Posted by: elenadbutler | September 29, 2008

Symmetry in Evolutionary Biology

As a rare Applied Math/Organismic & Evolutionary Biology concentrator, I thought I would dedicate my post to examples of (a)symmetry found in nature and biology. In my travels through the internet, I learned some cool stuff, which I will share with you now. Most of this information can be found in a really interesting (and short!) article, “The science of symmetry in biology,” by Michal Polak and Robert Trivers, which was published in Trends in Ecology & Evolution in 1994.

In biology, there are three kinds of bilateral asymmetry (bilateral symmetry is the kind humans have–we can be split down the middle into two approximately congruent sides). First, there’s directional asymmetry, which has a normal distribution that is shifted away from zero. One example of this kind of asymmetry are handedness in humans (most of us are right-handed).

Secondly, there’s antisymmetry, when symmetry is uncommon but there is no directional pattern in the asymmetry. A cool example of this is male fiddler crabs, which have one giant claw, which may be either left or right.

Another kind of asymmetry has normal distribution around zero; this kind is called “fluctuating asymmetry.” Fluctuating asymmetry (FA) is often used to explain disruptions in development that come from a non-ideal environment or genetic problems. In humans, FA is used to model the distribution of Down’s syndrome, inbreeding, and maternal alcoholism. In pigtailed macaques (ED: a kind of monkey), repeatedly catching and releasing the mother around the time of pregnancy increases FA in her offspring.

Since it is optimal for organisms to be perfectly symmetrical (symmetry maximizes surface area to volume ratios and the functionality of locomotors, to name two reasons it’s beneficial), FA is associated with lower fitness. There seems to be sexual selection for individuals with a high degree of symmetry. It is also possible that there is sexual selection toward male characteristics that do a good job of showing off a male’s symmetry. Generally, sexually-selected traits (like colorful feathers or horns) show 5-10 times more FA than non sexually selected traits.

Parasites also play an important role in organisms’ symmetry and their chances of being sexually selected for. High parasite levels correlate with high levels asymmetry; the latter indicates to the opposite sex that the host may be a bad mate choice. Another unfavorable characteristic, genetic homozygosity, which leaves organisms more vulnerable to disease, is linked to FA. This phenomenon has been studied in rainbow trout.  Inbreeding may also lead to increased FA in some species, such as cheetahs, who have higher FAs than leopards, perhaps because the cheetahs are more inbred.

I really enjoyed this article because it summarized how biological deviations from perfect symmetry can affect an organism’s influence on its population’s gene pool. I had also never thought about why perfect symmetry is optimal for living things. Imagine how much harder it would be to walk or fly without symmetrical limbs!

I’m still trying to figure out why traits that are selected for sexually would demonstrate a higher rate of FA. I would think that since males that are passing on their genes tend to be more symmetrical in those traits, those traits would achieve perfect symmetry. I’m also confused about how handedness could have a normal distribution, since I think of handedness as a pretty black and white situation. I mean, when you ask people which hand they write with, they almost always say “left” or “right,” not “70% right, 30% left.” Do you think the researchers were measuring how dexterous people were with each of their hands? That’s the only way I can imagine getting a continuous distribution.

If you have any thoughts on these topics, please comment!

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Responses

  1. Small correction: pigtailed macaques are monkeys, not birds.

    A wonderful book which touches on some of these things is Dawkins’ “The Ancestor’s Tale” which is a tour through evolution (he stays away from haranguing about anything). One thing he talks about is non-bilateral symmetry, such as that seen in starfish, and how it may have evolved.

    As to some of the your questions: I imagine aboslutely perfect symmetry is not desireable, because besides evolving organisms, we’ve also evolved a system of evolution. We have a certain mutation rate which is optimal for our species (and so does each other species) and so we always have some diversity. If we didn’t, we would be too vulnerable to sudden extinction (variations in fitness insure against everyone having the same fatal flaw), and we would evolve and adapt more slowly. Even for sexual selection, I’m guessing something similar holds. Also, I don’t know whether sexual selection or environmental pressures is a stronger selection pressure most of the time. It must vary a lot.

    Thanks for all the interesting information!


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