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From a simple patch
of light-sensitive
cells, a structure
like an eye can
develop.
View in
QuickTime |
RealPlayer |
Building
an eye |
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| 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. |
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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.
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The chambered
nautilus has
eyes that are
relatively
simple, yet they
still can sharply
focus light. |
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