Now the argument goes that disadvantageous mutations do not matter in the process of evolution, because they are eliminated again by selection. The decisive aspect is said to be, that mutations were advantageous from time to time. These mutations would are assumed to have prevailed within the populations and the process could be continued by further positive mutations.
This argumentation is basically correct. Still the occurring of advantageous mutations is not equivalent to the development of new genetic information, but normally also goes back to defects, which turn out to be positive under certain environmental factors.
An example shall help to illustrate this: Some of the insects that live on islands exposed to strong winds have vestigial wings or even none at all (pic. 58). For the indigenous insects this is an advantageous change, because a violent storm can blow them far out to the open sea while flying. If the wind does not turn, the insects mostly will not have the energy to fly back by their own and will die. So in this case it is better if the insects are unable to fly from the beginning. Also, the loss of motility is bearable, because there normally are fewer natural enemies on the islands than onshore. So all in all, the loss of wings is advantageous. Alone, it will not do any good to the understanding of evolution, because the advantage is based on a loss, that means the reduction of an organ – no upward development. Moreover, the loss of flight is positive only under specific circumstances; it is disadvantageous elsewise and such mutants are eliminated by selection immediately.
The following conditions are known from biochemistry. If by means of mutation an enzyme can metabolize a new substance (new substrate affinity), this normally is at the expense of the (former) specificity of the substrate affinity: The exact coordination between enzyme and substrate gets lost. Newly acquired toxitolerance or antibiotic resistances can be based on metabolic defects that prevent the toxin from being infiltrated into essential metabolic processes.
We can state that mutations that are only “positive” under certain circumstances do not explain an evolution of new structures, because they imply losses. So it is not the question whether or not advantageous mutations exist, but rather if new genetic material and new structures develop.