Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry (are asymmetric).
In nature and biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half.
BILATERAL SYMMETRY
In bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves (with respect to external appearance only, see situs solitus). Thus there is approximate reflection symmetry. Often the two halves can meaningfully be referred to as the right and left halves, e.g. in the case of an animal with a main direction of motion in the plane of symmetry.
Animals
Most animals are bilaterally symmetric, including humans (see also facial symmetry), and belong to the group Bilateria. The oldest known bilateral animal is the Vernanimalcula. Most bilateral animals have an identical shape on either side, as if bisected by a mirror.
Bilateral symmetry permits streamlining, favors the formation of a central nerve center, contributes to cephalization, and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and vertebrates.
Bilateral symmetry is not easily broken. In experiments using the fruit fly, Drosophila, in contrast of other traits (where laboratory selection experiments always yield a change), right or left-sidedness in eye size, or eye facet number, wing-folding behavior (left over right) show a lack of response. [2]
Plants
Flowers such as members of the orchid and pea families are bilaterally symmetrical (also known as zygomorphic). The leaves of most plants are also superficially bilaterally symmetrical. A careful examination of leaf vein patterns often shows imperfect bilateral symmetry. Also, the pattern of leaves on a branch or stem may often show glide symmetry, with left, right alternation, rather than perfect bilateral symmetry. Cases where otherwise bilateral plant organs are transformed into seemingly helical shapes are known under the term helical growth.
