Symmetry (biology)
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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. 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.
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[edit] Radial symmetry
These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only.
[edit] Animals
Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as starfish (see special forms of radial symmetry). Animals in the phyla cnidaria and echinodermata exhibit radial symmetry. Cnidarians are one of the simplest life forms on this planet.
[edit] Plants
Many flowers are radially symmetric (also known as actinomorphic). Roughly identical petals, sepals, and stamen occur at regular intervals around the center of the flower.
[edit] Special forms of radial symmetry
[edit] Tetramerism
Many jellyfish have four radial canals and thus exhibit tetramerous radial symmetry.
[edit] Pentamerism
This variant of radial symmetry (also called pentaradial and pentagonal symmetry) arranges roughly equal parts around a central axis at orientations of 72° apart.
- Animals
Members of the phyla echinodermata (like starfish) arrange parts around the axis of the mouth in five equal sectors. The radiolarians demonstrate a remarkable array of pentamerism forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae).
- Plants
Flowering plants demonstrate symmetry of five more frequently than any other form.
Around 1510–1516 A.D., Leonardo da Vinci determined that in many plants a sixth leaf stands above the first. This arrangement later became known as 2/5 phyllotaxy, a system where repetitions of five leaves occur in two turns of the axis. This is the most common of all patterns of leaf arrangement.
[edit] Hexamerism and Octamerism
Corals and sea anemones (class Anthozoa) are divided into two groups based on their symmetry. The most common corals in the subclass Hexacorallia have a hexameric body plan; their polyps have six-fold internal symmetry and the number of their tentacles is a multiple of six.
Corals belonging to the subclass Octocorallia have polyps with eight tentacles and octameric radial symmetry.
[edit] 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.
[edit] 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.
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.
[edit] 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 bilaterally symmetrical.
[edit] Asymmetry
The notable exception among animals are the Porifera (sea sponges) which have no symmetry.
[edit] Evolution of asymmetry
Features of the symmetry are determined by the environment. Maximal extent of organism symmetry corresponds to a completely isotropic ecological niche.
First organisms on Earth floating in the depths of water (unicellular and lower multicellular organisms) have the maximum possible spherical symmetry. They appeared approximately 3.5 billon years ago. Asymmetrization along the “top – bottom” axis occurred under the influence of gravity. This led to the appearance of the attached, low-mobility forms (plants and coelenterates) that had radial symmetry.
Asymmetrization along the “front – back” axis occurred due to the interaction with space, when rapid motion was required (to escape from the predator, or to chase a prey). As a result, the main receptors and the brain were moved to the front of the body. Organisms with bilateral symmetry were dominating last 650-800 million years. These are crustaceans, fish as well as the most progressive forms, i.e., mammals, birds, and insects.
V. N. Beklemishev (1964) distinguished three types of symmetry (spherical, radial, and bilateral) and arranged them into evolutionary array. Forth type of triaxial asymmetry he assigned to a primitive organism (amoeba) and placed at the beginning of the array. Organisms of bilateral symmetry he considered the “crown” of evolution.[1] Based on the growing number of facts of laterality found in modern progressive forms (functional asymmetry of a brain, right handedness in humans, unilateral ovulation and unihemispheric sleep of dolphins) V. Geodakyan proposed in his theory of Asymmetrization of Organisms, Brain and Body that the evolution of the organisms consistently goes from symmetry to asymmetry.[2][3]
Each transition changes one axis from symmetry to asymmetry with triaxial asymmetry placed at the end of the array (spherical → radial → bilateral → triaxial). According to V. Geodakyan the organisms of triaxial and not bilateral asymmetry should be considered the most evolutionary advanced type.
The trend towards asymmetrization can be followed in phylogeny of plant organs (flower, leaf, fruits, and seeds). It is known, that zygomorphic (bilateral symmetry) flowers [Gladiolus sp., Orchids, Eyebrights and Violets] are evolutionary more progressive, than actinomorphic (radial symmetry) flowers [Primula, Narcissus, Pyrola], but are less progressive, than triaxial asymmetric ones [Cannaceae and Valerianaceae]. The morphology of a leaf during evolution follows the same picture: spherical symmetry of chlorella, radial symmetry of pine needles, bilateral symmetry of Magnolia leafs, and triaxial asymmetry of Begonia or Elms leafs. The same trend can be found in embryogenesis—spherical zygote, radial gastrula, bilateral embryo and triaxial asymmetric child.
It was unknown what creates triaxial asymmetry. According to a new theory asymmetrization along the “left – right” axis is a consequence of asynchronous evolution and occurs in time.
[edit] See also
[edit] References
- ^ Beklemishev V. N. (1944) Osnovi sravnitel’noi anatomii bespozvonochnykh (The Foundations of Comparative Anatomy of. Invertebrates). Moscow, Sovetskaya Nauka. 490 p.
- ^ Geodakyan V. A. (1992) Evolutionary Logic of the Functional Asymmetry of the Brain. “Doklady Biological Sciences” 324 N 1-6, 283–287.
- ^ Geodakyan V. A., Geodakyan K. V. (1997) A New Concept on Lefthandedness. “Doklady Biological Sciences” 356 450-454.
- Fact Monster
- Heads, Michael. "Principia Botanica: Croizat's Contribution to Botany." Tuatara 27.1 (1984): 26-48.
- Zoology a website by the Monaco educational service
[edit] External links
- Live Science.com article called "Symmetry in Nature: Fundamental Fact or Human Bias?" By Ker Than
- Evolutionary Theories of Asymmetrization of Organisms, Brain and Body
