This post began turning out longer than I intended, so I’m going to divide it in two, with the second part up tomorrow.
When we consider major organs or features of animals, they can be bilaterally symmetrical, with the traits the same on both sides, or bilaterally asymmetrical, with differences between left and right. And there are two major forms of bilateral asymmetry.
In antisymmetry (which can be considered a macroscopic form of “fluctuating asymmetry” see here), there is no directionality to the trait, so the asymmetry is random with respect to the side of the body. One example of this is the lobster claws, in which one becomes a “crusher” claw and the other a “cutter”, as in this individual.
Now this asymmetry is adaptive in the sense that it’s useful for a lobster to have one claw that can crush and another that can cut; it’s like a crustacean Swiss Army knife that can do multiple things. But if you look at lobsters, you’ll find that the crusher claw is on the right as often as it is on the left; in other words, the asymmetry is random in direction among individuals.
This is still an evolved trait, as it’s clearly adaptive to have the two functions, but it doesn’t really matter to the lobster which side does which.
We know how this asymmetry develops—at least proximally. What happens is that the claw that is used most often after the fourth molt develops into the crusher claw, and the other one into the cutter. (I recommend having a look at the link, which details some clever experiments.) This means that there is some developmental program in the lobster’s genes that turns on “crusher” genes in the most stimulated claw, and that, in turn, may activate genes (or repress genes) on the other side of the body leading to the development of the cutting claw. In this case the environment itself, or rather the behavior of the animal interacting with the environment, activates the genetic program, and since it’s apparently random which claw is most stimulated, we get half the lobsters with a crusher on the left, and half with the crusher on the right.
Here’s another example of antisymmetry, the big vs. small claws of the male fiddler crab, Uca deichmanni (the females’ claws are the same size). There are equal proportions of right-clawed and left-clawed males:
Another type of asymmetry is directional, that is, the left and right sides differ, but always in the same direction. We’re familiar with this in our own bodies, in which the heart and viscera are directionally asymmetrical. The bulk of the heart, for instances is on the left side of the body, which is why you feel your heartbeat on that side. Quirks of Human Anatomy gives more examples:
Our right lung has three lobes but our left lung only two. Our heart is shifted to left, our spleen is located on the left, and our stomach bulges to the left, whereas our liver is shifted to the right. Our colon curls into a question mark, although its exact path can vary from person to person.
There are rare individuals in which every directionality like this is reversed due to a condition called situs inversus; these individuals are usually normal, but their innards are mirror images of the much more frequent “normal” individuals.
These kinds of directional asymmetries are not infrequent. Male narwhals, for instance, have a grossly enlarged canine tooth (up to 3 meters long) that forms a tusk, and it’s always on the left side, as shown in this photograph below. (Females don’t usually have tusks, which might imply sexual selection via male-male competition, but it looks as if the tooth/tusk is a sensory organ that males, use to communicate with each other when they rub tusks.)
Another example of directionality are some nocturnal owls in which the ear openings are asymmetrical; this helps them localize prey. In the barn owl, the left ear opening is higher than the right. Here’s another owl, the boreal owl, showing directional asymmetry in the skull:
Flounder species show both forms of asymmetry. In some species of flounders, which begin swimming vertically, they subsequently flatten so that they lie on their left side, with the left eye migrating over the head to the right side, while other species lie on their right side with the eye migrating the other way. These species are directionally asymmetrical, but in opposite ways. Still other species of flounders also flatten, but in a random direction, so some individuals lie on their left sides, and others on their right; this, of course, is antisymmetry.
Like antisymmetry, directional symmetry is often adaptive in that it’s useful to have only one side enlarged, and if you have to enlarge a tooth to make a tusk, it’s got to lead to asymmetry. Here, however, we face a genetic problem: the induction of the tooth on a given side is not due to random environmental stimuli, but is somehow to the genes themselves. There must be a genetic program in narwhals, for example, that says “make left tooth grow large,” regardless of the environment. And that means this: those genes know whether they’re on the right or left side of the body! It’s easier to envision genes knowing whether they’re in the front or back half of the body, as an anterior-posterior gradient is set up in the egg or early zygote. But such gradients aren’t obvious for the right versus left sides of the body in animals that are, by and large, bilaterally symmetrical.
Let me add first that while it may be important to be asymmetrical, as with the lobster, there probably aren’t many cases in which directional asymmetry is more important than antisymmetry (can you think of examples?). In these cases which form of asymmetry evolves may just be a result of whether the genetic variation promoting asymmetry is of the antisymmetric or the directional sort.
When I was younger I pondered this question at length. Yes, you can determine front and back in the egg, and then top vs. bottom (dorsal versus ventral), but, unless there’s some directional left-right gradient set up in the egg (and I wasn’t sure how that would work), I couldn’t see how a gene would know, from its internal environment, which side of the body it was on. (Draw a box with a front-back and a top-bottom chemical gradient; you’ll see that the concentrations of the “morphogen” chemicals are the same on the left and the right.) How, then, I wondered, could directional asymmetry, which must involve genes taking cues from their local environments, ever evolve?
Well, if we start with a single trait being directionally asymmetrical, that would be all that is required for subsequent traits to cue on that, or on each other, to themselves evolve directional asymmetries. And even organisms that look pretty bilaterally symmetrical, like Drosophila, can have subtle directional asymmetry (flies have asymmetrical guts and the male genitalia rotate in a given direction during development.)
But that still leaves a problem: Assuming that organisms evolved from a common ancestor that was completely bilaterally symmetrical (right vs left), how did the very first directional asymmetry evolve? With gradients the same on both sides of the organisms, how could gene variants accumulate that would be activated (or silenced) on a consistent side of the body?
I’ll leave this for readers to ponder. If you’re a biologist, you may already know some of the answers. I’ll discuss some solutions (and some selection experiments) in the next installment.
For more on directional versus antisymmetry, go to Rich Palmer’s website at the University of Alberta.