Alternatives to evolution by natural selection,
also described as non-Darwinian mechanisms
of evolution, have been proposed by scholars
investigating biology since classical times
to explain signs of evolution and the relatedness
of different groups of living things.
The alternatives in question do not deny that
evolutionary changes over time are the origin
of the diversity of life, nor deny that the
organisms alive today share a common ancestor
from the distant past (or ancestors, in some
proposals); rather, they propose alternative
mechanisms of evolutionary change over time,
arguing against mutations acted on by natural
selection as the most important driver of
evolutionary change. (In most cases, they
do not deny that mutations or natural selection
occur, or that they play a role in evolutionary
change, but instead deny that they are fully
sufficient primary causes for the evidence
of evolutionary change that is observed in
the natural world.)
This distinguishes them from certain other
kinds of arguments that deny that large scale
evolution of any sort has taken place, as
in some forms of creationism, which do not
propose alternative mechanisms of evolutionary
change but instead deny that evolutionary
change has taken place at all. Not all forms
of creationism deny that evolutionary change
takes places; notably, proponents of theistic
evolution, such as the biologist Asa Gray,
assert that evolutionary change does occur
and is responsible for the history of life
on Earth, with the proviso that this process
has been influenced by a god or gods in some
meaningful sense.
Where the fact of evolutionary change was
accepted but the mechanism proposed by Charles
Darwin, natural selection, was denied, explanations
of evolution such as Lamarckism, catastrophism,
orthogenesis, vitalism, structuralism and
mutationism (called saltationism before 1900)
were entertained. Different factors motivated
people to propose non-Darwinian mechanisms
of evolution. Natural selection, with its
emphasis on death and competition, did not
appeal to some naturalists because they felt
it immoral, leaving little room for teleology
or the concept of progress in the development
of life. Some who came to accept evolution,
but disliked natural selection, raised religious
objections. Others felt that evolution was
an inherently progressive process that natural
selection alone was insufficient to explain.
Still others felt that nature, including the
development of life, followed orderly patterns
that natural selection could not explain.
By the start of the 20th century, evolution
was generally accepted by biologists but natural
selection was in eclipse. Many alternative
theories were proposed, but biologists were
quick to discount theories such as orthogenesis,
vitalism and Lamarckism which offered no mechanism
for evolution. Mutationism did propose a mechanism,
but it was not generally accepted. The modern
synthesis a generation later claimed to sweep
away all the alternatives to Darwinian evolution,
though some have been revived as molecular
mechanisms for them have been discovered.
== Unchanging forms ==
Aristotle did not embrace either divine creation
or evolution, instead arguing in his biology
that each species (eidos) was immutable, breeding
true to its ideal eternal form (not the same
as Plato's theory of Forms). Aristotle's suggestion
in De Generatione Animalium of a fixed hierarchy
in nature - a scala naturae ("ladder of nature")
provided an early explanation of the continuity
of living things. Aristotle saw that animals
were teleological (functionally end-directed),
and had parts that were homologous with those
of other animals, but he did not connect these
ideas into a concept of evolutionary progress.In
the Middle Ages, Scholasticism developed Aristotle's
view into the idea of a great chain of being.
The image of a ladder inherently suggests
the possibility of climbing, but both the
ancient Greeks and mediaeval scholastics such
as Ramon Lull maintained that each species
remained fixed from the moment of its creation.By
1818, however, Étienne Geoffroy Saint-Hilaire
argued in his Philosophie anatomique that
the chain was "a progressive series", where
animals like molluscs low on the chain could
"rise, by addition of parts, from the simplicity
of the first formations to the complication
of the creatures at the head of the scale",
given sufficient time. Accordingly, Geoffroy
and later biologists looked for explanations
of such evolutionary change.Georges Cuvier's
1812 Recherches sur les Ossements Fossiles
set out his doctrine of the correlation of
parts, namely that since an organism was a
whole system, all its parts mutually corresponded,
contributing to the function of the whole.
So, from a single bone the zoologist could
often tell what class or even genus the animal
belonged to. And if an animal had teeth adapted
for cutting meat, the zoologist could be sure
without even looking that its sense organs
would be those of a predator and its intestines
those of a carnivore. A species had an irreducible
functional complexity, and "none of its parts
can change without the others changing too".
Evolutionists expected one part to change
at a time, one change to follow another. In
Cuvier's view, evolution was impossible, as
any one change would unbalance the whole delicate
system.Louis Agassiz's 1856 "Essay on Classification"
exemplified German philosophical idealism.
This held that each species was complex within
itself, had complex relationships to other
organisms, and fitted precisely into its environment,
as a pine tree in a forest, and could not
survive outside those circles. The argument
from such ideal forms opposed evolution without
offering an actual alternative mechanism.
Richard Owen held a similar view in Britain.The
Lamarckian social philosopher and evolutionist
Herbert Spencer, ironically the author of
the phrase "survival of the fittest" adopted
by Darwin, used an argument like Cuvier's
to oppose natural selection. In 1893, he stated
that a change in any one structure of the
body would require all the other parts to
adapt to fit in with the new arrangement.
From this, he argued that it was unlikely
that all the changes could appear at the right
moment if each one depended on random variation;
whereas in a Lamarckian world, all the parts
would naturally adapt at once, through a changed
pattern of use and disuse.
== Alternative explanations of change ==
Where the fact of evolutionary change was
accepted by biologists but natural selection
was denied, including but not limited to the
late 19th century eclipse of Darwinism, alternative
scientific explanations such as Lamarckism,
orthogenesis, structuralism, catastrophism,
vitalism and theistic evolution were entertained,
not necessarily separately. (Purely religious
points of view such as young or old earth
creationism or intelligent design are not
considered here.) Different factors motivated
people to propose non-Darwinian evolutionary
mechanisms. Natural selection, with its emphasis
on death and competition, did not appeal to
some naturalists because they felt it immoral,
leaving little room for teleology or the concept
of progress in the development of life. Some
of these scientists and philosophers, like
St. George Jackson Mivart and Charles Lyell,
who came to accept evolution but disliked
natural selection, raised religious objections.
Others, such as the biologist and philosopher
Herbert Spencer, the botanist George Henslow
(son of Darwin's mentor John Stevens Henslow,
also a botanist), and the author Samuel Butler,
felt that evolution was an inherently progressive
process that natural selection alone was insufficient
to explain. Still others, including the American
paleontologists Edward Drinker Cope and Alpheus
Hyatt, had an idealist perspective and felt
that nature, including the development of
life, followed orderly patterns that natural
selection could not explain.Some felt that
natural selection would be too slow, given
the estimates of the age of the earth and
sun (10–100 million years) being made at
the time by physicists such as Lord Kelvin,
and some felt that natural selection could
not work because at the time the models for
inheritance involved blending of inherited
characteristics, an objection raised by the
engineer Fleeming Jenkin in a review of Origin
written shortly after its publication. Another
factor at the end of the 19th century was
the rise of a new faction of biologists, typified
by geneticists like Hugo de Vries and Thomas
Hunt Morgan, who wanted to recast biology
as an experimental laboratory science. They
distrusted the work of naturalists like Darwin
and Alfred Russel Wallace, dependent on field
observations of variation, adaptation, and
biogeography, as being overly anecdotal. Instead
they focused on topics like physiology and
genetics that could be investigated with controlled
experiments in the laboratory, and discounted
less accessible phenomena like natural selection
and adaptation to the environment.
=== Vitalism ===
Vitalism holds that living organisms differ
from other things in containing something
non-physical, such as a fluid or vital spirit,
that makes them live. The theory dates to
ancient Egypt.
Since Early Modern times, vitalism stood in
contrast to the mechanistic explanation of
biological systems started by Descartes. Nineteenth
century chemists set out to disprove the claim
that forming organic compounds required vitalist
influence. In 1828, Friedrich Wöhler showed
that urea could be made entirely from inorganic
chemicals. Louis Pasteur believed that fermentation
required whole organisms, which he supposed
carried out chemical reactions found only
in living things. The embryologist Hans Driesch,
experimenting on sea urchin eggs, showed that
separating the first two cells led to two
complete but small blastulas, seemingly showing
that cell division did not divide the egg
into sub-mechanisms, but created more cells
each with the vital capability to form a new
organism. Vitalism faded out with the demonstration
of more satisfactory mechanistic explanations
of each of the functions of a living cell
or organism. By 1931, biologists had "almost
unanimously abandoned vitalism as an acknowledged
belief."
=== 
Theistic evolution ===
The American botanist Asa Gray used the name
"theistic evolution" for his point of view,
presented in his 1876 book Essays and Reviews
Pertaining to Darwinism. He argued that the
deity supplies beneficial mutations to guide
evolution. St George Jackson Mivart argued
instead in his 1871 On the Genesis of Species
that the deity, equipped with foreknowledge,
sets the direction of evolution by specifying
the (orthogenetic) laws that govern it, and
leaves species to evolve according to the
conditions they experience as time goes by.
The Duke of Argyll set out similar views in
his 1867 book The Reign of Law. According
to the historian Edward Larson, the theory
failed as an explanation in the minds of late
19th century biologists as it broke the rules
of methodological naturalism which they had
grown to expect. Accordingly, by around 1900,
biologists no longer saw theistic evolution
as a valid theory. In Larson's view, by then
it "did not even merit a nod among scientists."
Biologists might argue about mechanisms, but
they were in no doubt that a mechanistic explanation
was needed. In the 20th century, theistic
evolution could take other forms, such as
the orthogenesis of Teilhard de Chardin.
=== Orthogenesis ===
Orthogenesis is the hypothesis that life has
an innate tendency to change, developing in
a unilinear fashion in a particular direction,
or simply making some kind of definite progress.
Many different versions have been proposed,
some such as that of Teilhard de Chardin openly
spiritual, others such as Theodor Eimer's
apparently simply biological. These theories
often combined orthogenesis with other supposed
mechanisms. For example, Eimer believed in
Lamarckian evolution, but felt that internal
laws of growth determined which characteristics
would be acquired and would guide the long-term
direction of evolution.Orthogenesis was popular
among paleontologists such as Henry Fairfield
Osborn. They believed that the fossil record
showed unidirectional change, but did not
necessarily accept that the mechanism driving
orthogenesis was teleological (goal-directed).
Osborn argued in his 1918 book Origin and
Evolution of Life that trends in Titanothere
horns were both orthogenetic and non-adaptive,
and could be detrimental to the organism.
For instance, they supposed that the large
antlers of the Irish elk had caused its extinction.Support
for orthogenesis fell during the modern synthesis
in the 1940s when it became apparent that
it could not explain the complex branching
patterns of evolution revealed by statistical
analysis of the fossil record. Work in the
21st century has supported the mechanism and
existence of mutation-biased adaptation (a
form of mutationism), meaning that constrained
orthogenesis is now seen as possible. Moreover,
the self-organizing processes involved in
certain aspects of embryonic development often
exhibit stereotypical morphological outcomes,
suggesting that evolution will proceed in
preferred directions once key molecular components
are in place.
=== Lamarckism ===
Jean-Baptiste Lamarck's 1809 evolutionary
theory, transmutation of species, was based
on a progressive (orthogenetic) drive toward
greater complexity. Lamarck also shared the
belief, common at the time, that characteristics
acquired during an organism's life could be
inherited by the next generation, producing
adaptation to the environment. Such characteristics
were caused by the use or disuse of the affected
part of the body. This minor component of
Lamarck's theory became known, much later,
as Lamarckism. Darwin included Effects of
the increased Use and Disuse of Parts, as
controlled by Natural Selection in On the
Origin of Species, giving examples such as
large ground feeding birds getting stronger
legs through exercise, and weaker wings from
not flying until, like the ostrich, they could
not fly at all. In the late 19th century,
neo-Lamarckism was supported by the German
biologist Ernst Haeckel, the American paleontologists
Edward Drinker Cope and Alpheus Hyatt, and
the American entomologist Alpheus Packard.
Butler and Cope believed that this allowed
organisms to effectively drive their own evolution.
Packard argued that the loss of vision in
the blind cave insects he studied was best
explained through a Lamarckian process of
atrophy through disuse combined with inheritance
of acquired characteristics. Meanwhile, the
English botanist George Henslow studied how
environmental stress affected the development
of plants, and he wrote that the variations
induced by such environmental factors could
largely explain evolution; he did not see
the need to demonstrate that such variations
could actually be inherited. Critics pointed
out that there was no solid evidence for the
inheritance of acquired characteristics. Instead,
the experimental work of the German biologist
August Weismann resulted in the germ plasm
theory of inheritance, which Weismann said
made the inheritance of acquired characteristics
impossible, since the Weismann barrier would
prevent any changes that occurred to the body
after birth from being inherited by the next
generation.In modern epigenetics, biologists
observe that phenotypes depend on heritable
changes to gene expression that do not involve
changes to the DNA sequence. These changes
can cross generations in plants, animals,
and prokaryotes. This is not identical to
traditional Lamarckism, as the changes do
not last indefinitely and do not affect the
germ line and hence the evolution of genes.
=== Catastrophism ===
Catastrophism is the hypothesis, argued by
the French anatomist and paleontologist Georges
Cuvier in his 1812 Recherches sur les ossements
fossiles de quadrupèdes, that the various
extinctions and the patterns of faunal succession
seen in the fossil record were caused by large-scale
natural catastrophes such as volcanic eruptions
and, for the most recent extinctions in Eurasia,
the inundation of low-lying areas by the sea.
This was explained purely by natural events:
he did not mention Noah's flood, nor did he
ever refer to divine creation as the mechanism
for repopulation after an extinction event,
though he did not support evolutionary theories
such as those of his contemporaries Lamarck
and Geoffroy Saint-Hilaire either. Cuvier
believed that the stratigraphic record indicated
that there had been several such catastrophes,
recurring natural events, separated by long
periods of stability during the history of
life on earth. This led him to believe the
Earth was several million years old.Catastrophism
has found a place in modern biology with the
Cretaceous–Paleogene extinction event at
the end of the Cretaceous period, as proposed
in a paper by
Walter and Luis Alvarez in 1980. It argued
that a 10 kilometres (6.2 mi) asteroid struck
Earth 66 million years ago at the end of the
Cretaceous period. The event, whatever it
was, made about 70% of all species extinct,
including the dinosaurs, leaving behind the
Cretaceous–Paleogene boundary. In 1990,
a 180 kilometres (110 mi) candidate crater
marking the impact was identified at Chicxulub
in the Yucatán Peninsula of Mexico.
=== Structuralism ===
Biological structuralism objects to an exclusively
Darwinian explanation of natural selection,
arguing that other mechanisms also guide evolution,
and sometimes implying that these supersede
selection altogether. Structuralists have
proposed different mechanisms that might have
guided the formation of body plans. Before
Darwin, Étienne Geoffroy Saint-Hilaire argued
that animals shared homologous parts, and
that if one was enlarged, the others would
be reduced in compensation. After Darwin,
D'Arcy Thompson hinted at vitalism and offered
geometric explanations in his classic 1917
book On Growth and Form. Adolf Seilacher suggested
mechanical inflation for "pneu" structures
in Ediacaran biota fossils such as Dickinsonia.
Günter P. Wagner argued for developmental
bias, structural constraints on embryonic
development. Stuart Kauffman favoured self-organisation,
the idea that complex structure emerges holistically
and spontaneously from the dynamic interaction
of all parts of an organism. Michael Denton
argued for laws of form by which Platonic
universals or "Types" are self-organised.
In 1979 Stephen J. Gould and Richard Lewontin
proposed biological "spandrels", features
created as a byproduct of the adaptation of
nearby structures. Gerd Müller and Stuart
Newman argued that the appearance in the fossil
record of most of the current phyla in the
Cambrian explosion was "pre-Mendelian" evolution
caused by plastic responses of morphogenetic
systems that were partly organized by physical
mechanisms. Brian Goodwin, described by Wagner
as part of "a fringe movement in evolutionary
biology", denied that biological complexity
can be reduced to natural selection, and argued
that pattern formation is driven by morphogenetic
fields. Darwinian biologists have criticised
structuralism, emphasising that there is plentiful
evidence from deep homology that genes have
been involved in shaping organisms throughout
evolutionary history. They accept that some
structures such as the cell membrane self-assemble,
but question the ability of self-organisation
to drive large-scale evolution.
=== Saltationism, mutationism ===
Saltationism held that new species arise as
a result of large mutations. It was seen as
a much faster alternative to the Darwinian
concept of a gradual process of small random
variations being acted on by natural selection.
It was popular with early geneticists such
as Hugo de Vries, who along with Carl Correns
helped rediscover Gregor Mendel's laws of
inheritance in 1900, William Bateson, a British
zoologist who switched to genetics, and early
in his career, Thomas Hunt Morgan. These ideas
developed into mutationism, the mutation theory
of evolution. This held that species went
through periods of rapid mutation, possibly
as a result of environmental stress, that
could produce multiple mutations, and in some
cases completely new species, in a single
generation, based on de Vries's experiments
with the evening primrose, Oenothera, from
1886. The primroses seemed to be constantly
producing new varieties with striking variations
in form and color, some of which appeared
to be new species because plants of the new
generation could only be crossed with one
another, not with their parents. However,
Hermann Joseph Muller showed in 1918 that
the new varieties de Vries had observed were
the result of polyploid hybrids rather than
rapid genetic mutation.Initially, de Vries
and Morgan believed that mutations were so
large as to create new forms such as subspecies
or even species instantly. Morgan's 1910 fruit
fly experiments, in which he isolated mutations
for characteristics such as white eyes, changed
his mind. He saw that mutations represented
small Mendelian characteristics that would
only spread through a population when they
were beneficial, helped by natural selection.
This represented the germ of the modern synthesis,
and the beginning of the end for mutationism
as an evolutionary force.Contemporary biologists
accept that mutation and selection both play
roles in evolution; the mainstream view is
that while mutation supplies material for
selection in the form of variation, all non-random
outcomes are caused by natural selection.
Masatoshi Nei argues instead that the production
of more efficient genotypes by mutation is
fundamental for evolution, and that evolution
is often mutation-limited. The endosymbiotic
theory implies rare but major events of saltational
evolution by symbiogenesis. Carl Woese and
colleagues suggested that the absence of RNA
signature continuum between domains of bacteria,
archaea, and eukarya shows that these major
lineages materialized via large saltations
in cellular organization.
Saltation at a variety of scales is agreed
to be possible by mechanisms including polyploidy,
which certainly can create new species of
plant, gene duplication, lateral gene transfer,
and transposable elements (jumping genes).
=== Genetic drift ===
The neutral theory of molecular evolution,
proposed by Motoo Kimura in 1968, holds that
at the molecular level most evolutionary changes
and most of the variation within and between
species is not caused by natural selection
but by genetic drift of mutant alleles that
are neutral. A neutral mutation is one that
does not affect an organism's ability to survive
and reproduce. The neutral theory allows for
the possibility that most mutations are deleterious,
but holds that because these are rapidly purged
by natural selection, they do not make significant
contributions to variation within and between
species at the molecular level. Mutations
that are not deleterious are assumed to be
mostly neutral rather than beneficial.The
theory was controversial as it sounded like
a challenge to Darwinian evolution; controversy
was intensified by a 1969 paper by Jack Lester
King and Thomas H. Jukes, provocatively but
misleadingly titled "Non-Darwinian Evolution".
It provided a wide variety of evidence including
protein sequence comparisons, studies of the
Treffers mutator gene in E. coli, analysis
of the genetic code, and comparative immunology,
to argue that most protein evolution is due
to neutral mutations and genetic drift.According
to Kimura, the theory applies only for evolution
at the molecular level, while phenotypic evolution
is controlled by natural selection, so the
neutral theory does not constitute a true
alternative.
== Combined theories ==
The various alternatives to Darwinian evolution
by natural selection were not necessarily
mutually exclusive. The evolutionary philosophy
of the American palaeontologist Edward Drinker
Cope is a case in point. Cope, a religious
man, began his career denying the possibility
of evolution. In the 1860s, he accepted that
evolution could occur, but, influenced by
Agassiz, rejected natural selection. Cope
accepted instead the theory of recapitulation
of evolutionary history during the growth
of the embryo - that ontogeny recapitulates
phylogeny, which Agassiz believed showed a
divine plan leading straight up to man, in
a pattern revealed both in embryology and
palaeontology. Cope did not go so far, seeing
that evolution created a branching tree of
forms, as Darwin had suggested. Each evolutionary
step was however non-random: the direction
was determined in advance and had a regular
pattern (orthogenesis), and steps were not
adaptive but part of a divine plan (theistic
evolution). This left unanswered the question
of why each step should occur, and Cope switched
his theory to accommodate functional adaptation
for each change. Still rejecting natural selection
as the cause of adaptation, Cope turned to
Lamarckism to provide the force guiding evolution.
Finally, Cope supposed that Lamarckian use
and disuse operated by causing a vitalist
growth-force substance, "bathmism", to be
concentrated in the areas of the body being
most intensively used; in turn, it made these
areas develop at the expense of the rest.
Cope's complex set of beliefs thus assembled
five evolutionary philosophies: recapitulationism,
orthogenesis, theistic evolution, Lamarckism,
and vitalism. Other palaeontologists and field
naturalists continued to hold beliefs combining
orthogenesis and Lamarckism until the modern
synthesis in the 1930s.
== Rebirth of natural selection, with continuing
alternatives ==
By the start of the 20th century, during the
eclipse of Darwinism, biologists were doubtful
of natural selection, but equally were quick
to discount theories such as orthogenesis,
vitalism and Lamarckism which offered no mechanism
for evolution. Mutationism did propose a mechanism,
but it was not generally accepted. The modern
synthesis a generation later, roughly between
1918 and 1932, broadly swept away all the
alternatives to Darwinism, though some including
forms of orthogenesis, epigenetic mechanisms
that resemble Lamarckian inheritance of acquired
characteristics, catastrophism, structuralism,
and mutationism have been revived, such as
through the discovery of molecular mechanisms.Biology
has become Darwinian, but belief in some form
of progress (orthogenesis) remains both in
the public mind and among biologists. Ruse
argues that evolutionary biologists will probably
continue to believe in progress for three
reasons. Firstly, the anthropic principle
demands people able to ask about the process
that led to their own existence, as if they
were the pinnacle of such progress. Secondly,
scientists in general and evolutionists in
particular believe that their work is leading
them progressively closer to a true grasp
of reality, as knowledge increases, and hence
(runs the argument) there is progress in nature
also. Ruse notes in this regard that Richard
Dawkins explicitly compares cultural progress
with memes to biological progress with genes.
Thirdly, evolutionists are self-selected;
they are people, such as the entomologist
and sociobiologist E. O. Wilson, who are interested
in progress to supply a meaning for life.
== See also ==
Coloration evidence for natural selection
History of evolutionary thought
Objections to evolution
Extended evolutionary synthesis
Lysenkoism
== Notes
