Alan R. Rogers wrote a book The Evidence For Evolution. In this book he has a section on the evolution of the eye, a chapter on the evidence for common descent in support of the evolution of the eye, and some scattered comments about what we know about eye evolution and how we know it. I take up the discussion in Chapter 4. Can Evolution Explain Design in the section A complex adaptation: the eye on page 37.
Rogers begins by describing some of the intricate inter-working of various elements required for proper function and concludes the opening paragraph with the statement that “If any one of these components had failed, you would not be reading.” He opens the next paragraph with a question “How could all this have evolved by natural selection.” He states that “…the eye has always been central to the argument against his [Darwin’s] theory.”
I want to pause here and say that this is the question: “How could all this have evolved by natural selection?” This is important because I want to ask whether Rogers actually ever answers this question. It certainly appears from other statements that he makes in the book that he thinks the matter has been settled. Take for example the following:
“…we understand in considerable detail how eyes were assembled by natural selection.” (p. 49)
“We saw on p. 42 that a complex eye can evolve from an eye spot in about 360,000 years.” (p. 80)
Rogers references two “critics” of Darwin’s theory who objected to Darwin’s claim that natural selection could create the eye, Charles Pritchard and Joseph John Murphy. He characterizes their arguments as being about plausibility. And of such arguments he says:
The nice thing about such arguments is that one can attack them without any data at all, merely by inventing a plausible story about how complex eyes might evolve. The story only needs to be plausible; it does not need to be true. Thus, the first piece of Darwin’s argument was a made-up story about how eyes might have evolved. I often tell my own version to introductory classes.
Rogers then shares his own story, of which he later writes:
This story is of course a fabrication. The cartoons in Figure 4.3 are just that–cartoons. By what right do I claim the story is plausible? Could creatures with eyes like those ever live? The answer to this question is an unambiguous “yes,” for all these sorts of eye can be found in creatures alive today.
Notice the subtle shift from “how could all this have evolved by natural selection” to “could creatures with eyes like those ever live.” Presumably we are being asked to reason that if creatures with eyes like these can live, then it must be the case that all this could have evolved by natural selection. That, of course, is a non-sequitur. The question is not whether creatures like this can ever live, the question is whether evolution took that path from one sort of “eye” to the other and whether that can be attributed to natural selection.
I also note the potential conflation of two distinct arguments. One is a response to a plausibility argument and the other is a claim about what actually happened. I take from this that when examining these sorts of arguments we need to attempt to discern just what it is that the argument is intended to show. Countering the “implausibilty” argument does not answer the question of what actually happened. Rogers acknowledges this:
So far I have argued more or less as Darwin did, and I hope I have convinced you that eyes might plausibly evolve. This is enough to demolish the arguments of Pritchard and Murphy, but it does not tell us whether eyes really did evolve. We cannot answer that question by making up stories. We need real evidence - evidence that Darwin did not have. If eyes did evolve, then closely related species should have similar eyes. Their eyes, in other words, should show traces of common descent.
Notice that we have yet another subtle shift. A shift from the mechanism of evolution to the subject of common descent, as if the truth of common descent can establish the mechanism. Even if common descent is true that does not compel the conclusion that “this all evolved by natural selection.”
I hate posts that are too long and this one is already too long, so I will end here.