Devolution, de-evolution, or backward evolution
is the notion that species can revert to supposedly
more primitive forms over time.
The concept relates to the idea that evolution
has a purpose (teleology) and is progressive
(orthogenesis), for example that feet might
be better than hooves or lungs than gills.
However, evolutionary biology makes no such
assumptions, and natural selection shapes
adaptations with no foreknowledge of any kind.
It is possible for small changes (such as
in the frequency of a single gene) to be reversed
by chance or selection, but this is no different
from the normal course of evolution.
In the 19th century, when belief in orthogenesis
was widespread, zoologists (such as Ray Lankester
and Anton Dohrn) and the palaeontologists
Alpheus Hyatt and Carl H. Eigenmann advocated
the idea of devolution.
The concept appears in Kurt Vonnegut's 1985
novel Galápagos, which portrays a society
that has evolved backwards to have small brains.
Dollo's law of irreversibility, first stated
in 1893 by the palaeontologist Louis Dollo,
denies the possibility of devolution.
The evolutionary biologist Richard Dawkins
explains Dollo's law as being simply a statement
about the improbability of evolution's following
precisely the same path twice.
== Context ==
The idea of devolution is based on the presumption
of orthogenesis, the view that evolution has
a purposeful direction towards increasing
complexity.
Modern evolutionary theory, beginning with
Darwin at least, poses no such presumption.
and the concept of evolutionary change is
independent of either any increase in complexity
of organisms sharing a gene pool, or any decrease,
such as in vestigiality or in loss of genes.
Earlier views that species are subject to
"cultural decay", "drives to perfection",
or "devolution" are practically meaningless
in terms of current (neo-)Darwinian theory.
Early scientific theories of transmutation
of species such as Lamarckism perceived species
diversity as a result of a purposeful internal
drive or tendency to form improved adaptations
to the environment.
In contrast, Darwinian evolution and its elaboration
in the light of subsequent advances in biological
research, have shown that adaptation through
natural selection comes about when particular
heritable attributes in a population happen
to give a better chance of successful reproduction
in the reigning environment than rival attributes
do.
By the same process less advantageous attributes
are less "successful"; they decrease in frequency
or are lost completely.
Since Darwin's time it has been shown how
these changes in the frequencies of attributes
occur according to the mechanisms of genetics
and the laws of inheritance originally investigated
by Gregor Mendel.
Combined with Darwin's original insights,
genetic advances led to what has variously
been called the modern evolutionary synthesis
or the neo-Darwinism of the 20th century.
In these terms evolutionary adaptation may
occur most obviously through the natural selection
of particular alleles.
Such alleles may be long established, or they
may be new mutations.
Selection also might arise from more complex
epigenetic or other chromosomal changes, but
the fundamental requirement is that any adaptive
effect must be heritable.The concept of devolution
on the other hand, requires that there be
a preferred hierarchy of structure and function,
and that evolution must mean "progress" to
"more advanced" organisms.
For example, it could be said that "feet are
better than hooves" or "lungs are better than
gills", so their development is "evolutionary"
whereas change to an inferior or "less advanced"
structure would be called "devolution".
In reality an evolutionary biologist defines
all heritable changes to relative frequencies
of the genes or indeed to epigenetic states
in the gene pool as evolution.
All gene pool changes that lead to increased
fitness in terms of appropriate aspects of
reproduction are seen as (neo-)Darwinian adaptation
because, for the organisms possessing the
changed structures, each is a useful adaptation
to their circumstances.
For example, hooves have advantages for running
quickly on plains, which benefits horses,
and feet offer advantages in climbing trees,
which some ancestors of humans did.The concept
of devolution as regress from progress relates
to the ancient ideas that either life came
into being through special creation or that
humans are the ultimate product or goal of
evolution.
The latter belief is related to anthropocentrism,
the idea that human existence is the point
of all universal existence.
Such thinking can lead on to the idea that
species evolve because they "need to" in order
to adapt to environmental changes.
Biologists refer to this misconception as
teleology, the idea of intrinsic finality
that things are "supposed" to be and behave
a certain way, and naturally tend to act that
way to pursue their own good.
From a biological viewpoint, in contrast,
if species evolve it is not a reaction to
necessity, but rather that the population
contains variations with traits that favour
their natural selection.
This view is supported by the fossil record
which demonstrates that roughly ninety-nine
percent of all species that ever lived are
now extinct.People thinking in terms of devolution
commonly assume that progress is shown by
increasing complexity, but biologists studying
the evolution of complexity find evidence
of many examples of decreasing complexity
in the record of evolution.
The lower jaw in fish, reptiles and mammals
has seen a decrease in complexity, if measured
by the number of bones.
Ancestors of modern horses had several toes
on each foot; modern horses have a single
hooved toe.
Modern humans may be evolving towards never
having wisdom teeth, and already have lost
most of the tail found in many other mammals
- not to mention other vestigial structures,
such as the vermiform appendix or the nictitating
membrane.
In some cases, the level of organization of
living creatures can also “shift” downwards
(e.g., the loss of multicellularity in some
groups of protists and fungi).A more rational
version of the concept of devolution, a version
that does not involve concepts of "primitive"
or "advanced" organisms, is based on the observation
that if certain genetic changes in a particular
combination (sometimes in a particular sequence
as well) are precisely reversed, one should
get precise reversal of the evolutionary process,
yielding an atavism or "throwback", whether
more or less complex than the ancestors where
the process began.
At a trivial level, where just one or a few
mutations are involved, selection pressure
in one direction can have one effect, which
can be reversed by new patterns of selection
when conditions change.
That could be seen as reversed evolution,
though the concept is not of much interest
because it does not differ in any functional
or effective way from any other adaptation
to selection pressures.
As the number of genetic changes rises however,
one combinatorial effect is that it becomes
vanishingly unlikely that the full course
of adaptation can be reversed precisely.
Also, if one of the original adaptations involved
complete loss of a gene, one can neglect any
probability of reversal.
Accordingly, one might well expect reversal
of peppered moth colour changes, but not reversal
of the loss of limbs in snakes.
== History ==
The concept of degenerative evolution was
used by scientists in the 19th century, at
this time it was believed by most biologists
that evolution had some kind of direction.
In 1857 the physician Bénédict Morel influenced
by Lamarckism claimed that environmental factors
such as taking drugs or alcohol would produce
social degeneration in the offspring of those
individuals, and would revert those offspring
to a primitive state.
Morel, a devout Catholic, had believed that
mankind had started in perfection, contrasting
modern humanity to the past, Morel claimed
there had been "Morbid deviation from an original
type".
His theory of devolution was later advocated
by some biologists.
According to Roger Luckhurst:
Darwin soothed readers that evolution was
progressive, and directed towards human perfectibility.
The next generation of biologists were less
confident or consoling.
Using Darwin's theory, and many rival biological
accounts of development then in circulation,
scientists suspected that it was just as possible
to devolve, to slip back down the evolutionary
scale to prior states of development.
One of the first biologists to suggest devolution
was Ray Lankester, he explored the possibility
that evolution by natural selection may in
some cases lead to devolution, an example
he studied was the regressions in the life
cycle of sea squirts.
Lankester discussed the idea of devolution
in his book Degeneration: A Chapter in Darwinism
(1880).
He was a critic of progressive evolution,
pointing out that higher forms existed in
the past which have since degenerated into
simpler forms.
Lankester argued that "if it was possible
to evolve, it was also possible to devolve,
and that complex organisms could devolve into
simpler forms or animals".Anton Dohrn also
developed a theory of degenerative evolution
based on his studies of vertebrates.
According to Dohrn many chordates are degenerated
because of their environmental conditions.
Dohrn claimed cyclostomes such as lampreys
are degenerate fish as there is no evidence
their jawless state is an ancestral feature
but is the product of environmental adaptation
due to parasitism.
According to Dohrn if cyclotomes would devolve
further then they would resemble something
like an Amphioxus.The historian of biology
Peter J. Bowler has written that devolution
was taken seriously by proponents of orthogenesis
and others in the late 19th century who at
this period of time firmly believed that there
was a direction in evolution.
Orthogenesis was the belief that evolution
travels in internally directed trends and
levels.
The paleontologist Alpheus Hyatt discussed
devolution in his work, using the concept
of racial senility as the mechanism of devolution.
Bowler defines racial senility as "an evolutionary
retreat back to a state resembling that from
which it began."Hyatt who studied the fossils
of invertebrates believed that up to a point
ammonoids developed by regular stages up until
a specific level but would later due to unfavourable
conditions descend back to a previous level,
this according to Hyatt was a form of lamarckism
as the degeneration was a direct response
to external factors.
To Hyatt after the level of degeneration the
species would then become extinct, according
to Hyatt there was a "phase of youth, a phase
of maturity, a phase of senility or degeneration
foreshadowing the extinction of a type".
To Hyatt the devolution was predetermined
by internal factors which organisms can neither
control or reverse.
This idea of all evolutionary branches eventually
running out of energy and degenerating into
extinction was a pessimistic view of evolution
and was unpopular amongst many scientists
of the time.Carl H. Eigenmann an ichthyologist
wrote Cave vertebrates of America: a study
in degenerative evolution (1909) in which
he concluded that cave evolution was essentially
degenerative.
The entomologist William Morton Wheeler and
the Lamarckian Ernest MacBride (1866–1940)
also advocated degenerative evolution.
According to Macbride invertebrates were actually
degenerate vertebrates, his argument was based
on the idea that "crawling on the seabed was
inherently less stimulating than swimming
in open waters."
== 
Dollo's law ==
It has been observed that complex body parts
evolve in a lineage over many generations;
once lost, they are unlikely to re-evolve.
This observation is sometimes generalized
to a hypothesis known as Dollo's law, which
states that evolution is not reversible.
This does not imply that similar engineering
solutions cannot be found by natural selection:
the tails of the cetacea—whales, dolphins
and porpoises, which are evolved from formerly
land-dwelling mammals—represent an adaptation
of the spinal column for propulsion in water.
Unlike the tails of the mammal's marine ancestor,
the Sarcopterygii, and of the teleosts, which
move from side to side, the cetacean's tail
moves up and down as it flexes its mammalian
spine: the function of the tail in providing
propulsion is remarkably similar.
== In literature ==
Kurt Vonnegut's 1985 novel Galápagos is set
a million years in the future, where humans
have "devolved" to have much smaller brains.
== See also ==
Deletional bias, bacteria research observing
that rate of genetic deletion exceeds insertion,
such that genomes tend to reduce in size
Dysgenics
Evolution of complexity
Great chain of being
International Society for Krishna Consciousness
views on evolution
Ulas family
Yeridat ha-dorot
