Living organisms are extremely robust. The analogy to software breaks down even for systems where no evolution is demanded. A single bitflip can stop a software program from functioning properly. (Think of cosmic rays and spacecraft.) A genome, on the other hand, can withstand a lot of changes without a change in function. An accumulation of such changes, when accompanied by a single change high up in the hierarchy of the genetic network will result in a massive change at the expression level. A smooth accumulation of functional changes may happen for one feature, but not for another. Gene sets which code for limbs, eyes, or limbs, for example, can be controlled as modules and the functioning intermediates need never exist. Plus, extensive genetic networks ensure that small changes in one area are diffused over the entire organism.