Q. How can a common organ such as the eye be used to trace evolution?
-The
main function of our eyes is to gather light and focus the light onto a retina.
The retina consists of special cells called rods and cones. These rods and
cones are specialized light absorbing cells, that respond to a certain form of
light and send nerve impulses to the brain. The brain in turn interprets the
pattern of nerve signals into we an image.
- One can trace the evolutionary history of
an eye from simple light gathering cells on the surface of a planaria to the highly intricate eyes of an octopus. However the
molecules that actually absorb the light energy and covert it in to an electrical
impulse, which can start a nerve transmission are called opsins. Opsins are
found in majority of the organisms; humans, insects, clams, and scallops all
use opsins.
- Evolutionary
principles can be used to make predictions. One would predict that organisms
that are more closely related to humans would have similar vision systems. Which
in fact is true. Humans have three genes producing opsin, thus giving us a a
large spectrum of color vision. This is indeed the case, with the majority of
mammals have two genes for color vision and the higher primates having three.
- There
is an intricate relationship among eyes in the animal kingdom. There is a
noticeable difference between vertebrate ‘camera-style’ eyes and invertebrate ‘compound’
eyes. Until 2001, this might have been considered an unbridgeable gap and many
prominent biologists assumed that eyes developed twice in the history of
multi-celled life on the Earth.
- But in 2001, Detlev Arendt found that a polychaete worm had the basics both kinds of vision systems. The
worm has a ‘normal’ invertebrate eye with connections to the neurons and opsins
but, underneath the skin, were tiny photoreceptors with vertebrate opsins and
tiny bristle like projections that matches basic rods and cones. This animal has
the precursors to both vision systems in use by the animal kingdom.
- Research
on fruit flies produced the discovery of a mutated gene that causes a reduction
or the elimination of eyes in fruit flies. The eyeless gene was unique. By turning on this gene in cells in
various places in the fruit fly, research could grown an eye there. Whole
generations of fruit flies were born with eyes on their legs and abdomen and
mouths and wings. Some of these eyes, could even respond to light and
considering they aren’t actually connected to the nervous system.
- Another
gene was discovered in mice, Pax6. It’s the mouse equivalent of eyeless. With some creative genetic
engineering, they found that Pax6 could be used to grow an eye in fruit
flies (again, anywhere the researchers wanted), but it was still a fruit fly
eye. These two genes, one in a fly and one in a
mouse, were so similar that either of them can trigger the complex
developmental cascade that results in the formation of an eye.
No comments:
Post a Comment