Eyes from Distal vs. proximal and Don't Forget

From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Fri Apr 27 2001 - 13:05:21 EDT

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    >Can anyone seriously doubt that the eye in its many manifestations was designed?

    The problem of defining design has already been raised. Biblically, everything is part of God's plan and thus designed in that sense. However, despite its popularity the eye seems to me a relatively poor choice as an example of likely miraculous intervention, as the intermediate stages are functional and useful.

    The simplest eye is simply a spot of light-sensitive pigment and a response mechanism (e.g., move if it is too bright or dark), found in many protists. This ability is obviously advantageous to photosythetic organisms, and can help any organism avoid overly strong solar radiation, detect passing shadows, or determine approximate time of day. Multicellular organisms may have more complex mechanisms. Some animals have single photosensitive cells associated with particular neurons. Increasing the complexity of the eye can improve its function for these purposes (e.g., a lens increasing the light-gathering ability; a cup-shaped eye enabling direction determination). It also may provide the opportunity for new functions, such as formation of images. Many eye proteins have known functions (or closely related proteins of known function) unrelated to vision, so the molecular raw material was largely in place.

    The presence of similar genes involved in eye formation throughout the Bilateria has two possible evolutionary explanations. On the one hand, the ancestral bilaterian could have already had simple eyes, which were the basis of subsequent evolution. On the other hand, the gene could have been doing some other function in the ancestor but was particularly well-suited for exaptation to vision-related functions. The apparently independent use of closely related proteins for the structure of the lens in cephalopods and vertebrates would be an example of this. Eye evolution might thus be better regarded as parallel evolution than convergent evolution, but to claim that this single example is a threat to the concept of convergent evolution, much less to the concept of biological evolution as a whole, is absurd hyperbole.

    >By the way, how would I access Medline?


    It has a wide variety of biomedical references available either as abstracts or complete references, though strongly emphasizing English-language sources.

        Dr. David Campbell
        "Old Seashells"
        Biology Department
        Saint Mary's College of Maryland
        18952 E. Fisher Road
        St. Mary's City, MD 20686-3001 USA
        dcampbell@osprey.smcm.edu, 301 862-0372 Fax: 301 862-0996
    "Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!

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