Everything about Vestigial totally explained
Vestigiality describes
homologous characters of
organisms which have lost all or most of their original
function in a species through
evolution. These may take various forms such as
anatomical structures,
behaviors and
biochemical pathways. Some of these disappear early in
embryonic development, but others are retained in adulthood. All such characters can, in turn, be traced to the
genes which code for such characters. Some genes no longer code for anything, and can thus be called vestigial themselves, or
junk DNA.
Vestigial structures are often called
vestigial organs, although many of them are not actually
organs. These are typically in a degenerate, atrophied, or rudimentary condition, and tend to be much more
variable than similar parts. Although structures usually called "vestigial" are largely or entirely functionless, a vestigial structure may retain lesser functions or develop minor new ones. However, care must be taken not to apply the label of vestigiality to
exaptation, in which a structure originally used for one purpose is modified for a new one. For example, the wings of
penguin wouldn't be vestigial, as they've been modified for a substantial new purpose (underwater locomotion), while those of an
emu would be, as they've no major purpose anymore (not even for display as in
ostriches).
Vestigial characters range on a continuum from detrimental through neutral to marginally useful. Some may be of some limited utility to an organism but still degenerate over time; the important point isn't that they're without utility, but that they don't confer a significant enough advantage in terms of
fitness to avoid the random force of disorder that's
mutation. It is difficult however to say that a vestigial character is detrimental to the organism in the long term - the future is unpredictable, and that which is of no use in the present may develop into something useful in the future. Vestigiality is one of several lines of evidence for biological evolution.
History
Vestigial structures have been noticed since ancient times and are very old, and the reason for their existence was long speculated upon before
Darwinian evolution provided a widely-accepted explanation. In the 4th century BC,
Aristotle was one of the earliest writers to comment, in his
History of Animals, on the vestigial eyes of moles, calling them "stunted in development". However, only in recent centuries have anatomical vestiges become a subject of serious study. In 1798,
Étienne Geoffroy Saint-Hilaire noted on vestigial structures:
Jean-Baptiste Lamarck, named a number of vestigial structures in his 1809 book
Philosophie Zoologique. Lamarck noted "
Olivier's
Spalax, which lives underground like the mole, and is apparently exposed to daylight even less than the mole, has altogether lost the use of sight: so that it shows nothing more than vestiges of this organ."
Charles Darwin was very familiar with the concept of vestigial structures, though the term for them didn't yet exist. He listed a number of them in
The Descent of Man, including the
muscles of the ear,
wisdom teeth, the
appendix, the
tail bone,
body hair, and the
semilunar fold in the corner of the
eye. Darwin also noted, in
The Origin of Species, that a vestigial structure could be useless for its primary function, but still 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."
Darwin however still often refers to the 'use and disuse' of structures having some role in heredity, with
inheritance of acquired characters being treated as an important aspect besides the central force of natural selection. In the final chapter of
The Origin of Species he describes the process: "This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts". Since his time, the function of some of these structures has been discovered, while other anatomical vestiges have been unearthed, making the list primarily of interest as a record of the knowledge of human anatomy at the time. Later versions of Wiedersheim's list were expanded to as many as 180 human "vestigial organs". This is why the zoologist Newman stated in the
Scopes Monkey Trial that "There are, according to Wiedersheim, no less than 180 vestigial structures in the human body, sufficient to make of a man a veritable walking museum of antiquities."
Common descent and evolutionary history
Vestigial structures are often
homologous to structures that are functioning normally in other species. Therefore, vestigial structures can be considered evidence for
evolution, the process by which beneficial heritable traits arise in populations over an extended period of time. The existence of vestigial traits can be attributed to changes in the environment and behavior patterns of the organism in question. As the function of the trait is no longer beneficial for survival, the likelihood that future offspring will inherit the "normal" form of it decreases. In some cases the structure becomes detrimental to the organism (for example the eyes of a mole can become infected
Vestigial traits are still considered
adaptations. This is because an adaptation is defined as a trait that has been favored by natural selection. Adaptations therefore need not be
adaptive, as long as they were at some point.
Examples
Animals
In
whales and other
cetaceans, one can find small vestigial leg bones deeply buried within the back of the body. These are remnants of their land-living ancestors' legs. Many whales also have undeveloped, unused,
pelvis bones in the
anterior part of their torsos.
The wings of
ostriches,
emus, and other
flightless birds are vestigial; they're remnants of their flying ancestors' wings.
The eyes of certain
cavefish and
salamanders are vestigial, as they no longer allow the organism to see, and are remnants of their ancestors' functional eyes.
Boas and
pythons have vestigial pelvis remnants which are externally visible as two small
anal spurs on each side of the cloaca. These spurs are sometimes used in copulation, but are not essential, as no colubroid snake (the vast majority of species) possesses these remnants. Furthermore, in most snakes the left lung is greatly reduced or absent.
Amphisbaenians, which independently evolved limblessness, also retain vestiges of the pelvis as well as the pectoral girdle, and have lost their right lung.
Crabs have small tails tucked between their rear legs that are probably vestigial, as they're no longer in use. The working version of these tails can be found in their close
crustacean relative, the
lobster.
Certain species of
moths (for example the
Gypsy moth) have females that, although flightless, still carry small wings. These wings have no use, and are vestigial to the versions in species whose females can fly.
The
fruit fly can be bred in high school experiments to produce off-spring with vestigial wings, to better understand basic genetics in biology.
Humans
Human vestigiality is related to
human evolution, and includes a variety of characters occurring in the
human species. Many of these are also vestigial in other
primates and related animals. The
vermiform appendix is perhaps the most commonly used example of vestigiality in humans. While many functions for the appendix have been
hypothesized, none has been
empirically demonstrated. Other structures that still are considered vestigial include the
coccyx, or tailbone (a remnant of a lost
tail); the
plica semilunaris on the inside corner of the
eye (a remnant of the
nictitating membrane); and, as pictured,
muscles in the
ear and other parts of the body.
Humans also bear some vestigial behaviors and reflexes. The formation of
goose bumps in humans under
stress is a vestigial
reflex; its function in human ancestors was to raise the body's hair, making the ancestor appear larger and scaring off predators. Raising the hair is also used to trap an extra layer of air, keeping an animal warm. This reflex formation of goosebumps when cold isn't vestigial in humans, but the reflex to form them under stress is.
Infants are also able to support their own weight while hanging from a rod, responding to certain tacticle stimuli. An ancestral primate would have had sufficient body hair for an infant to cling to, allowing its mother to escape from danger, such as climbing up a tree in the presence of a
predator.
There are also vestigial molecular structures in humans, which are no longer in use but may indicate common ancestry with other species. One example of this is
L-gulonolactone oxidase, a gene, that's functional in most other mammals, which produces an
enzyme that can make
vitamin C. A mutation inactivated the gene in an ancestor of the current group of
primates, and it now remains in the
human genome as a vestigial sequence called a
pseudogene.
Plants and other organisms
Plants also have vestigial parts. For example,
dandelions and other
asexually reproducing plants produce unneeded flower petals, which were once used to attract pollinating insects.
Further Information
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