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paleoanthropology, genetics and evolution

vision

  • Quote: Dalton's colorblindness

    Sat, 2013-01-12 20:59 -- John Hawks

    I'm writing up some text on the evolution of trichromatic vision, which I always present in my intro class. As I'm writing I got curious about the history of how scientists discovered the evolutionary story of trichromacy and have been reading some historical references. Very interesting is a book by Sir William de Wiveleslie Abney, who gave a series of lectures about color vision in 1894 [1]. His description of the discovery of colorblindness as a phenotype in the 18th century is fun, including a passage describing how the chemist John Dalton recognized his own colorblindness and proceeded to rigorously quantify it. Here's an excerpt:

    Mr. Babbage, in Scientific London (1874), gives an account of Dalton's presentation at Court.

    Firstly, he was a Quaker, and would not wear a sword, which is an indispensable appendage to ordinary Court-dress. Secondly, the robe of a Doctor of Civil Laws was known to be objectionable on account of its color - scarlet, being one forbidden by the Quakers. Luckily, it was recollected that Dalton was affected with that peculiar colour blindness which bore his name, and that as cherries and the leaves of a cherry-tree were to him of the same colour, the scarlet gown would present no extraordinary appearance. So perfect evidence was the colour blindness, that the most modest and simple of men, after having received the Doctor's gown at Oxford, actually wore it for several days in happy unconsciousness of the effect he produced in the street.

    References

    1. de Wiveleslie Abney W. Colour vision: being the Tyndall lectures delivered in 1894 at the Royal Institution. London: Wm. Wood & Co.; 1895.
    Tags: 
    trichromacy
    vision

  • Human tetras

    Mon, 2012-06-18 10:04 -- John Hawks

    Ah, I was just writing about the evolution of color vision and this interesting article came across my Twitter (via Ed Yong): "The humans with super human vision".

    What would it be like to see through cDa29’s eyes? Unfortunately, she cannot describe how her color vision compares with ours, any more than we can describe to a dichromatic person what red looks like. “This private perception is what everybody is curious about,” Jordan says. “I would love to see that.” Jordan’s next challenge is discovering why cDa29 is different from the other women she tested. “We now know tetrachromacy exists,” Jordan says. “But we don’t know what allows someone to become functionally tetrachromatic, when most four-coned women aren’t.”

    Potential tetrachromats are female relatives of colorblind males, based on the presence of a fraction of mutant cones. I'd be more interested in knowing about more extreme tetrachromats, who may have allelic dimorphism in any of the normal three beyond the normal trichromatic range.

    Tags: 
    vision

  • Anthropology 105, lecture 7: Eyes

    Sat, 2012-02-25 17:03 -- John Hawks
    Synopsis: 
    Illustrating phylogeny and evolutionary convergence using trichromacy and eye development

    Out of all the lectures in the course, this was one of my favorites to put together. I return to the topic of evolutionary developmental biology, first raised in the "Vertebrae" lecture, by extending from the Hox genes to toolkit genes, focusing on the role of Pax6 in eye development. Again, we see how model organisms like fruit flies and zebrafish are relevant to understanding human biology.

    Then, we zoom closer into the phylogeny of primates, considering the superfamilies and reminding students that New World monkeys, Old World monkeys and hominoids are all anthropoid primates. The anthropoids have a tremendously interesting difference with respect to color vision. Many New World monkey species have trichromacy in some individuals but many remain able only to see two colors. This is because one of the genes that codes for color-detecting pigments has different alleles. Heterozygotes can see three colors, homozygotes can see only two. By contrast, Old World monkeys and hominoids have trichromatic vision by virtue of a gene duplication in our ancestry, which generated two different genes that diverged in sequence to be sensitive to different wavelengths of light.

    The convergence of trichromatic vision reflects its adaptive value in anthropoids, which emerged from diurnal activity pattern, the need to detect young leaves for their protein content and low toxicity, and a coevolution of color vision with mating displays. At the same time, owl monkeys lost two-color vision in parallel with lorises and galagos, in this case reflecting the low adaptive value of color vision in nocturnal primates.

    Last, I discuss the polymorphism of eye color in living humans, which emerges due to the regulation of OCA2 in the surface layers of the iris.

    Study questions: 
    • Would you predict that the common ancestors of New World monkeys, Old World monkeys and hominoids had three-color or two-color vision?
    • Why is two-color vision so often lost in lineages that are active nocturnally?
    • Would it be possible to use zebrafish and fruit flies as models to understand human biology if we did not share common ancestors with these species? Why or why not?
    Study terms: 
    pigmentation
    melanin
    trichromatic
    monochromatic
    dichromatic
    toolkit gene
    Tags: 
    pigmentation
    phylogeny
    primates
    vision
    evo-devo
    development

  • Eye and visual cortex development

    Mon, 2008-11-03 11:56 -- John Hawks

    Neurophilosophy reviews an interesting paper that traces the directional preferences of visual cortex neurons in developing ferrets:

    Now researchers from Duke University Medical Center have observed how early visual experience drives maturation of the visual cortex. Using sophisticated in vivo imaging techniques, they have monitored the changes in the functional properties of visual cortical neurons which occur immediately following eye opening in ferrets. In this way, they show how the first stimuli to enter the eye lead to the emergence of direction selectivity in visually naïve neurons and to the organization of the cells into groups which respond to a preferred direction.

    Tags: 
    development
    brain
    vision
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Neandertals

For years, I've worked on their bones. Now I'm working on their genes. Read more about the science studying these ancient people.

Denisova

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Acceleration

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Malapa

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