Is the Eye Irreducibly Complex?

Among the original writings on intelligent design are those of William Paley, who laid the foundation for intelligent design and specifically spoke of the eye in his book, Natural Theology, back in 1802 (follow the link for an online edition of the book). I suggest reading the first three chapters of Paley's book. Chapters I and II create his "argument," and chapter III applies the "argument" to the eye, among other things. This same logic is embraced by the intelligent design movement today, and William Paley is their Charles Darwin. Contrast Paley's logic with that of Karl Popper, especially the part on The Growth of Human Knowledge

The standard example of the irreducibly complex eye has been the human eye, or the vertebrate eye. We'll refer to this as the "camera eye," which is also observed in other phyla, e.g. cephalopod molluscs, box jellyfish, and possibly dinoflagellates as "eye organelles" (see Pennisi 2002 and Gehring 2005 for illustrations of a dinoflagellate "eye organelles"). The hypothesis that the "camera eye" (i.e. an eye that includes a lens) is irreducibly complex and that  it is impossible for it  to have evolved by Darwinian evolution (e.g. link) can be falsified by two counterexamples: 1. a theoretical model that demonstrates that it is possible theoretically for the camera eye to have evolved by Darwinian evolution; and, 2. empirical observations that species exist with less than complete "camera eyes;" i.e. with a subset of the components of the most advanced eyes. Of course, the more interesting question to evolutionary biology is how did the "camera eye" evolve, not could the "camera eye" evolve? 

Regarding models of eye evolution, see Nilsson and Pelger (1994). For examples of reducibly complex eyes, check out this video clip from the PBS Evolution series, on Nilsson's body of work, which appears here in both QuickTime and Real Player formats (link); the flatworm has "cup eyes," the chambered nautilus has "pinhole camera eyes," and the squid and the octopus have "camera eyes." For a novel hypothesis for eye evolution, with a lot of examples of reducibly complex eyes, check out Gehring (2005). This hypothesis basically begins with photosynthetic cyanobacteria and three rounds of endosymbiosis, first with cyanobacteria as primary choloplasts in red algae, second with red algae as secondary choloplasts in dinoflagellates, which later become eye organelles, and finally transferred to cnidaria via endosymbiotic dinoflagellates (Gehring 2005).

Here are links to the Lund Vision Research Group (Nilsson's institution), and their research themes

References:

Gehring WJ. 2005. New perspectives on eye development and the evolution of eyes and photoreceptors. J Hered. 2005 May-Jun;96(3):171-84. Epub 2005 Jan 13. (PubMed, Heredity, pdf)

WALTER J. GEHRING 2002. The genetic control of eye development and its implications for the evolution of the various eye-types. Int. J. Dev. Biol. 46: 65-73. (pdf, PubMed)

Thorsten Hadrys, Rob DeSalle, Sven Sagasser, Nina Fischer and Bernd Schierwater. The Trichoplax PaxB Gene: A Putative Proto-PaxA/B/C Gene Predating the Origin of Nerve and Sensory Cells. Mol Biol Evol. 2005 Jul;22(7):1569-78. Epub 2005 Apr 27. (abstract, PubMed )

Nordstrom K, Wallen R, Seymour J, Nilsson D. A simple visual system without neurons in jellyfish larvae. Proc R Soc Lond B Biol Sci. 2003 Nov 22;270(1531):2349-54. (Journal, PubMed)

Nilsson DE , Gislen L, Coates MM, Skogh C, Garm A. 2005. Advanced optics in a jellyfish eye.  Nature. 2005 May 2;435(7039):201-5. (PubMed, Nature)

Nilsson D.-E. & Pelger S. (1994) A pessimistic estimate of the time required for an eye to evolve. Proc R Soc Lond B 256: 53-58. Available online in JSTOR archives, (also check this link).

Todd H. Oakley 2003. The eye as a replicating and diverging, modular developmental unit. Trends in Ecology and Evolution. 18(12):623-627. (pdf)

Elizabeth Pennisi 2002. News Focus, EVOLUTION OF DEVELOPMENTAL DIVERSITY: Evo-Devo Devotees Eye Ocular Origins and More. Science, Vol 296, Issue 5570, 1010-1011 , 10 May 2002 (Science)

Schierwater B. My favorite animal, Trichoplax adhaerens. Bioessays. 2005 Dec;27(12):1294-1302. (BioEssays, PubMed)

Trilobite Eyes

Some thought questions

  1. What is the logic behind hypothesis testing and falsification in science? (Hint: consult the Stanford University Encyclopedia of Philosophy on Karl Popper, especially the part on The Growth of Human Knowledge). 

  2. How does this logic differ from Paley's and that of modern proponents of intelligent design? Does intelligent design offer any testable hypotheses? What predictions do they make?

  3. What would you have to know in order to build a testable theory of intelligent design?

  4. Do a Google search on "God of the Gaps." What is this argument, and how is it relevant to the logic of intelligent design?

  5. Do a PubMed search on Pax genes and eyes, and read several abstracts. How phylogenetically broad is the involvement of Pax genes in eye development? What else do Pax genes do?