In humans the vermiform appendix is a vestigial structure; it has lost much of its ancestral function.
Vestigiality is the retention during the process of sexual reproduction of genetically determined structures or attributes that have lost some or all of their ancestral function in a given species. Assessment of the vestigiality must generally rely on comparison with homologous
features in related species. The emergence of vestigiality occurs by
normal evolutionary processes, typically by loss of function of a
feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against
more urgently when its function becomes definitively harmful, but if
the lack of the feature provides no advantage, and its presence provides
no disadvantage, the feature may not be phased out by natural selection
and persist across species.
Examples of vestigial structures are the loss of functional wings in island-dwelling birds; the human appendix and vomeronasal organ; and the hindlimbs of the snake and whale.
Overview
The Darwin-tubercle (left) is a vestigial form of the ear tip (right) in the mammalian ancestors of humans—here shown in a crab-eating macaque.
Vestigial features may take various forms; for example, they may be
patterns of behavior, anatomical structures, or biochemical processes.
Like most other physical features, however functional, vestigial
features in a given species may successively appear, develop, and
persist or disappear at various stages within the life cycle of the organism, ranging from early embryonic development to late adulthood.
Vestigial hindlegs (spurs) in a boa constrictor
Vestigiality, biologically speaking, refers to organisms retaining
organs that have seemingly lost their original function. The issue is
controversial and not without dispute; nonetheless, vestigial organs are
common evolutionary knowledge. In addition, the term vestigiality
is useful in referring to many genetically determined features, either
morphological, behavioral, or physiological; in any such context,
however, it need not follow that a vestigial feature must be completely
useless. A classic example at the level of gross anatomy is the human vermiform appendix—though vestigial in the sense of retaining no significant digestive function, the appendix still has immunological roles and is useful in maintaining gut flora.
Similar concepts apply at the molecular level—some nucleic acid sequences in eukaryotic genomes have no known biological function; some of them may be "junk DNA",
but it is a difficult matter to demonstrate that a particular sequence
in a particular region of a given genome is truly nonfunctional. The
simple fact that it is noncoding DNA
does not establish that it is functionless. Furthermore, even if an
extant DNA sequence is functionless, it does not follow that it has
descended from an ancestral sequence of functional DNA. Logically such
DNA would not be vestigial in the sense of being the vestige of a
functional structure. In contrast pseudogenes
have lost their protein-coding ability or are otherwise no longer
expressed in the cell. Whether they have any extant function or not,
they have lost their former function and in that sense, they do fit the
definition of vestigiality.
Vestigial structures are often called vestigial organs, although many of them are not actually organs. Such vestigial structures typically are degenerate, atrophied, or rudimentary,[3] and tend to be much more variable
than homologous non-vestigial parts. Although structures commonly
regarded "vestigial" may have lost some or all of the functional roles
that they had played in ancestral organisms, such structures may retain
lesser functions or may have become adapted to new roles in extant
populations.
It is important to avoid confusion of the concept of vestigiality with that of exaptation.
Both may occur together in the same example, depending on the relevant
point of view. In exaptation, a structure originally used for one
purpose is modified for a new one. For example, the wings of penguins
would be exaptational in the sense of serving a substantial new purpose
(underwater locomotion), but might still be regarded as vestigial in
the sense of having lost the function of flight. In contrast Darwin
argued that the wings of emus
would be definitely vestigial, as they appear to have no major extant
function; however, function is a matter of degree, so judgments on what
is a "major" function are arbitrary; the emu does seem to use its wings
as organs of balance in running. Similarly, the ostrich uses its wings in displays and temperature control, though they are undoubtedly vestigial as structures for flight.
Vestigial characters range from detrimental through neutral to
favorable in terms of selection. Some may be of some limited utility to
an organism but still degenerate over time if they do not confer a
significant enough advantage in terms of fitness to avoid the effects of genetic drift or competing selective pressures. Vestigiality in its various forms presents many examples of evidence for biological evolution.
History
The blind mole rat (Spalax typhlus) has tiny eyes completely covered by a layer of skin.
Vestigial structures have been noticed since ancient times, 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" due to the fact that moles can scarcely see. However, only in recent centuries have anatomical vestiges become a subject of serious study. In 1798, Étienne Geoffroy Saint-Hilaire noted on vestigial structures:
| “ | Whereas useless in this circumstance, these rudiments... have not been eliminated, because Nature never works by rapid jumps, and She always leaves vestiges of an organ, even though it is completely superfluous, if that organ plays an important role in the other species of the same family. | ” |
His colleague, 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 familiar with the concept of vestigial structures, though the term
for them did not 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 On 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."
In the first edition of On the Origin of Species, Darwin briefly mentioned inheritance of acquired characters under the heading "Effects of Use and Disuse",
expressing little doubt that use "strengthens and enlarges certain
parts, and disuse diminishes them; and that such modifications are
inherited". In later editions he expanded his thoughts on this,
and in the final chapter of the 6th edition concluded that species have
been modified "chiefly through the natural selection of numerous
successive, slight, favorable variations; aided in an important manner
by the inherited effects of the use and disuse of parts".
In 1893, Robert Wiedersheim published The Structure of Man, a book on human anatomy and its relevance to man's evolutionary history. The Structure of Man
contained a list of 86 human organs that Wiedersheim described as,
"Organs having become wholly or in part functionless, some appearing in
the Embryo alone, others present during Life constantly or inconstantly.
For the greater part Organs which may be rightly termed Vestigial."
Since his time, the function of some of these structures have 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 Horatio Newman said in a written statement read into evidence in the Scopes 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 theory
Vestigial structures are often homologous
to structures that are functioning normally in other species.
Therefore, vestigial structures can be considered the 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. Through an examination of these various traits,
it is clear that evolution had a hard role in the development of
organisms. Every anatomical structure or behavior response has origins
in which they were, at one time, useful. As time progressed, the ancient
common ancestor organisms did as well. Evolving with time, natural
selection played a huge role. More advantageous structures were
selected, while others were not. With this expansion, some traits were
left to the wayside. 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). In many cases the structure is of no direct harm, yet all structures require extra energy in terms of development, maintenance, and weight, and are also a risk in terms of disease (e.g., infection, cancer), providing some selective
pressure for the removal of parts that do not contribute to an
organism's fitness. A structure that is not harmful will take longer to
be 'phased out' than one that is. However, some vestigial structures
may persist due to limitations in development, such that complete loss
of the structure could not occur without major alterations of the
organism's developmental pattern, and such alterations would likely
produce numerous negative side-effects. The toes of many animals such as
horses, which stand on a single toe, are still evident in a vestigial form and may become evident, although rarely, from time to time in individuals.
The vestigial versions of the structure can be compared to the
original version of the structure in other species in order to determine
the homology of a vestigial structure. Homologous structures indicate common ancestry with those organisms that have a functional version of the structure. Douglas Futuyma has stated that vestigial structures make no sense without evolution, just as spelling and usage of many modern English words can only be explained by their Latin or Old Norse antecedents.
Vestigial traits can still be considered adaptations.
This is because an adaptation is often 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
Non-human animals
Letter c in the picture indicates the undeveloped hind legs of a baleen whale.
Vestigial characters are present throughout the animal
kingdom, and an almost endless list could be given. Darwin said that
"it would be impossible to name one of the higher animals in which some
part or other is not in a rudimentary condition."
The wings of ostriches, emus, and other flightless birds are vestigial; they are 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. Animals that reproduce
without sex (via asexual reproduction) generally lose their sexual traits, such as the ability to locate/recognize the opposite sex and copulation behavior.
Boas and pythons have vestigial pelvis remnants, which are externally visible as two small pelvic spurs
on each side of the cloaca. These spurs are sometimes used in
copulation, but are not essential, as no colubrid 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.
Vestigial attachement clamps in various genera of protomicrocotylids. Accessory sclerites (black) are present in normal clamps but absent in simplified clamps. Lethacotyle (right) has no clamp at all.
A case of vestigial organs was described in polyopisthocotylean Monogeneans (parasitic flatworms). These parasites usually have a posterior attachment organ with several clamps, which are sclerotised organs attaching the worm to the gill of the host fish. These clamps are extremely important for the survival of the parasite. In the family Protomicrocotylidae, species have either normal clamps, simplified clamps, or no clamps at all (in the genus Lethacotyle). After a comparative study of the relative surface of clamps in more than 100 Monogeneans,
this has been interpreted as an evolutionary sequence leading to the
loss of clamps. Coincidentally, other attachment structures (lateral
flaps, transverse striations) have evolved in protomicrocotylids.
Therefore, clamps in protomicrocotylids were considered vestigial organs.
In the foregoing examples the vestigiality is generally the (sometimes incidental) result of adaptive evolution. However, there are many examples of vestigiality as the product of drastic mutation,
and such vestigiality is usually harmful or counter-adaptive. One of
the earliest documented examples was that of vestigial wings in Drosophila. Many examples in many other contexts have emerged since.
Humans
The muscles connected to the ears of a human do not develop enough to have the same mobility allowed to many animals.
Human vestigiality is related to human evolution, and includes a variety of characters occurring in the human species. Many examples of these are vestigial in other primates and related animals, whereas other examples are still highly developed. The human caecum is vestigial, as often is the case in omnivores, being reduced to a single chamber receiving the content of the ileum into the colon. The ancestral caecum would have been a large, blind diverticulum in which resistant plant material such as cellulose would have been fermented in preparation for absorption in the colon.
Analogous organs in other animals similar to humans continue to perform
similar functions. An alternative explanation would be the possibility
that natural selection selects for larger appendices because smaller
and thinner appendices would be more susceptible to inflammation and
disease. The coccyx,
or tailbone, though a vestige of the tail of some primate ancestors, is
functional as an anchor for certain pelvic muscles including: the
levator ani muscle and the largest gluteal muscle, the gluteus maximus.
Other structures that are vestigial include 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. Other organic structures (such as the occipitofrontalis muscle) have lost their original functions (keep the head from falling) but are still useful for other purposes (facial expression).
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. The arrector pili
muscle, which is a band of smooth muscle that connects the hair follicle
to connective tissue, contracts and creates the goosebumps on skin.
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 a gene that is functional in most other
mammals and which produces L-gulonolactone oxidase, an enzyme that can make vitamin C. A documented mutation deactivated the gene in an ancestor of the modern infraorder of monkeys, and apes, and it now remains in their genomes, including the human genome, as a vestigial sequence called a pseudogene.
The shift in human diet towards soft and processed food over time
caused a reduction in the number of powerful grinding teeth, especially
the third molars or wisdom teeth, which were highly prone to impaction.
Plants and fungi
Plants also have vestigial parts, including functionless stipules and carpels, leaf reduction of Equisetum, paraphyses of Fungi.
Well known examples are the reductions in floral display, leading to
smaller and/or paler flowers, in plants that reproduce without outcrossing, for example via selfing or obligate clonal reproduction.
