What Are Vestigial Structures and Organs?

Biologically, vestigiality refers to organisms retaining organs and structures that lost their original function in a species during the process of evolution,

“A true vestigial organ is indeed an organ that has lost its original structure. It has also lost its function. Individuals possessing a vestigial organ don’t differ in fitness from those without it. The organ is simply a leftover” (1).

Vestigiality occurs when an organ loses its value in a changing environment. Although the feature may be selected against when its function becomes harmful, if the feature’s presence provides no disadvantage then it is possible that it may not be weeded out by natural selection and therefore persist across species. 

Various examples of vestigiality are cited such as the hindlimbs of whales, legless lizards, and snakes, and the loss of functional wings in island-dwelling birds. The blind mole rat has tiny eyes completely covered by a layer of skin. In The Origin of Species (1872 [1859), Charles Darwin noted vestigial structures and identified a list that included wisdom teeth, the appendix, muscles of the ear, the tailbone, body hair, and the semilunar fold in the corner of the eye. Darwin recognized that these structures are replete across the animal kingdom stating that “it would be impossible to name one of the higher animals in which some part or other is not in a rudimentary condition” (2).

The eyes of mole rats, spiders, crayfish, beetles, and certain salamanders and cavefish are vestigial since they no longer allow the organism to see and are the debris of their ancestors’ functional eyes. The wings of ostriches and other flightless birds like the kiwis and cassowaries are vestigial. Similarly, the nonfunctional hind wings in beetles such as carabus solieri are vestigial.

Frequently cited vestigial structures in humans include the muscles in the ear and wisdom teeth, the latter of which was caused by a shift in diet towards soft and processed food over time. This caused a reduction in the number of powerful grinding teeth, especially the third molars or wisdom teeth. Goose bumps are considered a vestigial reflex whose function in human ancestors was to raise the body’s hair, making the ancestor appear larger and scaring off predators. 

Contrary to what some believe, vestigiality does not mean a vestigial feature is completely useless. Darwin, in On the Origin of Species (1859), maintained that a vestigial structure could be useless for its primary function yet retain secondary anatomical roles,

“An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose, and remain perfectly efficient for the other… [A]n organ may become rudimentary for its proper purpose, and be used for a distinct object” (3).

Biologists consider vestigial structures evidence for evolution. Theodosius Dobzhansky, a prominent geneticist and evolutionary biologist whose work had a major influence on 20th-century thought and research on genetics, stated that “There is, indeed, no doubt that vestigial rudimentary organs silently proclaim the fact of evolution” (4).

As natural selection unfolded over time advantageous structures were selected, while others were not. If the structure is unadvantageous and no longer beneficial for survival, it becomes less likely that the organism’s offspring will inherit the structure.

Although vestigial structures are often harmless, they can be a risk for an organism, such as in contracting infections or cancer. A vestigial structure that poses no harm to the organism takes a longer time to be phased out than one that is.

References

  1. Conrad, Ernest C. 1982. True Vestigial Structures in Whales and Dolphins. Available.
  2. Darwin, Charles. 1872 [1859]. The Origin of Species: By Means of Natural Selection Or the Preservation of Favored Races in the Struggle for Life. Modern Library. p. 346.
  3. Darwin, Charles. 1872 [1859]. Ibid. p. 347.
  4. Wiley, John. 1995. “Evolution, Genetics, and Man”. p. 242.
    McMahon, April and McMahon, Robert 2012. “Evidence for evolution”. In Evolutionary Linguistics, 23-50. Cambridge University Press.

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