You probably don’t look like your dog.

Hypothetical mechanisms of trichromatic vision evolution in primates

-OR-

Why Rainbows are Awesome.

OK, some people do look like their dog.

What we mean is, you don’t SEE like your dog. Most placental mammals have dichromatic vision—what we would call ‘colorblind.’

“3-color vision”

• Our vision works like a computer monitor in reverse.
• Our eyes receive (rather than emit) three wavelengths of light: red, green and blue (RGB).
• There is a single receptor for each wavelength
• 1 receptor =1 gene.
• All 3 receptors(RGB)= 3 genes.
• The combination of these three colors is what defines our ‘rainbow’.

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Computer monitor through a magnifying lens

Mixing colors of light to make new color perceptions

“2-color vision”

• ALL placental mammals had only 2-color receptors at some point
• Now, most humans have 3-color vision.
• How did that happen?

RGB monitor using three colors of light to creating the image of an eye on a face. What would happen if we removed one color entirely?

What does the world look like ??

2-color vision

3-color vision

Sex-Linked Inheritance.

• Women have 2 “X” chromosomes, XX
• Men have an X and a Y chromosome, XY.
• Each parent contributes one chromosome to the child
• This is why it is said that men ‘determine’ the sex of the child.

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Color-vision Genes are on the X Chromosome, not the Y.

• X** Y
• X** X**
• X*** Y
• X*** X***
• X** X***
• Y Y

• There are either 2 (X**) or 3(X***) of them on the X chromosome.
• There are always none (Y) on the Y chromosome.
• You can have 6 different combinations:

• Male , 2-color vision
• Female, 2-color vision
• Male, 3-color vision
• Female, 3-color vision
• Female, 3-color vision
• Trick question! non-viable

“Would you like to play a game?”�-Joshua the Computer, from the 1983 Film ‘War Games’

• You are not an Old World Monkey but you will track one in this simulation. In fact, you will track several.
• We will simulate several generations.
• Your primates will live, love, die, and live again as a new primate in each successive generation.

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Generation Zero

1. Grab a ‘Generation Zero’ genotype on a post-it.
• This is a mostly stable 2-color troop- but there are a few 3-color immigrants from another troop.
2. Pair/share with a partner, and decide if your primate is:
• Male, 2-color vision
• Female, 2-color vision
• Male, 3-color vision or
• Female, 3-color vision
3. LATER, after we make Generation 1, you will put these back on the graph, in the appropriate columns.

Making Generation 1: A Shumit and Amelia tutorial.

This is Shumit.

He has a female 2-color vision primate

He also has a penny.

Lucky Shumit!

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Shumit Labels his Chromosomes!

• One Heads, one Tails!
• Then, he attaches a ‘Generation 1’ post-it to the bottom. He is careful with the color!
• He is ready to reproduce!

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First he must find someone with a Male Primate

• Remember, Shumit has a female!
• Amelia has a Male Primate!
• She has also labeled her Heads/Tails correctly, and has her ‘Generation 1’ post-it on there.
• This is a good sign!

Now they need to flip coins!

• They decide to make Shumit’s progeny first.
• He flips his penny and gets heads.
• He moves the X** chromosome to his Generation 1 primate!

Now Amelia flips a coin!

• She also gets Heads!
• She moves her X*** chromosome to Shumit’s Generation 1 primate.
• It’s a Girl!!
• She has 3-color vision.

“But Wait!” says Amelia.

• “I also need a primate!”
• “Right!” says Shumit
• They both flip again
• This time they both get tails!
• They move their chromosomes to Amelia’s gen 1 primate
• It’s a Boy!
• He has 2-color vision.

Keep that Data!

• Amelia & Shumit put their old Generation Zero primates in the correct columns!
• When these guys grow up, they will be ready to make the next generation!

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Generation 1:�

• Make Generation 1 now!
• Put your Generation 0 post-it back on the poster when you are finished.
• Hold on to your new Generation 1 primate, but DON’T MAKE THE NEXT GENEATION JUST YET.

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Generation 2:�

• Do exactly the same as Generation 1
• You may need to mix and match in mating. You don’t need to stay with the same partner every time!
• STOP when you have created your Generation 2 primate. DO NOT go on to the next generation.

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Before Generation #3

• Now for some…..

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SELECTIVE PRESSURE!

Why does it matter? Because Green Eggs and Ham.

This is a simulation of deuteranopia. Can you tell which of these meats is the most well done?

Why does it matter?

With trichromatic vision, the answer is obvious (#4).

MEAT�

• If you chose #4: Good for you! Very well done meat. You survive.

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• If you chose #1 or #3: Boy, you did get sick. But you got through it. You survive.

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• If you chose #2: Sorry. Parasitic Trichinella worms.

YOU MEAT YOUR MAKER!

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Through the Bottleneck.

• If your primate expired this round, PUT IT BACK ON THE DATA CHART.
• If your primate survived, you can begin to reproduce! You may need to reproduce a little extra this round, because…..
• If your primate did expire, get a new post-it anyway, and find some primates that survived…
• They can flip their coins and fill out your new post-it for you!

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Which fruit is ripe?

For Trichromats Only….

FRUIT�

• If you chose D: Well, they were edible, but not quite ripe. You ate enough to still have offspring though. You survive.

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• If you chose B: Nicely done! Extra Food! Have as many kids as you want. You survive.

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• If you chose A or C: Whoops. Those are poisonous when they aren’t ripe.

YOU BUY THE FARM.

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Pick a frog to eat (or kiss, if you are vegetarian).

Warning coloration!

FROGS

• If you chose C or B: Safe. You survive.

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• If you chose A or D: They were poisonous. So……

YOU CROAK.

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• Make Generation 5.

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Example outcome:

Science Talk:

• PAIR-SHARE: Find an elbow partner and discuss these questions:
• Did you change your mating behavior during the simulation? When and why?
• Looking at the graphs, where do you see stability and change in the proportions of vision type?
• Do we see any difference in the changes between male and female primates? If so, can we say something about stability and change in terms of gender?

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• Bonus question:.
• What CAUSED the shift from 2-color to 3-color vision? Is it a simple or complex answer?

Bonus Slides

• A little more about the science of color vision

FAQ: “How real were the simulations?”

• For the meat, primates don’t cook roasts.
• For the frogs (or snakes, or other poisonous creatures) warning coloration certainly matters- but it’s not clear if that drove the evolution of more complex color vision or vice versa. Interesting fact: placental mammals in Australia have 3-color vision, a continent where everything is purported to be poisonous. Coincidence?
• For the fruit, this one is based on actual research, although the researchers believe reddish edible foliage was also a driver, along with actual fruit.

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Colorblindness

• As the activity illustrated, humans typically have 3 types of cones, that are tuned to three different wavelengths of light.

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• Colorblindness results when one (or multiple) of those cone types malfunctions or isn’t present.

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• It is far more common in men, because the genes for the cone types are carried on the x chromosome, so men have no back up. (Worldwide, 8% of men, and .5% of women have some type of color vision deficiency)

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Types of color blindness

• Deuteranopia/Deuteranomaly – the green-sensitive cone is absent or malfunctioning. (6% of XY population)

• Protanopia/Protanomaly – the red-sensitive cone is absent or malfunctioning (2% of XY population)

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• Tritanopia/Tritanomaly – the blue-sensitive cone is absent or malfunctioning. (>.00002 of gen. population)

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• Monochromacy- only one type of cone functions and/or is present. (Very rare)

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• Achromatopsia – no cones work, only rods. People with this type perceive no color, and have difficulty with brightness of daylight. (Very rare)

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All the colorblindness simulations…..

Normal Color Vision

Deuteranopia

Protanopia

Tritanopia

And then, there’s ‘anomalous trichromacy’…

• People with anomalous trichromacy have three functioning cones types, but one of their cones is tuned to a slightly different frequency of light than the rest of the population.

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• The color vision deficiency it produces is usually slight; it doesn’t significantly interfere with a persons ability to distinguish colors and its often discovered by accident.

YOUR AVERAGE BEAR

ANOMALOUS TRICHROMAT

Tetrachromacy (4-color vision) is a result of �anomalous trichromacy

• Tetrachromats have four different cone types, instead of the usual 3. People with this mutation may see MILLIONS more colors than you or I do.

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• Its estimated that around 12% of women may have the genes for four distinct cone types; however, vision researcher Jay Neitz speculates that perhaps only 1-2% of women possess two of the same cone that are significantly different enough to produce a real change in color vision.

How does it work?

• Remember, genes for cones are on the X chromosome.
• Let’s say Juliette is the daughter of Mike and Linda;

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Mike is an anomolous trichromat:

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Linda is a normal trichromat:

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Juliette gets four distinctly different cones,

making her a tetrachromat:

What is it like?

Normal Color Vision

Tetrachromacy?

It’s impossible to truly understand, but known tetrachromats can give us a glimpse of what they see.

Painting by Concetta Antico, tetrochromat and artist