple mutations at active sites inevitably do more harm than good.Explore Evolution, p. 103
There are a number of problems with this claim. First, not all mutations are associated with a fitness cost relative to the wild type in the original environment, and in those mutations that do reduce fitness with respect to the wild type, subsequent compensatory mutations often restore fitness, sometimes to above wild type levels. Therefore, a large number of mutations can be tolerated.
This "loss of fitness" is always in relation to the wild-type organism in the original environment. In an environment with antibiotics, the wild-type organism's fitness is very low, and the antibiotic resistant mutant is much fitter. Accumulation of mutations may indeed mean that an organism is restricted to the new environment, and cannot survive in the old environment. This is irrelevant. Penguins and polar bears have adapted to cold environments, and cannot survive in the environments their ancestors came from. Our ancestors came from the sea, but we are remarkably unfit in the sense of being able to breath underwater. There are many examples like this. Explore Evolution implies there is some universal standard of fitness, but fitness is always relative to the environment the organism is in.