The term "contingent" is sometimes used to describe this sort of explanation. The idea is that evolution's designs are not a consequence of necessity or good design, but rather the vagaries of historical accidents. Far from answering the question, this explanation simply raises even more questions and problems.
One problem is that this doctrine that biological variation, from which natural selection does its choosing, is blind and independent of fitness, though long a staple of evolutionary theory, is false.
Another problem is that strikingly similar designs, which could not be due to a common ancestor, are common. Biological designs are clearly not contingent on a capricious process of historical accidents.
Yet another problem is that the contingency explanation for evolutionary designs cuts both ways. For if this is the explanation for why eyes did not evolve in the back of the head, we could then just as easily ask, why then did eyes evolve in the front of the head?
It is all one big tautology. Eyes did not evolve in the back of the head because they did not accidentally arise there. On the other hand, eyes did evolve in the front of the head because they did accidentally arise there. This reminds me of a debate I was in where the evolutionist explained that the purpose of science is to explain nature, and that evolution is good science because it explains biology.
Given the absurdities it is not surprising that Olshansky changes gears. Instead of "it all depends on what happens to arise accidentally," Olshansky retools the explanation, this time with selection doing the heavy lifting:
Although light-sensitive cells are likely to have appeared on different parts of early forms of life, selection seems to favor those that enable creatures to detect light in the direction they are headed rather than the direction from which they came. Forward locomotion probably was a driving force for the current location of light-sensitive cells.
In the space of a few paragraphs, Olshansky has completely reversed himself. I guess we should think of it as a menu of explanations from which to choose your favorite. You may have contingency or you may have necessity. You can limit biological variation, leaving selection with little flexibility, or you can expand the powers of variation and use selection to winnow back the many choices. Evolution is not merely one tautology--it has multiple tautologies.
Another problem is that if biological variation is blind, then how do nature's intricate designs arise? Olshansky glosses over this problem, assuring the reader that:
The first light-sensitive cell most certainly arose through random mutation among the earliest multicellular creatures.
In fact, even the simplest light-sensitive cells in nature are phenomenally complex. The idea that they "arose through random mutation" is "certainly" not motivated by the science. And that is only the beginning. Evolutionists such as Olshansky forget that light sensitivity, even if it could magically arise on its own, would do the creature no good without a host of concomitant capabilities to take advantage of the windfall. For the newly available sensory data must be processed, transmitted, and ultimately integrated into the creature's cognitive processes. Even primitive versions of these requirements render evolution silly.