Editing Irreducible complexity (section)

=== Eye ===
{{Main|Evolution of the eye}}
[[File:Stages in the evolution of the eye.png|thumb|300px|Stages in the evolution of the eye<br />(a) A pigment spot<br />(b) A simple pigment cup<br />(c) The simple optic cup found in [[abalone]]<br />(d) The complex lensed eye of the marine snail and the octopus]]
The [[eye]] is frequently cited by intelligent design and creationism advocates as a purported example of irreducible complexity. Behe used the "development of the eye problem" as evidence for intelligent design in ''Darwin's Black Box''. Although Behe acknowledged that the evolution of the larger anatomical features of the eye have been well-explained, he pointed out that the complexity of the minute biochemical reactions required at a molecular level for light sensitivity still defies explanation. Creationist [[Jonathan Sarfati]] has described the eye as evolutionary biologists' "greatest challenge as an example of superb 'irreducible complexity' in God's creation", specifically pointing to the supposed "vast complexity" required for transparency.<ref name="aig">[[Jonathan Sarfati|Sarfati, Jonathan]] (2000). [http://www.answersingenesis.org/home/area/re2/chapter10.asp Argument: 'Irreducible complexity'] {{webarchive|url=https://web.archive.org/web/20051123070148/http://www.answersingenesis.org/home/area/re2/chapter10.asp|date=2005-11-23}}, from ''[[Refuting Evolution]]'' ([[Answers in Genesis]]).</ref>{{failed verification|redirects to http://www.answersingenesis.org/get-answers/topic/design-features, an unauthored blog|date=June 2012}}{{primary source inline|date=September 2020}}

In an often misquoted<ref>{{cite web|url=http://www.talkorigins.org/indexcc/CA/CA113_1.html|title=CA113.1: Evolution of the eye.|first=Mark|last=Isaak|website=www.talkorigins.org|access-date=7 May 2018|url-status=live|archive-url=https://web.archive.org/web/20171003062117/http://www.talkorigins.org/indexcc/CA/CA113_1.html|archive-date=3 October 2017}}</ref> passage from ''[[On the Origin of Species]]'', [[Charles Darwin]] appears to acknowledge the eye's development as a difficulty for his theory. However, the quote in context shows that Darwin actually had a very good understanding of the evolution of the eye (see [[fallacy of quoting out of context]]). He notes that "to suppose that the eye ... could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree". Yet this observation was merely a [[procatalepsis|rhetorical device]] for Darwin. He goes on to explain that if gradual evolution of the eye could be shown to be possible, "the difficulty of believing that a perfect and complex eye could be formed by natural selection ... can hardly be considered real". He then proceeded to roughly map out a likely course for evolution using examples of gradually more complex eyes of various species.<ref>[[Charles Darwin|Darwin, Charles]] (1859). ''[[The Origin of Species|On the Origin of Species]]''. London: John Murray. [http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=204 pages 186ff, Chapter VI] {{webarchive|url=https://web.archive.org/web/20070927190830/http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=204 |date=2007-09-27 }}</ref>

[[File:Evolution eye.svg|thumb|left|200px|The eyes of vertebrates (left) and invertebrates such as the [[octopus]] (right) developed independently: vertebrates evolved an inverted [[retina]] with a [[blind spot (vision)|blind spot]] over their [[optic disc]], whereas octopuses avoided this with a non-inverted retina. (1 photo-receptors, 2 neural tissue, 3 optic nerve, 4 blind spot)]]

Since Darwin's day, the eye's ancestry has become much better understood. Although learning about the construction of ancient eyes through fossil evidence is problematic due to the soft tissues leaving no imprint or remains, genetic and comparative anatomical evidence has increasingly supported the idea of a common ancestry for all eyes.<ref>{{cite journal |vauthors=Halder G, Callaerts P, Gehring WJ |title=New perspectives on eye evolution |journal=Current Opinion in Genetics & Development |volume=5 |issue=5 |pages=602–9 |date=October 1995 |pmid=8664548 |doi=10.1016/0959-437X(95)80029-8 }}</ref><ref>{{cite journal |vauthors=Halder G, Callaerts P, Gehring WJ |title=Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila |journal=Science |volume=267 |issue=5205 |pages=1788–92 |date=March 1995 |pmid=7892602 |doi=10.1126/science.7892602 |bibcode=1995Sci...267.1788H|s2cid=9646449 }}</ref><ref>{{cite journal |vauthors=Tomarev SI, Callaerts P, Kos L, etal |title=Squid Pax-6 and eye development |journal=Proc Natl Acad Sci USA |volume=94 |issue=6 |pages=2421–6 |date=March 1997 |pmid=9122210 |pmc=20103 |doi=10.1073/pnas.94.6.2421 |bibcode=1997PNAS...94.2421T|doi-access=free }}</ref>

Current evidence does suggest possible evolutionary lineages for the origins of the anatomical features of the eye. One likely chain of development is that the eyes originated as simple patches of [[photoreceptor cell]]s that could detect the presence or absence of light, but not its direction. When, via random mutation across the population, the photosensitive cells happened to have developed on a small depression, it endowed the organism with a better sense of the light's source. This small change gave the organism an advantage over those without the mutation. This genetic trait would then be "selected for" as those with the trait would have an increased chance of survival, and therefore progeny, over those without the trait. Individuals with deeper depressions would be able to discern changes in light over a wider field than those individuals with shallower depressions. As ever deeper depressions were advantageous to the organism, gradually, this depression would become a pit into which light would strike certain cells depending on its angle. The organism slowly gained increasingly precise visual information. And again, this gradual process continued as individuals having a slightly shrunken [[aperture]] of the eye had an advantage over those without the mutation as an aperture increases how [[collimated]] the light is at any one specific group of photoreceptors. As this trait developed, the eye became effectively a [[pinhole camera]] which allowed the organism to dimly make out shapes—the [[nautilus]] is a modern example of an animal with such an eye. Finally, via this same selection process, a protective layer of transparent cells over the aperture was differentiated into a crude [[lens (anatomy)|lens]], and the interior of the eye was filled with humours to assist in focusing images.<ref>Fernald, Russell D. (2001). [http://www.karger.com/gazette/64/fernald/art_1_1.htm The Evolution of Eyes: Why Do We See What We See?] {{webarchive|url=https://web.archive.org/web/20060319045945/http://www.karger.com/gazette/64/fernald/art_1_1.htm |date=2006-03-19 }} ''Karger Gazette'' 64: "The Eye in Focus".</ref><ref>{{cite book |author=Fernald, RD |chapter=Aquatic Adaptations in Fish Eyes |editor=Atema, J |title=Sensory biology of aquatic animals |publisher=Springer-Verlag |location=Berlin |year=1988 |isbn=978-0-387-96373-0}}</ref><ref>{{cite journal |author=Fernald, RD |title=The evolution of eyes |journal=Brain Behav. Evol. |volume=50 |issue=4 |pages=253–9 |year=1997 |pmid=9310200 |doi=10.1159/000113339|s2cid=46796856 }}</ref> In this way, eyes are recognized by modern biologists as actually a relatively unambiguous and simple structure to evolve, and many of the major developments of the eye's evolution are believed to have taken place over only a few million years, during the [[Cambrian explosion]].<ref>{{cite book |author=Conway-Morris, S |title=The Crucible of Creation: The Burgess Shale and the Rise of Animals |publisher=Oxford University Press |location=Oxford [Oxfordshire] |year=1999 |isbn=978-0-19-286202-0 |url=https://archive.org/details/crucibleofcreati00conw }}</ref> Behe asserts that this is only an explanation of the gross anatomical steps, however, and not an explanation of the changes in discrete biochemical systems that would have needed to take place.<ref>{{Cite book|title=Darwin's Black Box|last=Behe|first=Michael|publisher=Free Press|year=2006|isbn=978-0-7432-9031-9|pages=38}}</ref>

Behe maintains that the complexity of light sensitivity at the molecular level and the minute biochemical reactions required for those first "simple patches of photoreceptor[s]" still defies explanation, and that the proposed series of infinitesimal steps to get from patches of photoreceptors to a fully functional eye would actually be considered great, complex leaps in evolution if viewed on the molecular scale. Other intelligent design proponents claim that the evolution of the entire visual system would be difficult rather than the eye alone.<ref>{{cite book|title=A Meaningful World|url=https://archive.org/details/meaningfulworldh00wike|url-access=limited|year=2006|first1=Benjamin |last1=Wiker |first2=Jonathan |last2=Witt |page=[https://archive.org/details/meaningfulworldh00wike/page/n41 44]|publisher=InterVarsity Press |isbn=978-0-8308-2799-2}}</ref>