Practical Morphometrics of Bilateral Symmetry and Heatmap

Description

Bilateral symmetry is a type of symmetry in which the arrangement of the body parts such that a single plane, passing between the upper and lower surfaces and through the longitudinal axis of an organism into right and left mirror images (e.g. human).  Asides bilateral symmetry, other types of symmetry exist such as rotational symmetry spiral or helical symmetry, biradial symmetry, translational symmetry, etc.

In the study and observation of bilateral symmetric structures in morphometrics, two methods have been distinguished: matching symmetry and object symmetry. When symmetry is studied by matching the pairs, such as in the case of fly wings, the structure of interest is present in two separate copies as mirror images on the left and right side of the body. In contrast, in object symmetry, the structure is already symmetric because of its internal plane or line of symmetry (the midline in two-dimensional representations), resulting in the left and right halves being mirror images of one another, as in the case of a human head.

Simply said, asymmetry is the absence or lack of symmetry. Three different forms of asymmetry patterns have historically been identified in nature, depending on the distribution of left-right disparities in a population: directional asymmetry, fluctuating asymmetry, and antisymmetry.

When a character develops more fully on one side of the plane or planes of symmetry than on the other, this is referred to as directional asymmetry (DA) between two morphological structures (Valen, 1962). The average values of a trait that differ systematically between the two sides can be used to identify DA. The difference between the left and right sides may be tiny or enormous, but the mean left-right difference is not zero (Klingenberg, 2015). Animal and human organs like the liver and lungs, which are regularly asymmetric, are examples of DA

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