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thumbnail image of human eye life's grand design
green tree frog with bright red eyes
A tree frog's eyes are adapted for night vision.
One improvement at a time  
The pathway by which evolution can produce complex structures has been brilliantly explained in The Blind Watchmaker by Richard Dawkins, a biologist at Oxford University. The essence of Dawkins’s explanation is simple. Given enough time (thousands of years) and material (millions of individuals in a species), many genetic changes will occur that result in slight improvements in a system or structure such as the eye. However slight that improvement, as long as it is genuine, natural selection will favor its spread throughout the species over several generations.  
  head of bird with large, blue-rimmed eye
Birds have highly
developed eyes, in
some ways even
more so than
humans.
  Little by little, one improvement at a time, the system becomes more and more complex, eventually resulting in the fully functioning, well-adapted organ that we call the eye. The retina and the lens did not have to evolve separately because they evolved together.
 
  Evolution can be used as an explanation for complex structures if we can imagine a series of small, intermediate steps leading from the simple to the complex. Further, because natural selection will act on every one of those intermediate steps, no single one can be justified on the basis of the final structure toward which it may be leading. Each step must stand on its own as an improvement that confers an advantage on the organism that possesses it.
   
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