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thumbnail image of human eye life's grand design
drawing of a simple, cup-shaped eye
From a simple patch
of light-sensitive
cells, a structure
like an eye can
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Building an eye  
This step-by-step criterion can easily be applied to a complex organ like the eye. We begin with the simplest possible case: a small animal with a few light-sensitive cells. We could then ask, at each stage, whether natural selection would favor the incremental changes that are shown, knowing that if it would not, the final structure could not have evolved, no matter how beneficial. Starting with the simplest light-sensing device, a single photoreceptor cell, it is possible to draw a series of incremental changes that would lead directly to the lens-and-retina eye. None of the intermediate stages are unreasonable, since each requires nothing more than an incremental change in structure: an increase in cell number, a change in surface curvature, a slight increase in transparency.  

This incremental process is the real reason why it is unfair to characterize evolution as mere chance. Chance plays a role in presenting random genetic variations. But natural selection, which is not random, determines which variations will become fixed in the species.

Critics might ask what good that first tiny step, perhaps only five percent of an eye, might be. As the saying goes, in the land of the blind the one-eyed man is king. Likewise, in a population with limited ability to sense light, every improvement in vision, no matter how slight, would be favored -- and favored dramatically -- by natural selection.

chambered nautilus
The chambered
nautilus has
eyes that are
simple, yet they
still can sharply
focus light.
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