Friday, March 25, 2011
Transition of Eyes
Beginning at page 154, Neil Shubin talks about how the vertebrates and invertebrates have different kinds of eyes, and that the Polychaetes were the turning point in the differences of eyes in vertebrates and invertebrates. Explain which conditions and characteristics of the Polychaetes might have influenced the change/evolution of eyes from invertebrate eyes to vertebrate ones. John Park (wisejsm@yahoo.com)
Subscribe to:
Post Comments (Atom)
It is hard to differentiate vertebrate and non-vertebrate eyes on first glance, but the major difference would be the orientation of the retina. In vertebrates, the eye is formed by invagination of the optic vesicle, and the retina is reversed with sensory parts (the rods and cones) further pushed outwards, and incoming light passes through all outer layers before reaching the rods and cones. In contrast, invertebrate eyes position the light-sensitive retina on the inside surface facing incoming light, so that light will always strike the retina before other parts of the eye. The differences between these two types of eyes was necessary in natural selection due to potential changes in the environments for vertebrates and invertebrates; for example, while vertebrates possess blind-spots (a spot on the optic disc of the eye that lacks light-detecting photoreceptor cells) and a retina that adjusts pupil size to light exposure,invertebrates has neither (but can, however, adjust lens sizes at lower water depths). Perhaps the environment of vertebrates generally contain more harmful light rays than invertebrates, so vertebrates gradually developed their eye characteristics over time to allow eyes to adjust well to high amounts of light (and the blind-spot was a tradeoff stemming from the reversal of the retina). Conversely, there might have been less penetration in the aquatic environment of invertebrates like squid, so they had to make the trade-off of improved vision (lack of blind-spot) for inability to adjust pupil size to light, resulting in dimmer lighting.
ReplyDeleteIn regards to the second part of the question, Polychaetes (primitive worms) inhabit shallow marine environments such as sandy Southeast Asian beaches and intertidal lagoons, which places their environment right in the center of the spectrum between the deep ocean environment of invertebrates (squid, octopi, cnidarians) and the terrestrial environment of certain vertebrates (mammals), so their exposure to light rays may vary more. Polychaetes most likely were the first to develop qualities of both types of eyes because they could transition from environments of lower to higher light exposure more easily than other organisms, and thus developed vertebrates' photoreceptor cells and invertebrates' blind spot-lacking structure in order to compensate for this environmental situation.
Sources referenced:
1. http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab11/Eye/NOTES/EYENOTE.HTM
2. http://www.d.umn.edu/~olse0176/Evolution/mammals.html
3. http://rmbr.nus.edu.sg/polychaete/Introworms.html
^ Christine Lin
ReplyDeletechoco_cat11@comcast.net
Adding on to what Christine said, another major difference between vertebrate eyes and invertebrate eyes is that vertebrate eyes include a muscled eye ball. Vertebrates also have retinal circuitry (Myers). Another difference between invertebrate and vertebrate eyes is the type of receptors. Invertebrates have rhabdomeric receptors and vertebrates have photoreceptors. Since opsin and color receptors didn't appear until later in vertebrate evolution, it is probably that invertebrates were not able to see in color (Myers).
ReplyDeleteIn addition to what Christine said, many polychaetes are also predators that need to capture prey (1). Therefore, it is likely that they developed more advanced vertebrate eyes so that they can have better vision to help them find food more easily. While thinking about evolution, it is advantageous for vertebrates to have opsin and photoreceptors for color vision. Since polychaetes evolved later than many other invertebrate and have other adavanced features such as a closed circulatory system, it makes sense that they would have more advanced eyes. Since the eyes form as outpocket of the brain, and brains have increased in size through evolution, it makes sense that this would cause eyes to also become more complex (Myers).
Sources:
http://cas.bellarmine.edu/tietjen/images/annelids.htm
http://scienceblogs.com/pharyngula/2007/12/evolution_of_vertebrate_eyes.php
Marissa Lobl (marissa.lobl@gmail.com)