This final installment of the response to National Geographic‘s recent evolution article will discuss both Carl Zimmer’s scientific arguments regarding the evolution of the eye, and his theological arguments which he uses to claim the eye was not designed. Before Zimmer discussed “conservation” among genes controlling eye development in widely different types of eyes (reminiscent of common design), he does some blocking by using theological arguments against eye design. Up to this point, Zimmer avoided typical evolutionary icons, but once he started to make the dysteleological argument that the eye is “far from perfect,” he slipped into classical Darwinist iconography.

Zimmer cites 3 lines of evidence which he thinks count against design of the vertebrate eye: (1) our retinas may become detached after “a sharp punch to the head”; (2) light-gathering cells point inward, not outward towards the light; (3) the optic nerve creates a blind spot because it starts in front of the retina before going into the brain. Based on this evidence, Zimmer concludes that “[e]volution, with all its blunders, made the eye.”

Zimmer’s argument is based upon 2 fallacies: (a) that “imperfect” or “suboptimal” design implies no design, and (b) that the eye has an obviously suboptimal design.

(a) Firstly, Zimmer’s claim that an eye is not designed because our retinas may become detached after “a sharp punch to the head” is not a scientific argument: intelligent design doesn’t require “perfection,” nor does it require that a system always survive malicious physical attacks. Was the Ford Pinto, with all its imperfections revealed in crash tests, not designed?

Zimmer thus presents a straw-man argument against intelligent design, based upon his view that a designer must design things to withstand a certain type of malicious physical attack. This is not a scientific objection, but a theological objection. As a scientific theory, intelligent design does not require that systems always survive malicious physical beatings: as a science, ID requires the detection of specified complexity, and the moral purposes of the designer or the “perfection” of the design are irrelevant when determining whether an object was designed. But Carl Zimmer’s personal theological views have no bearing upon the science of intelligent design. A more interesting question is, Why has National Geographic become a mouthpiece for a view of theology that states that a designer must design things to withstand certain types of physical attacks?

(b) Secondly, the design of the eye actually isn’t inefficient, refuting this tired icon. Both the presence of the blind-spot and the inward orientation of light-recepting cells–cited by Zimmer as evidence against “perfection”–are actually the result of an eye design which appears optimized for high visual acuity. George Ayoub explains why vertebrate eye design does not even appear to be suboptimal:

It has been commonly claimed that the vertebrate eye is functionally suboptimal, because photoreceptors in the retina are oriented away from incoming light. However, there are excellent functional reasons for vertebrate photoreceptors to be oriented as they are. Photoreceptor structure and function is maintained by a critical tissue, the retinal pigment epithelium (RPE), which recycles photopigments, removes spent outer segments of the photoreceptors, provides an opaque layer to absorb excess light, and performs additional functions. These aspects of the structure and function of the vertebrate eye have been ignored in evolutionary arguments about suboptimality, yet they are essential for understanding how the eye works.

[…]

[I]ndeed, our thought experiment has taken the vertebrate eye rapidly downhill. In trying to eliminate the blind spot, we have generated a host of new and more severe functional problems to solve. Our “repair” seems far worse than the apparent flaw we wanted to fix.

(George Ayoub, “On the Design of the Vertebrate Retina,” Origins & Design 17:1; read the full article to get an appreciation for the argument)

Carl Zimmer’s dysteleological argument makes inappropriate theological assumptions that a designer must make things to withstand certain physical attacks. The argument also makes inappropriate scientific assumptions which assumes that the presence of the “blind spot” implies suboptimal design. Both assumptions are false. It’s time to put to rest the evolutionary icon of “poor design of the eye’s retina.” Just like the panda’s thumb, the vertebrate eye functions quite well.

Eye – Carumba
Carl Zimmer does make one scientific argument about the eye. His article has a diagram showing various types of eyes, asserting that the gradations of extant eye types imply that step-wise evolution of the eye is possible. This type of diagram is common among ID-critics, and it is almost becoming an icon in-and-of itself. It brings to mind an excellent passage from Thomas Woodward’s new book Darwin Strikes Back: Defending the Science of Intelligent Design:

I once asked the renowned Princeton biologist John Tyler Bonner how he would explain the macroevolution of such complex organs. In response, he directed me to George Gaylord Simpson’s book, The Meaning of Evolution. Simpson’s answer–looking at a suggestive sequence or gradation of different eyes, from simple to complex–was not much advanced on Darwin’s. Do we really know that natural selection can accomplish the drastic morphological transitions between these different types of eyes, with all the knitting and organizing of new complex proteins? (Thomas Woodward, Darwin Strikes Back (Baker, 2006))

Thank you for reading this series of responses. Readers with questions or comments may contact me at cluskin@discovery.org.