Chapter 9: Vision

  1. Humans and Old World monkeys have similar vision – explain the similarity and reasons for it.

Humans and Old World monkeys have similar vision due in major part to the analogous structures of the optical systems. The evolution of vivid color perception beginning with old world primates that is now shared with humans is another major component of the similarities between the two groups. Both humans and the Old World monkeys, which include Gabons, macaques, and baboons, are enabled to have this vivid color vision because of the three unique light receptors that enable the perception of different kinds of light. The majority of mammals have only two light receptors which inhibits color perception. The reason for the ability for humans and the old world monkeys to perceive color is based around the change in plant life. Increased diversity in plant life gave the old world monkeys an evolutionary benefit to being able to perceive different colors in order to differentiate different fruits, berries and such. It is estimated that color vision as we now know it arose approximately 55 million years ago. Evidence of this is the change is seen through the change in fossils in forests with a noticeable transition from bland colored palms and figs to bright fruits and berries.

 

  1. What do eyeless and Pax 6 genes do and where can they be found?

Eyeless genes were originally discovered when Mildred Hoge observed fruit flies that lacked eyes in their entirety. This gene when present in other species can lead to deformities in the eyes of the given species. Deformities in humans due to the eyeless gene can cause conditions such as anaridia. It has been observed that eyelessness in flies , mice and humans all stems from similar sequences of DNA of the eyeless genes. These eyeless genes and the DNA fragment primarily responsible have been studied a great deal. Biologist Walter Gehring was able to use the eyeless gene to active the DNA sequence similar in other areas of the body (ie legs, antenna, wings). In areas of activation of the gene, an eye would grow. This was an astonishing discovery. The analogous structure to eyeless genes in mice, pax 6, showed similar results when activated. Based on these findings it can be determined that eyeless, or Pax 6 is the overarching control gene of eye production in all animals that have eyes.

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4 Comments

  1. Why would there be a gene the limits the development of eyes entirely? I understand not wanting eyes to develop anywhere except in eye sockets, but could it not be possible to simply premanently methylate the eye-making gene after it has been expressed once (where the eyes should be)?

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  2. ngjenny

     /  May 1, 2012

    The development of more advanced ‘levels’ of vision is fascinating. It’s fairly logical to consider how the genes underlying the animal sense of vision are so similar. And yet, outside of evolutionary biology, eyes seem to be so specially developed to each organism; we consider eyes to be aesthetically and culturally divergent among things.

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  3. Max – I doubt that the eye-making gene is expressed only once. I would think that, if that were the control mechanism, the eyes of the organism would be either non-existent or vestigial. I am sure that those proteins are transcribed multiple times in order to stimulate the structural changes accompanying the formation of eyes.

    The origin of eyes, then, is a perfect example of co-evolution between plants and animals. The question I have is, which precipitated which? Did the enhanced color vision of old-world monkeys lead them to select for brightly colored fruits, or did the existence of brightly colored fruits make it advantageous for old-world apes to have color vision? Co-evolution seems to be a common trend throughout evolutionary history.
    Arman

    Reply
  4. Two things: 1) Go here http://www.pbs.org/wgbh/evolution/library/03/4/l_034_04.html and watch the video the clearly illustrates the concepts of common ancestry, and gene transfer. Implied is the idea that different transcription factors (from different tissues OR different species) can act on the same DNA sequence, gene, and get “tinkered”, altered results. In this case the eyeless gene in mice produces mice eyes, but the same gene in fruit flies produces ff eyes. I assume if the ff transcription factors were replaced with mice transcription factors in a ff embryo, it would grow mice eyes, not ff eye.
    2) A popular argument against evolution is that something like the eye is just too complex to happen by chance. As Sean Carrol says in the video referenced above, it’s actually a simpler process than once thought. Each species does not reinvent the eye. They share the one gene needed. Then through the combinations of LOTS of time, accumulation of random mutations among transcription factors and natural selection, diversity arises. ML

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