directed by:mohamad mehdi shafiei
Types of Eyes
The Simple Eye (3 Main Kinds):
1. The Positive Lens or Camera Eye Found in vertebrates and some invertebrates. This is the type of eye humans have. You can even poke it with your finger right now.
The Simple Eye
2. The Concave Mirror Eye
Found in the clam Pecten and a few ostracod crustaceans. This produces bright but reasonably hazy picture.
3. The Pinhole Eye
Pit or Cup eyes are found mainly in mollusks and can only resolve location of objects.
Made up of ommatidia receptors, each of which functions as a separate visual receptor.
The Apposition Eye
Ommatidia function independently.
The Superposition Eye
Ommatidia cooperate to produce a brighter, superimposed image on the retina.
The Two Kinds of Photoreceptors
All photoreceptors use a light sensitive pigment derived from vitamin A which is bound to an opsin. After being exposing the photopigment to light the opsin binds to a G-protein (common neurotransmitter). These similarities suggest a shared ancestry.
Rhabdomeric Photoreceptors (in protosomes)
Found mainly in the compund eyes of arthropods.
Ciliary Photoreceptors (in deuterosomes)
Common in vertebrates.
Rhabdomeric Photoreceptors
Increase their surface area by throwing up their apical surfaces into numerous folds (think Bart Simpson’s hair). Many morphologies for this receptor exist.
Ciliary Photoreceptor
Increases membrane surface area by modifying the cilium. The ciliary membrane is expanded and thrown into deep folds, so that the actuala receptor region of the cell looks like a stack of discs.
Evidence of Common Receptor Ancestry
Rods and Cones seem to have evolved from common ciliary photoreceptor precursors, while retinal ganglion, amacrine and horizontal cells seem to have evolved from a rhabdomeric precursor.
How Could Eyes Evolve?
Creationists often use the eye as a debate point to show that evolution is flawed, citing that the eye is too complex and perfect to have evolved. Darwin himself noted that “To suppose that the eye… could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree,” (The Origin of Species). He follows, however, with the assertion that eyes could likely evolve from light sensitive neurons. This seems to be the case.
The Evolution of the Eye
Research by Dan-E Nilsson and Susanne Pelger indicates that it is in fact easier to estimate the number of generations necessary to evolve an eye than complex organs. This is because these changes can be viewed as quantitative local modifications to a pre-existing tissue.
In order to determine the number of generations needed to evolve an eye, Nilsson simply made calculations outlining the plausible sequence of alterations leading from a light sensitive spot to a fully developed lens eye.
The Evolution of the Eye
Nilsson assumed an organism with a light sensitive patch of cells resting on a dark pigmented background and placed in a selection for spatial recognition. The first method to create a spatial recognition is either for a depression to form in the center of the patch, or for the edges of the patch to constrict and raise. This cupping would allow for the vague correlation of light to position where the exposure of an area on the patch is dependent on its angle to the light source.
The Evolution of the Eye
This cupping evolution should first favor the formation of a depression in the patch, than the constriction of an aperture via the raising and constriction of the surrounding pigment epithelium. This results in a sunken eye cup that resembles that of some mollusks.
This pinhole-like eye is not very good at resolving detail and creates a very dim image. Because of this, any change that improves clarity and illumination will be favored. The two routes of change for this would be the development of a lens, or the increase in the size of the eye. Increasing the size of the eye, however, presents physical problems and less acute vision than a lens would.
The Simple Eye (3 Main Kinds):
1. The Positive Lens or Camera Eye Found in vertebrates and some invertebrates. This is the type of eye humans have. You can even poke it with your finger right now.
The Simple Eye
2. The Concave Mirror Eye
Found in the clam Pecten and a few ostracod crustaceans. This produces bright but reasonably hazy picture.
3. The Pinhole Eye
Pit or Cup eyes are found mainly in mollusks and can only resolve location of objects.
Made up of ommatidia receptors, each of which functions as a separate visual receptor.
The Apposition Eye
Ommatidia function independently.
The Superposition Eye
Ommatidia cooperate to produce a brighter, superimposed image on the retina.
The Two Kinds of Photoreceptors
All photoreceptors use a light sensitive pigment derived from vitamin A which is bound to an opsin. After being exposing the photopigment to light the opsin binds to a G-protein (common neurotransmitter). These similarities suggest a shared ancestry.
Rhabdomeric Photoreceptors (in protosomes)
Found mainly in the compund eyes of arthropods.
Ciliary Photoreceptors (in deuterosomes)
Common in vertebrates.
Rhabdomeric Photoreceptors
Increase their surface area by throwing up their apical surfaces into numerous folds (think Bart Simpson’s hair). Many morphologies for this receptor exist.
Ciliary Photoreceptor
Increases membrane surface area by modifying the cilium. The ciliary membrane is expanded and thrown into deep folds, so that the actuala receptor region of the cell looks like a stack of discs.
Evidence of Common Receptor Ancestry
Rods and Cones seem to have evolved from common ciliary photoreceptor precursors, while retinal ganglion, amacrine and horizontal cells seem to have evolved from a rhabdomeric precursor.
How Could Eyes Evolve?
Creationists often use the eye as a debate point to show that evolution is flawed, citing that the eye is too complex and perfect to have evolved. Darwin himself noted that “To suppose that the eye… could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree,” (The Origin of Species). He follows, however, with the assertion that eyes could likely evolve from light sensitive neurons. This seems to be the case.
The Evolution of the Eye
Research by Dan-E Nilsson and Susanne Pelger indicates that it is in fact easier to estimate the number of generations necessary to evolve an eye than complex organs. This is because these changes can be viewed as quantitative local modifications to a pre-existing tissue.
In order to determine the number of generations needed to evolve an eye, Nilsson simply made calculations outlining the plausible sequence of alterations leading from a light sensitive spot to a fully developed lens eye.
The Evolution of the Eye
Nilsson assumed an organism with a light sensitive patch of cells resting on a dark pigmented background and placed in a selection for spatial recognition. The first method to create a spatial recognition is either for a depression to form in the center of the patch, or for the edges of the patch to constrict and raise. This cupping would allow for the vague correlation of light to position where the exposure of an area on the patch is dependent on its angle to the light source.
The Evolution of the Eye
This cupping evolution should first favor the formation of a depression in the patch, than the constriction of an aperture via the raising and constriction of the surrounding pigment epithelium. This results in a sunken eye cup that resembles that of some mollusks.
This pinhole-like eye is not very good at resolving detail and creates a very dim image. Because of this, any change that improves clarity and illumination will be favored. The two routes of change for this would be the development of a lens, or the increase in the size of the eye. Increasing the size of the eye, however, presents physical problems and less acute vision than a lens would.
هیچ نظری موجود نیست:
ارسال یک نظر