How the Blind Watchmaker Made Eyes
By DONALD PROTHERO - ESKEPTIC
Added: Thu, 05 Jul 2012 03:47:34 UTC
Since the days of Darwin, eyes and evolution have been an irresistible topic for scientists and amateur authors alike. British biologist St. George Jackson Mivart was initially a supporter of Darwin, but when his Catholic religion caused conflict with Thomas Henry Huxley in 1871, he changed to a critic. Mivart’s critique focused on the issue of the perfection of the human eye and how he could not fathom how it could have evolved by natural selection and random chance (a point still raised by creationists today who know nothing about comparative biology).
In later editions of On the Origin of Species, Darwin specifically addressed Mivart’s criticism and carefully explained how the incipient stages of complex structures like the eye could be useful, and could have evolved by small steps; it did not require a giant leap to the complexity to develop the human eye. As Darwin first showed, nature is full of examples of every kind of photoreceptor, from simple light-sensitive cells to eyespots to simple eyes with no lenses, to a variety of solutions of seeing with more and more complex eyes. Once you arrange these solutions in an array, it is only a small step from one to the next, more complex eye. (Indeed, many animals actually show this transition during their embryonic development as their eyes change, and in some organisms, the eyes develop differently in males and females). In fact, the passages where Darwin talks about the eye are one of the most frequently “quote mined” by creationists trying to distort Darwin’s meaning, because they quote only the beginning of the paragraph where Darwin is setting up the creationist position in order to shoot it down the in the rest of the passage (which creationists never quote). Here is the first section that creationists quote (On the Origin of Species, 6th ed., 1872, 143–144):
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.
Here is the rest of the quote that creationists conveniently leave out:
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 any 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, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound.
The rest of Darwin’s chapter goes into great depth describing the full range of photoreceptor solutions in the animal kingdom, which creationists also conveniently fail to address.
Fast-forward 153 years later to the culmination of this line of argument, represented by Ivan Schwab’s outstanding book Evolution’s Witness: How Eyes Evolved.
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