Asymmetry in Biology

absence of or deviation from the regular disposition of similar parts of the body relative to a specific point, axis, or plane. Asymmetry usually arises as the result of a change in conditions, in particular, a change in the direction of gravity; such a change disturbs the symmetry originally laid down in the course of evolution. An example of asymmetry, arising because of the transition from active swimming to resting on the sea floor, is provided by the location of the flounder’s two eyes on the flat side of its body which faces the surface. Asymmetry is encountered, to a greater or lesser degree, in almost all organisms and is sometimes a characteristic trait of a specific species, genus, or family. In man asymmetry can be observed both in the structure of the body and in the location of many internal organs. The asymmetry of the head and face is due to the fact that the left half of the skull is larger than the right half and the left half of the face is longer than the right. Asymmetry of the extremities, which is usually absent at birth, appears during the course of life, and as a result of it most people’s right hand is thicker, longer, and stronger than their left. An example of the asymmetry of internal organs is the location of the aorta to the left of the plane of symmetry and the location of the large veins to the right of it. Pathological asymmetry—for example, a marked enlargement or diminution of the right or left half of the body—can be caused by developmental defects, partial gigantism, or a disturbance in the nutrition of or innervation of a certain part of the body.