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Eclectic Reflections
on Biological Asymmetry

(How many ways may symmetry be broken?)

Links to the scientific literature on:

Why asymmetry?

The 'breaking' of symmetry poses major challenges to both developmental and evolutionary biologists:

Minute deviations from bilateral symmetry also provide an attractive tool for quantifying developmental precision: if the genetic underpinning of the right and left sides is the same, then differences between them tell us how accurately the genotype translates into the phenotype.

Small versus large asymmetries: It makes a difference

Departures from bilateral symmetry tend to fall in one of two categories. Some are quite subtle, around 1% of trait size and thus not readily visible, while others are conspicuous. The development and evolution of these two types of asymmetry differs substantially.

Subtle asymmetriesConspicuous asymmetries
The most familiar of subtle asymmetries is fluctuating asymmetry, a term coined by the German biologist Wilhelm Ludwig in his monumental monograph on biological asymmetry (1932). These small, completely random departures from bilateral symmetry provide a surprisingly convenient measure of developmental precision: the more precisely each side develops the greater the symmetry. Rather remarkably, the level of fluctuating asymmetry (FA) appears to vary predictably with many interesting biological phenomena: Conspicuous asymmetries are readily visible to the naked eye, either as asymmetrical structures on otherwise bilaterally symmetrical animals (claws of lobsters & many crabs, narwhal's tusk, ears of owls), or as whole-body asymmetries (snails, flatfish). They fall in one of two categories that differ more profoundly than might be apparent at first glance:

  • Environmental quality
    (FA is higher in poorer quality habitats)
  • Stress
    (stress during development increases FA)
  • Hybridization
    (FA is higher in hybrids between species)
  • Inbreeding
    (inbreeding increases FA)
  • Heterozygosity
    (FA decreases with increasing heterozygosity)
  • Fitness
    (females prefer more symmetrical males)

  • Random asymmetry = Antisymmetry
    (right- and left-handed equally frequent in a species)

  • Fixed asymmetry = Directional asymmetry
    (only right- or only left-handed forms in a species)
Unfortunately because subtle asymmetries are often so small, they are exceedingly difficult to measure reliably. Both measurements and analyses must be done with great care to ensure a valid measure of developmental precision. Not only does measurement error often mask subtle asymmetries, but not all forms of subtle asymmetries reliably signal developmental precision. Extravagant claims about the predictive power of FA have engendered "bemused incredulity" among other scientists. These two different kinds of conspicuous asymmetry may signal very different ways in which symmetry is 'broken' during development. Random asymmetries suggest that symmetry is 'broken' during development by effects of the external environment. Fixed asymmetry, however, suggests that symmetry is 'broken' during development by pre-existing internal asymmetries in biological molecules or the cytoplasm of cell. The evolutionary implications are profound.
Links to the scientific literature: reviews, meta-analyses, disquieting revelations, analytical tools, follies.
Links to analytical tools. 		Links to the scientific literature: general, genetics, chordates, crustacea, molluscs, other taxa

Asymmetries most curious

Perhaps most entertaining of all are the many tales of biological asymmetries
that range from the mundane and idiosyncratic to the truly bizarre.

 
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asymmetrically inclined visitors since Jan. 1, 1997

Original material on this page copyright (c) 1998-2001 by A. Richard Palmer. All rights reserved.
(revised May 6, 2003)