Biology Reference
In-Depth Information
Fig. 5.4
water, creating the danger of clogging by detritus and parasitic organisms. These
dangers are averted if the pit opening becomes covered by a transparent layer of
skin. If the aperture of the pit reduces in diameter, the resolution of light direction
improves, but the amount of light that is admitted is also reduced, which would be
a disadvantage in low-light environments.
This problem is solved if a lens forms in the transparent jelly that now occupies
the chamber. This could result from the splitting of the transparent layer over the
eye opening into two layers. The outer layer forms the cornea and the inner layer
the lens. The formation of a lens creates two large advantages - more light can be
admitted, and it can be focussed onto the retina so that a detailed image is possible.
The formation of a cornea both protects the lens and assists in refracting light rays,
while the formation of an expandable iris permits the eye to operate under a wide
range of light intensities.
How long might it take for such a pathway to evolve, and is there any way we can
observe some of the changes in real time today? Biologists have constructed a model
for the evolution of the chambered eye that attempts to answer these questions. This
model assumes that each mutation causes a change of 1% in some character relevant
to eye formation - this might be the size of the light-sensitive patch or the concen-
tration of a particular protein. On this basis, the number of sequential mutations
required to form a chambered eye from a light-sensitive patch is 1829. Applying
population genetic equations to this number of mutations gives a figure of nearly
Search WWH ::




Custom Search