Biology Reference
In-Depth Information
transparency when present at high enough concentration to refract light rays This
is another example of a common theme in evolution - one thing leads to another.
Thus a structure can be used for a quite different function from the one for which it
initially evolved - in the section on transitional forms I mentioned how the appear-
ance of feathers for insulation allowed the subsequent development of flight. In the
mirror eye, a concave inner layer of cells reflects light back onto the layer of light-
sensitive cells. The reflecting material is made of crystals of the base guanine - one
of the four bases found in DNA. You will recall from your knowledge of physics
that there are two ways of focussing light used by the manufacturers of microscopes
and telescopes - either a convex lens or a concave mirror. Evolution discovered both
these ways long before humans appeared. Figure 5.3 illustrates some image-forming
eyes.
In the simplest compound eye each light-sensitive cell lies at the base of a tube
of pigment that restrict the light to a narrow angle, that can be as small as two
degrees. Figure 5.3 shows this type of eye found in the sea fans, a group of sessile
marine invertebrates related to jellyfish. A more sophisticated version of the com-
pound eye is called the apposition eye, common in insects such as the dragonfly.
Apposition means “standing side by side” and refers to the multiplicity of identical
light-sensitive units that are not in contact, as they are in the retina of chambered
eyes. Each unit may have its own lens and so forms a tiny image. There are also
superposition eyes, found in some nocturnal insects and deepwater crustaceans,
where many individual lenses co-operate to form a single image. There are even
superposition eyes that use reflecting layers rather than lenses - Figure 5.3 shows
the example of the lobster eye.
Plausible Evolutionary Possibilities
Eyes being soft-bodied structures, it is not surprising that the fossil record of extinct
eyes is so poor. But what we can do is to see if we can arrange the existing eye struc-
tures into a plausible pathway. The term “plausible” here means that the pathway
must include existing examples, and that we must be able to specify the advantage
of each stage over its presumed precursor - a requirement of evolution by natural
selection.
Figure 5.4 illustrates the main stages in a plausible pathway.
We start with an eyespot that expands into a flat patch of light-sensitive cells,
linked to one or more neurons that can generate an electrical impulse when light
falls on the patch. This simple arrangement enables the organism to detect light
intensity but not direction, and this is sufficient to enable it to move towards or
away from the light source. An improvement to this system would be for the patch
to be shaded on one side by cells containing light-blocking pigment granules. If
this shaded patch then deepens into a pit, the intensity of light can be measured in
different directions at the same time. If the chamber gets larger, more light-sensitive
cells are located together, creating a retina. This chamber will initially be filled with
