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The Art of Genes: How Organisms Make Themselves
The head has been produced by starting at one end of the object and, at each vertical level, deforming it to the same extent on each side while moving along the horizontal axis towards the other end. In the example shown, the front midline of the face has been left unchanged by the deformations: if you look at the head from the side, the frontal outline is the same as that of the fully radially symmetrical object it was derived from. Our new object now has asymmetries along two major axes: one horizontal and one vertical.
There is also only one plane of reflection symmetry remaining: the one that runs vertically through the midline and divides the object in two. Although it would be reasonable to call this one-fold radial symmetry, I shall use the more common name of bilateral symmetry to refer to this arrangement. Bilateral symmetry, then, is characterised by a single plane of reflection symmetry, and it arises when asymmetries are elaborated along two major axes, in this case a vertical top-bottom and horizontal front-back axis.
There are numerous objects with bilateral symmetry, including chairs, teacups, airplanes or knights in a chess game. To summarise, the symmetry of an object can be defined by the number of ways it can be transformed into itself. The most symmetrical objects, such as spheres, have the highest number of these transformations.
If we impose a single major axis of asymmetry on a sphere, we reduce the number of possible transformations to those of full radial symmetry, such as bottles. The extent of symmetry can be further reduced by imposing asymmetry along several minor axes at right angles to the first, giving lower degrees of radial symmetry five-fold, four-fold, etc. In the case of one-fold radial symmetry, more commonly termed bilateral symmetry, we end up with an object.
In practice, the symmetry class of an object is often related to the way it is used. Spherical symmetry, for example, predominates in objects that are used with little or no constraint from any specific direction, as with soccer balls that roll around in any way we choose. Radial symmetry, on the other hand, is common in cases where constraints operate along one major axis.
Bottles and stools, for instance, are designed to withstand the effects of gravity: bottles prevent liquids being spilt everywhere, and stools stop you from falling to the floor when you sit down. Both of these types of object have radial symmetry, with a single major axis running vertically, parallel to the gravitational force they are designed to oppose.
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By contrast, pouring jugs and chairs have bilateral symmetry, because they are designed to accommodate a second directional constraint in addition to gravity, the shape of a hand that comes from one side to lift them or the shape of a person who wishes his back to be supported while he is seated. One consequence of their matching the shape of the user in this way is that they can be approached from only one direction.
Compare, for example, the stool of fourfold radial symmetry with the bilaterally symmetrical chair shown in Fig. The stool can be sat upon from at least four directions whereas the chair can be comfortably occupied from only one. Just as the symmetry of artifacts is related to their use, so the symmetry of organisms is related to the way they are adapted to their environment. Humans, for example, are to a large extent bilaterally symmetrical in terms of their external appearance. We have a single plane of reflection symmetry running down our midline, as a result of asymmetry along two major axes: the asymmetry from head to foot and front to back.
The asymmetries along these axes reflect two important directional constraints on us: gravity and movement. If we lived in a free-floating world without gravity, standing would become meaningless and we would have lost much of the rationale behind asymmetry along the head-foot axis. Our second asymmetry, front-back, has more to do with movement. To generate a force that moves us in one direction, it helps to be. Many other distinctions between front and back follow on from this. For example, the position of our eyes at the front of our head allows us to see where we are going.
Most importantly, these two major aspects of our lifestyle, movement and gravity, are oriented at right angles to each other. If we normally moved parallel to gravity, only moving up or down like springs, we would have only one major direction to deal with and would no longer need to distinguish between, say, front and back.
Our asymmetries are related to our lifestyle and environment: we move at right angles to the force of gravity. For similar reasons, many other animals display bilateral symmetry. When comparing ourselves with them, however, there can be some confusion because of our upright posture.
Most animals move along the direction of their head-tail axis whereas we walk at right angles to ours. The 'back' of a horse might mean its tail end referring to the way it moves or the part we ride on corresponding to our back. To avoid confusion, biologists use dorsal to refer to our back and the upper part of most animals, and ventral belly to mean our front and the corresponding lower part of most other animals. From now on I shall refer to two major axes in animals as the head-tail axis and the dorsal-ventral axis.
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Humans move parallel to their dorsal-ventral axis whereas most other animals move parallel to their head-tail axis. It is because of our familiarity with bilateral symmetry in the living world that we tend to underestimate the asymmetry that lies behind it. When you take an irregular ink blot and fold the paper in half, you end up with an ordered symmetrical pattern that looks harmonious.
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Yet the original irregularity of the blot has not disappeared, it has simply been balanced by a duplicate image. The single mirror plane relieves tension by giving the blot a symmetry that we are much more familiar with. Not all animals have bilateral symmetry. A large group of soft-bodied aquatic animals, including jellyfish, sea anemones and Hydra the polyp mentioned in the previous chapter , have various degrees of radial symmetry.
Their lifestyle is correspondingly quite different from ours. They either drift with the water currents most of the time or stay stuck to a surface, like a rock or the sea floor. If they do move around, it is usually parallel to their major axis, as with jellyfish slowly swimming by rhythmic contractions of their bells. Hydra has a different solution: it can somersault its way along by bending its main axis round so that its tentacles touch the ground, and then straightening up again. It is perhaps not too surprising that these radially symmetrical animals are found in aquatic habitats, where the reduced effects of gravity are more conducive to these styles of locomotion.
Plant symmetries are also related to their lifestyle and environment. Plants explore their surroundings by growing; growth often plays a role in the lives of plants similar to movement in animals. But unlike animal movement, which is mainly horizontal, a large part of plant growth is oriented vertically, parallel to many of the environmental factors that dominate their lives, such as gravity and light. Many of their asymmetries are therefore elaborated along a single major axis. This is reflected in the typical radial symmetry of the main stems and roots.
Leaves, however, usually grow out more horizontally, roughly at right angles to the direction of the light they harvest. Consequently, leaves tend to be bilaterally symmetrical: their upper surface is usually quite different from the lower and their tip is different from their base. Flowers display a variety of symmetries, depending on how they are pollinated. Buttercups, for example, have five-fold radial symmetry, displaying five identical petals spaced out equally. Like stools, these flowers can be approached from several directions by their visitors.
Other flowers, such as those of Antirrhinum or orchids, are more discriminating, accommodating selected guests more precisely through bilateral symmetry. The upper and lower petals of an Antirrhinum flower have quite a different shape and are united for part of their length to form a tube. The lower petals provide a sort of platform for bees to land on, prise open the flower and enter the tube where the nectar is stored Fig. This ensures that the flowers are only visited by certain types of insect: a bumble-bee is large enough to prise open and enter an Antirrhinum flower but a smaller insect would fail.
The sex organs of the flower, the stamens and carpels, are carefully positioned within the tube to give and receive pollen from the bee's back. The bilateral symmetry of the flowers therefore allows their pollen to be targeted very effectively to a specific insect carrier. Media reviews. Developmental biology has been transformed from a field in which ingenious manipulative experiments generated speculations about unobservable underlying causes, such as gradients and prepatterns, to one in which we have a very detailed knowledge of what is actually going on at the molecular and cellular level.
Enrico Coen has written a book that attempts, with considerable success, to convey the essence of this revolution to the lay reader. It will also be of great interest to those biologists Current promotions. Bestsellers in Evolution.
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