Directional asymmetry: how does it develop and how did it evolve? Part 3. Artificial selection for handedness

February 9, 2017 • 1:04 pm

In the first part of this series, I discussed examples of asymmetry—both directional asymmetry (right-versus left-handedness) and  anti-symmetry (differences between sides, but in a random direction)—and raised the problem of how directional asymmetry, like the enlarged left tusk of the male narwhal or the higher left ear of the barn owl, could evolve. In other words, how would a gene know whether it was on the left side of an organism or the right?

In yesterday’s installment, I discussed two recent pieces of research, based on the directional movement of cilia and the asymmetrical operation of proteins, that could produce a directional gradient in a bilaterally symmetrical organism and thus lead to the evolution of handedness in a trait.

The potential difficulty of a gene somehow “knowing” it was on the right versus left side of an organism that’s bilaterally symmetrical led me to this question:

One question that occupied me when I was younger was this: if you take an organism that is, by and large, bilaterally symmetrical, like Drosophila (though there is a bit of handedness in a couple of its traits), could you impose artificial selection on it to produce handedness? That is, could you select for a line of flies whose right eyes were bigger than their left, or who had more bristles on their left side than on their right (and vice versa in both cases)? How hard would that be? Given the absence of marked bilateral asymmetries in species like Drosophila that could act as developmental cues for the successful selection of directional asymmetry, you might think it would be hard—even though virtually every other trait in Drosophila can be successfully changed by artificial selection. Tomorrow we’ll learn the answer to my question.

Today I’ll give the answer, which is that yes, it’s very hard to select for directional asymmetry in organisms. Drosophila, for example, are pretty bilaterally symmetrical, though there are some slight differences in morphology on the right versus left sides that are directional. I’d predict, then, that it would be hard to select for a line of flies that would be directionally different: say, one that had the right eye always bigger than the left, or had more bristles on the left side than on the right.  Or, at least harder to do that than to select for other traits that are obviously variable in populations, like a simple increase in the number of bristles, or the ability to move more toward the light than the dark.

In fact, of all the artificial selection experiments I know about in Drosophila, the only ones that have ever failed are those selecting for directional asymmetry. You can increase antisymmetry by selection fairly easily: that is, you can make flies more asymmetrical for traits like eye size and bristle number, but not directionally so.

The paper by Ashley Carter et al. given at the bottom gives a history of selection experiments in Drosophila for directional asymmetry, and then adds a new experiment (this was published in 2009).  Here are the experiments preceding that of Carter et al.; they are a dismal history of failures:

  • In 1960,  Maynard Smith and Sondhi published an experiment in which they tried to select for directional difference in the number of ocellar bristles (there are usually three, one anterior and two posterior, both on top of the head). They showed no significant increase in the directionality.
  • In 1965, Beardmore selected for directional asymmetry in the number of sternopleural bristles, which are located on the sides of the fly (below).  The two lines that were selected in opposite directions (more bristles on right vs. more bristles on left) diverged very slightly but significantly over 50 generations (a long time for selection). The selection response, however, was tiny compared to other experiments that selected simply for an increase or decrease in total bristle number regardless of the side.
Sternopleural bristles (in the red segment)
  • In 1973, Purnell and Thompson selected for directional asymmetry of wing folding. A given fly always folds its left wing over its right, or vice versa. But different flies have different folding directions, so this is a case of antisymmetry. These workers selected for a line of flies in which left folded over right consistently, and vice versa. While the authors claimed a modest directional response, Carter et al. say that there was in fact no difference achieved in either line.
  • In 1987, I published a paper in which I placed the eyeless mutant of Drosophila into a genetically variable line. This mutant makes the eyes very small, and often asymmetrical. I then selected for lines having the left eye bigger than the right and vice versa. As a check on the general presence of genetic variation, I also selected for reduced eyes on both sides of the head. The last experiment succeeded greatly, showing there was genetic variation for eye size, but both experiments selecting for directionality failed: there was no sign, after 30 generations, that I had produced a line with eyes consistently bigger on one versus the other side of the head.  Here’s some of the variation in the expression of the eyeless mutation:
Variation in the expression of the eyeless mutation in Drosophila
  • Finally, in 1990 Tuinstra et al. selected for a directional difference between left and right bristle numbers on the scutellum in a line containing a mutation that destabilizes bristle number. (There are usually four scutellar bristles, as shown below). After 12 generations they saw no response to selection for directionality, but the line was also depauperate in general genetic variation.
The normal phenotype: four scutellar bristles.

In the experiment of Carter et al., the authors selected for directionality in the distance between the posterior crossvein of each wing from the tip, trying to create lines in which the distance was larger on the left than on the right—and vice versa. Here’s a diagram of the trait they selected for: the average of the distances from the intersection of the posterior crossvein with the longitudinal veins on either side to the tip of the wing; in other words, the average of two distances shown in the dark black lines in the bottom figure.


Carter et al. practiced 15 generations of selection for right wing distances bigger than left, and in the opposite direction as well. There was no significant response. Again, selection for directional asymmetry (which could have reflected either a difference in either the vein configuration or the general size of the wing) failed.

So out of these six experiments for directional asymmetry in Drosophila, only one succeeded, and that was a very modest success.

Why the failure? The authors suggest three possibilities:

1). There is no left-right axis of asymmetry that could allow genes to cue on whether they’re on the left or right; therefore there could be no variant genes that could produce directional asymmetry. The authors reject this hypothesis because flies do show some directional asymmetry in their guts, genitals, and a very small amount in wing size.

2). The amount of genetic variation that exists for directional asymmetry—that is, genes that can recognize what side of the body they’re on given that some slight directional asymmetry already exists—is small. Given this lack of available genetic variation, selection would often be unsuccessful. This would be my favored hypothesis given the pervasive bilateral symmetry in Drosophila. There just aren’t many asymmetries for genes to cue in on.

3). It’s easier to evolve directional asymmetry if the trait is initially antisymmetrical, with one side different from the other, though in a random direction. This would already show that the trait is sensitive to developmental differences on the sides (though not directionally)—and perhaps that sensitivity could be leveraged into a directional response. This seems unlikely to me (and the authors) because at least two experiments (mine and Tuinstra’s) created big antisymmetry by destabilizing a trait via mutation. Despite that increase in antisymmetry, selection for directional asymmetry was unsuccessful in both cases.

This lack of success is in stark contrast to the pervasive success of other selection experiments in flies. I know of no failures of selection for other traits that don’t involve directional asymmetry, though of course some unsuccessful selection experiments might not have been published.

To conclude, then, we do see directional asymmetries in some organisms, so selection has, in some species, picked out genes that distinguish right from left. But in organisms like Drosophila that are bilaterally symmetrical on the whole, it’s hard to produce flies that are right-handed or left-handed for some traits. Carter et al., and I, suggest that this is because there are very few genetic variants that can take advantage of the very slight directional asymmetries that pre-exist in Drosophila.  This, in turn, would suggest that such selection might be more successful in species having more pronounced directional asymmetries to begin with—organisms like humans. But of course we can’t select on our own species, and even in mammals the long generation time (and lack of interest in directional asymmetry!) makes such experiments impractical. Nevertheless, I still find directional asymmetry fascinating, simply because I want to know how a gene can tell whether it’s on the right versus left side of the body.


Carter, A. J. R., E. Osborne, and D. Houle. 2009. Heritability of directional asymmetry in Drosophila melanogaster . International Journal of Evolutionary Biology 2009:7.

30 thoughts on “Directional asymmetry: how does it develop and how did it evolve? Part 3. Artificial selection for handedness

  1. Just a note in proof…

    “Drosophila, for example, are pretty bilaterally asymmetrical, though there are some slight differences in morphology on the right versus left sides that are directional.”

    I think you meant “Drosophila, for example, are pretty bilaterally symmetrical

  2. Gentle reminder! Today, 9 February, is Darwin’s birthday. It is also Abraham Lincoln’s birthday.



  3. and even in mammals the long generation time (and lack of interest in directional asymmetry!) makes such experiments impractical

    What about mice? You’d need five to six years for 30 generations, right? Perhaps not something to take lightly, but hardly out of the ordinary for lab experiments, no?

    After all, we’ve already had the Russian experiments on fox neonatality that revealed the behavioral connection….



  4. All’s well then, all I can think of is natural selection would not allow for to much messing with symmetries as we would (oganisms) end up being freeks and evolutionary dead ends. At least that’s what comes to mind, a low tolorence and a tight reign until proven to be of some advantage. So for certain traits, little variation for science fellows to play with.
    Great series… thanks NS.. and Prof (E)

  5. Question from the lobby. Do we know what takes place genetically to product right vs. left handed. Another thought, in the heart we have bicuspid and tricuspid values and normally we get the correct type in the correct place but sometimes we do not. Since I was one who got the wrong type aortic value, was wondering about that as well.

    1. I’m no expert, but I read somewhere (article in neuropsychology of some kind) that right handers are almost invariably left-brained (in the relevant sense) but left-handers are much more variable. So there is a “second order” asymmetry. I know this isn’t exactly an answer, but I find that datum interesting.

    1. [Chases Haggi into the distance. Mr Haggis-Botherer circulates the same hill, equally frustrated, but turnnwise instead of widdershins. No true Scotsman can stomach a widdershins haggis.]

  6. My favorite story of animal handedness concerns toads. Apparently toads show a marked preference for using their right forelimb when eating large food items or removing foreign items from their mouth or heads. Why?
    The suggestion was made that it’s related to their emetic behavior: when they vomit, they do so in such extreme fashion that their stomach actually everts and protrudes, inside-out, from the mouth. Because of the curvature of the digestive system, only the right ‘hand’ is useful in shoving the stomach back in and down!

    But then it turned out they also have hindlimb preferences, so who knowws.

  7. Left-handedness runs on one side of my family, and only in the females. There’s something there, but perhaps not something significant?

    1. It is horribly difficult to separate “culture” (in the non-Petri dish sense) from genetics in hoomins. As a left-hander who slipped past the “caned for using left hand” generation by only a few years, it is poignant that you have proper-handedness on the distaff side. Obviously they (and I) am are horrible deviants who are just a few notches down Trump’s list from the Mexicanos. Nice to know that I’ll be up against the wall (paid for by American’s ; billed to Mexico ;invoice returned) in company.
      What’s that Lassie? It won’t stop at lefties- against righties- ? Orangeness of skin might enter the xenophobia at some point? Or smallness of hands.

      1. As we say up here, Maintiens le droit. Sinister souls such as ourselves may someday once again get our left arms tied behind our backs in order to learn to write the right way and keep us from Satanic possession.

        Perhaps we can prevent American terrorists from infiltrating our Great White North by getting Trump to build a wall for us. Then it will be only be necessary to Maintiens le mur.

  8. Symmetry to anti-symmetry easy, to directional asymmetry hard.
    Fascinating, makes one wonder how easy or difficult it is to convert from directional to anti (I’m thinking e.g. of snails) or even to symmetry.

  9. Very interesting. I also agree with reason #2 as to the difficulty to get much directional asymmetry in Drosophila. But good efforts all around. One might do better with a species that has more asymmetry to begin with.

  10. Thanks for this three-part series on natural and artificial selection for asymmetrical morphology. I should have been more familiar with the literature, but much of this was new to me.
    All very useful to enrich my future teaching.

  11. This has been a great series. I never even thought before about the difficulties of distinguishing left from right from an evolutionary perspective. These posts have been eye-opening.

  12. Excellent stuff, we may comment more on the politics/atheism posts but it’s the science ones that are the killer app for the website.

    One slight quibble, I wouldn’t describe the earlier experiments as “failures”. Trying to do something in a certain way and not getting the result you hoped for is still successful in the sense of learning about the world.

    I mention this only because the idea that not identifying a difference in something is a “failure” is a contributory factor to (non-) publication bias.

    Its good to know that the results of the “failed” experiments were indeed published.

  13. A fascinating and enlightening trilogy. I found it very difficult to think of how one might achieve the final task you set. However, I was sure it was because of my lack of grasp on the subject, and was very interested to find out how it could be done. So I may have had a better grasp than I thought.

  14. Dear PCC(E) – perhaps this reveals how little I read, but it might be interesting if you posed some exam-style questions to this series.

  15. I just wanted to say “Interesting article”. I had to wait until the weekend to read all three, but I did it! I very much appreciate the science content: More work for you to write, more work for me to read, but worth it.

  16. “I want to know how a gene can tell whether it’s on the right versus left side of the body”

    As a layman I dont follow this, can you elaborate? I am sure you dont mean to say that there are actually genes on a certain side of the body. You probably mean genes that act on a particular side. IS that correct?

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