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Charles Darwin's Blog

  • Evolution of the Eye

    I constantly have people misquoting Darwin on the evolution of the eye.  I get countless people quoting me the following statement:

    "To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree."

    Unfortunately they are taking this quote out of its context.  Now let's look at the quote in its entirety.

    "To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection…"

    It is quite possible for a complex organ such as the vertebrate eye to evolve step by step.  In fact the vertebrate eye is actually wired backwards as Dawkins points in the following excerpt taken from page 93 of his book The Blind Watchmaker.

    "Any engineer would naturally assume that the photocells would point towards the light, with their wires leading backwards towards the brain.  He would laugh at any suggestion that the photocells might point away from the light, with their wires departing on the side nearest the light.  Yet this is exactly what happens in all vertebrate retinas.  Each photocell is, in effect, wired in backwards, with its wire sticking out on the side nearest the light.  The wire has to travel over the surface of the retina, to a point where it dives through a hole in the retina (the so-called 'blind spot' to join the optic nerve."

    If you don't believe your eye has a built in blind spot check the following picture and paragraph I got from: http://serendip.brynmawr.edu/bb/blindspot1.html

    Close your left eye and stare at the cross mark in the diagram with your right eye. Off to the right you should be able to see the spot. Don't LOOK at it; just notice that it is there off to the right (if its not, move farther away from the computer screen; you should be able to see the dot if you're a couple of feet away). Now slowly move toward the computer screen. Keep looking at the cross mark while you move. At a particular distance (probably a foot or so), the spot will disappear (it will reappear again if you move even closer). The spot disappears because it falls on the optic nerve head, the hole in the photoreceptor sheet.

    The other group of animals that has a camera like lens eye are the mollusks like the squids and octopus.  Their eyes are wired the right way.  If there is an intelligent designer why did he give mollusks a superior eye design?  If you believe in the creation of the eye you should feel ripped off that you didn't get a state of the art eye.  You got an eye that works well enough which is what you would expect from natural selection.  Our first vertebrate ancestors had photoreceptors that had the nerve on the surface.  Yes there is some distortion of light as it passes through the nerve but the ability to see far outweighed being blind.  Animals that had more photoreceptors had better vision and could find more food and escape predators better than those that did not offering a huge selective advantage.  The first mollusk eye was wired the right way to begin with so they do not have this flaw in their "design" as we do.  Another interesting fact for you; vertebrate eyes are derived from nerve tissue where as the mollusk eye is derived from skin tissue.  This really doesn't make a difference but it is another convincing piece of evidence that a complex structure like a camera lens eye evolved independently more than once. 

    The following text along with the video come from: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

    ..> ..> ..>..>

    When evolution skeptics want to attack Darwin's theory, they often point to the human eye. How could something so complex, they argue, have developed through random mutations and natural selection, even over millions of years?

    If evolution occurs through gradations, the critics say, how could it have created the separate parts of the eye -- the lens, the retina, the pupil, and so forth -- since none of these structures by themselves would make vision possible? In other words, what good is five percent of an eye?

    Darwin acknowledged from the start that the eye would be a difficult case for his new theory to explain. Difficult, but not impossible. Scientists have come up with scenarios through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes and complexities to form the human eye, with its many parts and astounding abilities.

    Through natural selection, different types of eyes have emerged in evolutionary history -- and the human eye isn't even the best one, from some standpoints. Because blood vessels run across the surface of the retina instead of beneath it, it's easy for the vessels to proliferate or leak and impair vision. So, the evolution theorists say, the anti-evolution argument that life was created by an "intelligent designer" doesn't hold water: If God or some other omnipotent force was responsible for the human eye, it was something of a botched design.

    Biologists use the range of less complex light sensitive structures that exist in living species today to hypothesize the various evolutionary stages eyes may have gone through.

    Here's how some scientists think some eyes may have evolved: The simple light-sensitive spot on the skin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade a predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that made "vision" a little sharper. At the same time, the pit's opening gradually narrowed, so light entered through a small aperture, like a pinhole camera.

    Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitive spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens formed at the front of the eye. It could have arisen as a double-layered transparent tissue containing increasing amounts of liquid that gave it the convex curvature of the human eye.

    In fact, eyes corresponding to every stage in this sequence have been found in existing living species. The existence of this range of less complex light-sensitive structures supports scientists' hypotheses about how complex eyes like ours could evolve. The first animals with anything resembling an eye lived about 550 million years ago. And, according to one scientist's calculations, only 364,000 years would have been needed for a camera-like eye to evolve from a light-sensitive patch.

    10:22 PM

  • Did Darwin Renounce the Theory of Evolution on his Deathbed?

     

     

    I get this question all the time and the answer is on my main page but apparently most people forget how to read when they see lots of pretty pictures so now I will just refer people to this blog.

     

    Darwin did not recant on his deathbed.  That rumor was started after he died by the evangelist Lady Elizabeth Hope.  This bit of Christian propaganda continues to float around to this day as fact when it was just a ruthless lie attacking Darwin when he could no longer fight fictitious rumors against him and his work.

     

    The passage below was taken from:

     

    http://www2.lucidcafe.com/lucidcafe/library/96feb/darwin.html

    (The same place I got Darwin's biography on the front page.)

     

    It has been supposed that Darwin renounced evolution on his deathbed. Shortly after his death, temperance campaigner and evangelist Lady Elizabeth Hope claimed she visited Darwin at his deathbed, and witnessed the renunciation. Her story was printed in a Boston newspaper and subsequently spread. Lady Hope's story was refuted by Darwin's daughter Henrietta who stated, "I was present at his deathbed ... He never recanted any of his scientific views, either then or earlier."

     

    Darwin's daughter did not have an agenda like Lady Hope who was not present at Darwin's deathbed as she so claimed.  Had Darwin actually recanted on his deathbed his daughter would have said so because Darwin's wife was extremely religious and I'm sure she would have told her mother the truth. 

    The following is from Wikipedia: http://en.wikipedia.org/wiki/Elizabeth_Hope

    Lady Elizabeth Reid Hope (1842-1922) was a British evangelist who is generally believed to be the Lady Hope who claimed in 1915 that she had visited the British naturalist Charles Darwin shortly before his death in 1882. Hope claimed that Darwin had recanted his theory of evolution on his deathbed, and accepted Jesus Christ as a savior.

    Charles Darwin's family denied the story, and insisted that Lady Hope "was not present during his last illness, or any illness." The Lady Hope Story is generally recognized, even by many Creationists, to be false or at least unverifiable and if true, probably exaggerated. The story remains a popular urban legend, even though it stands in sharp contrast to Darwin's published and known views about Christianity.

     

    I'm sure many fundamentalist Christians will start posting claims that Darwin recanted with links to Christian web sites.  Feel free to do so but know that you are just spreading a lie breaking one of your 10 commandments in the process.

     

    7:50 PM

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