In biology, saltation (from Latinsaltus 'leap, jump') is a sudden and large mutational change from one generation to the next, potentially causing single-step speciation. This was historically offered as an alternative to Darwinism. Some forms of mutationism were effectively saltationist, implying large discontinuous jumps.
Speciation, such as by polyploidy
in plants, can sometimes be achieved in a single and in evolutionary
terms sudden step. Evidence exists for various forms of saltation in a
variety of organisms.
Prior to Charles Darwin most evolutionary scientists had been saltationists. Jean-Baptiste Lamarck was a gradualist but similar to other scientists of the period had written that saltational evolution was possible. Étienne Geoffroy Saint-Hilaire
endorsed a theory of saltational evolution that "monstrosities could
become the founding fathers (or mothers) of new species by instantaneous
transition from one form to the next." Geoffroy wrote that environmental pressures could produce sudden transformations to establish new species instantaneously. In 1864 Albert von Kölliker revived Geoffroy's theory that evolution proceeds by large steps, under the name of heterogenesis.
With the publication of On the Origin of Species in 1859 Charles Darwin wrote that most evolutionary changes proceeded gradually but he did not deny the existence of jumps.
From 1860 to 1880 saltation had a minority interest but by 1890 had become a major interest to scientists. In their paper on evolutionary theories in the 20th century Levit et al wrote:
The advocates of saltationism deny the Darwinian idea of
slowly and gradually growing divergence of character as the only source
of evolutionary progress. They would not necessarily completely deny
gradual variation, but claim that cardinally new ‘body plans’ come into
being as a result of saltations (sudden, discontinuous and crucial
changes, for example, the series of macromutations). The latter are
responsible for the sudden appearance of new higher taxa including
classes and orders, while small variation is supposed to be responsible
for the fine adaptations below the species level.
In the early 20th century a mechanism of saltation was proposed as 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. Some of these geneticists developed it into the mutation theory of evolution. There was also a debate over accounts of the evolution of mimicry and if they could be explained by gradualism or saltation. The geneticist Reginald Punnett supported a saltational theory in his book Mimicry in Butterflies (1915).
The mutation theory of evolution 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. This mutationist view of evolution was
later replaced by the reconciliation of Mendelian genetics with natural selection into a gradualistic framework for the neo-Darwinian synthesis. It was the emergence of population thinking in evolution which forced many scientists to adopt gradualism in the early 20th century. According to Ernst Mayr, it wasn't until the development of population genetics
in the neo-Darwinian synthesis in the 1940s that demonstrated the
explanatory power of natural selection that saltational views of
evolution were largely abandoned.
Saltation was originally denied by the "modern synthesis" school of neo-Darwinism which favoured gradual evolution but has since been accepted due to recent evidence in evolutionary biology (see the current status section).In recent years there are some prominent proponents of saltation, including Carl Woese. Woese, and colleagues, suggested that the absence of RNA signature continuum between domains of bacteria, archaea, and eukarya
constitutes a primary indication that the three primary organismal
lineages materialized via one or more major evolutionary saltations from
some universal ancestral state involving dramatic change in cellular
organization that was significant early in the evolution of life, but in
complex organisms gave way to the generally accepted Darwinian
mechanisms. The geneticist Barbara McClintock introduced the idea of "jumping genes", chromosome transpositions that can produce rapid changes in the genome.
Saltational speciation, also known as abrupt speciation, is the discontinuity in a lineage that occurs through genetic mutations, chromosomal aberrations or other evolutionary mechanisms that cause reproductively isolated individuals to establish a new species population. Polyploidy, karyotypicfission, symbiogenesis and lateral gene transfer are possible mechanisms for saltational speciation.
The botanist John Christopher Willis
proposed an early saltationist theory of evolution. He held that
species were formed by large mutations, not gradual evolution by natural
selection.
The German geneticist Richard Goldschmidt
was the first scientist to use the term "hopeful monster". Goldschmidt
thought that small gradual changes could not bridge the hypothetical
divide between microevolution and macroevolution. In his book The Material Basis of Evolution
(1940) he wrote "the change from species to species is not a change
involving more and more additional atomistic changes, but a complete
change of the primary pattern or reaction system into a new one, which
afterwards may again produce intraspecific variation by micromutation."
Goldschmidt believed the large changes in evolution were caused by
macromutations (large mutations). His ideas about macromutations became
known as the hopeful monster hypothesis which is considered a type of
saltational evolution.
Goldschmidt's thesis however was universally rejected and widely ridiculed within the biological community, which favored the neo-Darwinian explanations of R.A. Fisher, J. B. S. Haldane and Sewall Wright. However, there has been a recent interest in the ideas of Goldschmidt in the field of evolutionary developmental biology as some scientists are convinced he was not entirely wrong.
Otto Schindewolf,
a German paleontologist, also supported macromutations as part of his
evolutionary theory. He was known for presenting an alternative
interpretation of the fossil record based on his ideas of orthogenesis,
saltational evolution and extraterrestrial impacts opposed to
gradualism but abandoned the view of macromutations in later
publications.
Søren Løvtrup, a biochemist and embryologist from Denmark, advocated a similar hypothesis of macromutation to Goldschmidt's in 1974. Lovtrup believed that macromutations interfered with various epigenetic
processes, that is, those which affect the causal processes in
biological development. This is in contrast to the gradualistic theory
of micromutations of Neo-Darwinism,
which claims that evolutionary innovations are generally the result of
accumulation of numerous very slight modifications. Lovtrup also
rejected the punctuated equilibria of Stephen Gould and Niles Eldredge,
claiming it was a form of gradualism and not a macromutation theory.
Lovtrup defended many of Darwin's critics including Schindewolf, Mivart, Goldschmidt, and Himmelfarb. Mae Wan Ho described Lovtrup's theory as similar to the hopeful monster theory of Richard Goldschmidt.
Goldschmidt presented two mechanisms for how hopeful monsters
might work. One mechanism, involved “systemic mutations”, rejected the
classical gene
concept and is no longer considered by modern science; however, his
second mechanism involved “developmental macromutations” in “rate genes”
or “controlling genes” that change early development and thus cause
large effects in the adult phenotype. These kind of mutations are
similar to the ones considered in contemporary evolutionary developmental biology.
On the subject of Goldschmidt Donald Prothero in his book Evolution: What the Fossils Say and Why It Matters (2007) wrote:
The past twenty years have vindicated Goldschmidt to some
degree. With the discovery of the importance of regulatory genes, we
realize that he was ahead of his time in focusing on the importance of a
few genes controlling big changes in the organisms, not small-scales
changes in the entire genome as neo-Darwinians thought. In addition, the
hopeful monster problem is not so insurmountable after all. Embryology
has shown that if you affect an entire population of developing embryos
with a stress (such as a heat shock) it can cause many embryos to go
through the same new pathway of embryonic development, and then they all
become hopeful monsters when they reach reproductive age.
In 2008 evolutionary biologist Olivia Judson in her article The Monster Is Back, and It’s Hopeful listed some examples which may support the hopeful monster hypothesis and an article published in the journal Nature in 2010 titled Evolution: Revenge of the Hopeful Monster reported that studies in stickleback populations in a British Columbia lake and bacteria
populations in a Michigan lab have shown that large individual genetic
changes can have vast effects on organisms "without dooming it to the
evolutionary rubbish heap". According to the article "Single-gene
changes that confer a large adaptive value do happen: they are not rare,
they are not doomed and, when competing with small-effect mutations,
they tend to win. But small-effect mutations still matter — a lot. They
provide essential fine-tuning and sometimes pave the way for explosive
evolution to follow."
A paper by (Page et al. 2010) have written that the Mexican axolotl (Ambystoma mexicanum)
could be classified as a hopeful monster as it exhibits an adaptive and
derived mode of development that has evolved rapidly and independently
among tiger salamanders. According to the paper there has been an
interest in aspects of the hopeful monster hypothesis in recent years:
Goldschmidt proposed that mutations
occasionally yield individuals within populations that deviate
radically from the norm and referred to such individuals as "hopeful
monsters". If the novel phenotypes of hopeful monsters arise under the
right environmental circumstances, they may become fixed, and the
population will found a new species. While this idea was discounted
during the Modern synthesis,
aspects of the hopeful monster hypothesis have been substantiated in
recent years. For example, it is clear that dramatic changes in
phenotype can occur from few mutations of key developmental genes and
phenotypic differences among species often map to relatively few genetic
factors. These findings are motivating renewed interest in the study of
hopeful monsters and the perspectives they can provide about the
evolution of development. In contrast to mutants that are created in the
lab, hopeful monsters have been shaped by natural selection and are therefore more likely to reveal mechanisms of adaptive evolution.
Günter Theissen, a German professor of genetics, has classified homeotic
mutants as "hopeful monsters" and has documented many examples of
animal and plant lineages that may have originated in that way.
American biologist Michael Freeling has proposed "balanced gene drive"
as a saltational mechanism in the mutationist tradition, which could
explain trends involving morphological complexity in plant and animal eukaryotic lineages.
Current status
Known mechanisms
Examples of saltational evolution include cases of stabilized hybrids that can reproduce without crossing (such as allotetraploids) and cases of symbiogenesis. Both gene duplication and lateral gene transfer have the capacity to bring about relatively large changes that are saltational. Polyploidy
(most common in plants but not unknown in animals) is saltational: a
significant change (in gene numbers) can result in speciation in a
single generation.
Claimed instances
Evidence of phenotypic saltation has been found in the centipede and some scientists have suggested there is evidence for independent instances of saltational evolution in sphinx moths. Saltational changes have occurred in the buccal cavity of the roundworm Caenorhabditis elegans. Some processes of epigenetic inheritance can also produce changes that are saltational. There has been a controversy over whether mimicry in butterflies and other insects can be explained by gradual or saltational evolution. According to Norrström (2006) there is evidence for saltation in some cases of mimicry. The endosymbiotic theory is considered to be a type of saltational evolution. Symonds and Elgar, 2004 have suggested that pheromone evolution in bark beetles is characterized by large saltational shifts. The mode of evolution of sex pheromones in Bactrocera has occurred by rapid saltational changes associated with speciation followed by gradual divergence thereafter. Saltational speciation has been recognized in the genus Clarkia (Lewis, 1966). It has been suggested (Carr, 1980, 2000) that the Calycadenia pauciflora could have originated directly from an ancestral race through a single saltational event involving multiple chromosome breaks. Specific cases of homeosis in flowers can be caused by saltational evolution. In a study of divergent orchid
flowers (Bateman and DiMichele, 2002) wrote how simple homeotic morphs
in a population can lead to newly established forms that become fixed
and ultimately lead to new species.
They described the transformation as a saltational evolutionary
process, where a mutation of key developmental genes leads to a profound
phenotypic change, producing a new evolutionary lineage within a
species.
Explanations
Reviewing the history of macroevolutionary theories, the American evolutionary biologist Douglas J. Futuyma notes that since 1970, two very different alternatives to Darwinian gradualism have been proposed, both by Stephen Jay Gould: mutationism, and punctuated equilibria.
Gould's macromutation theory gave a nod to his predecessor with an
envisaged "Goldschmidt break" between evolution within a species and
speciation. His advocacy of Goldschmidt was attacked with "highly
unflattering comments" by B. Charlesworth and Templeton. Futuyma concludes, following other biologists reviewing the field such as K.Sterelny and A. Minelli,
that essentially all the claims of evolution driven by large mutations
could be explained within the Darwinian evolutionary synthesis.
The mediaeval great chain of being as a staircase, implying the possibility of progress: Ramon Lull's Ladder of Ascent and Descent of the Mind, 1305
Alternatives to Darwinian evolution have been proposed by scholars investigating biology 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 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.
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 place; 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 (orthogenesis) 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.
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.
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.
Characteristics of groups do not fit on linear scale, as Aristotle observed. Teleology and homology recognised but not connected as evolution with adaptation; not spiritual
Varieties of theory from Ancient Egypt onwards, often spiritual. Dropped from biology with chemical synthesis of organic molecules e.g. of urea in 1828
Only at molecular level; fits in with natural selection at higher levels. Observed 'molecular clock' supports neutral drift; not a rival to natural selection, as does not cause evolution of phenotype
Lacked mechanisms of mutation and heredity until birth of genetics, 1900; Darwin instead proposed pangenesis and some degree of inheritance of acquired characteristics
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."
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." In the 20th century, theistic evolution could take other forms, such as the orthogenesis of Teilhard de Chardin.
Orthogenesis or Progressionism 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.
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 paleontologistsEdward Drinker Cope and Alpheus Hyatt, and the American entomologistAlpheus 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 is the hypothesis, argued by the French anatomist and paleontologistGeorges 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.
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 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).
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 mutantalleles 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
Multiple
explanations have been offered since the 19th century for how evolution
took place, given that many scientists initially had objections to
natural selection (dashed orange arrows). Many of these theories led
(blue arrows) to some form of directed evolution (orthogenesis), with or without invoking divine control (dotted blue arrows) directly or indirectly. For example, evolutionists like Edward Drinker Cope believed in a combination of theistic evolution, Lamarckism, vitalism, and orthogenesis, represented by the sequence of arrows on the extreme left of the diagram.
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.
1: directional selection: a single extreme phenotype favoured. 2, stabilizing selection: intermediate favoured over extremes. 3: disruptive selection: extremes favoured over intermediate. X-axis: phenotypic trait Y-axis: number of organisms Group A: original population Group B: after selection
Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait
value. This is thought to be the most common mechanism of action for
natural selection because most traits do not appear to change
drastically over time.
Stabilizing selection commonly uses negative selection (a.k.a.
purifying selection) to select against extreme values of the character.
Stabilizing selection is the opposite of disruptive selection.
Instead of favoring individuals with extreme phenotypes, it favors the
intermediate variants. Stabilizing selection tends to remove the more
severe phenotypes, resulting in the reproductive success of the norm or average phenotypes. This means that most common phenotype in the population is selected for and continues to dominate in future generations.
Depending
on the environmental conditions, a wolf may have an advantage over
wolves with other variations of fur color. Wolves with fur colors that
do not camouflage appropriately with the environmental conditions will
be spotted more easily by the deer, resulting in them not being able to
sneak up on the deer (leading to natural selection).
History
The Russian evolutionary biologist Ivan Schmalhausen
founded the theory of stabilizing selection, publishing a paper in
Russian titled "Stabilizing selection and its place among factors of
evolution" in 1941 and a monograph "Factors of Evolution: The Theory of
Stabilizing Selection" in 1945.
Influence on population structure
Stabilizing
selection causes the narrowing of the phenotypes seen in a population.
This is because the extreme phenotypes are selected against, causing
reduced survival in organisms with those traits. This results in a
population consisting of fewer phenotypes, with most traits representing
the mean value of the population. This narrowing of phenotypes causes a
reduction in genetic diversity in a population.
Maintaining genetic variation is essential for the survival of a
population because it is what allows them to evolve over time. In order
for a population to adapt to changing environmental conditions they must
have enough genetic diversity to select for new traits as they become
favorable.
Analyzing stabilizing selection
There
are four primary types of data used to quantify stabilizing selection
in a population. The first type of data is an estimation of fitness of
different phenotypes within a single generation. Quantifying fitness in a
single generation creates predictions for the expected fate of
selection. The second type of data is changes in allelic frequencies or
phenotypes across different generations. This allows quantification of
change in prevalence of a certain phenotype, indicating the type of
selection. The third type of data is differences in allelic frequencies
across space. This compares selection occurring in different populations
and environmental conditions. The fourth type of data is DNA sequences
from the genes contributing to observes phenotypic differences. The
combination of these four types of data allow population studies that
can identify the type of selection occurring and quantify the extent of
selection.
However, a meta-analysis of studies that measured selection in
the wild failed to find an overall trend for stabilizing selection.
The reason can be that methods for detecting stabilizing selection are
complex. They can involve studying the changes that causes natural
selection in the mean and variance of the trait, or measuring fitness
for a range of different phenotypes
under natural conditions and examining the relationship between these
fitness measurements and the trait value, but analysis and
interpretation of the results is not straightforward.
Examples
The
most common form of stabilizing selection is based on phenotypes of a
population. In phenotype based stabilizing selection, the mean value of a
phenotype is selected for, resulting a decrease in the phenotypic
variation found in a population.
Humans
Stabilizing selection is the most common form of nonlinear selection (non-directional) in humans.
There are few examples of genes with direct evidence of stabilizing
selection in humans. However, most quantitative traits (height,
birthweight, schizophrenia) are thought to be under stabilizing
selection, due to their polygenicity and the distribution of the
phenotypes throughout human populations.
Birth Weight − A classic example of this is human birth weight.
Babies of low weight lose heat more quickly and get ill from infectious
diseases more easily, whereas babies of large body weight are more
difficult to deliver through the pelvis. Infants of a more medium weight
survive much more often. For the larger or smaller babies, the baby
mortality rate is much higher. The bell curve of the human population peaks at a birth weight that the newly born babies exhibit the minimum death rate.
Plants
Height
− Another example of a trait, that might be acted on by stabilizing
selection, is plant height. A plant that is too short may not be able to
compete with other plants for sunlight. However, extremely tall plants
may be more susceptible to wind damage. Combined, these two selection
pressures select to maintain plants of medium height. The number of
plants of medium height will increase while the numbers of short and
tall plants will decrease.
Cacti Spine Number − Desert populations of spiny cacti experience predation by peccaries,
which consume the fleshy part of the cactus. This can be prevented by
increasing the number of spines on the cactus. However, there is also a
selection pressure in the opposite direction because there is a
parasitic insect that will lay its eggs in spines if they are densely
populated. This means that in order to manage both of these selection
pressures the cacti experiences stabilizing selection to balance the
appropriate number of spines to survive these different threats.
Insects
Bicyclus anynana with wing eyespot, which experiences stabilizing selection to avoid predation.Butterfly's Winged Eyespots − The African butterfly Bicyclus anynana exhibits stabilizing selection with its wing eyespots.
It has been suggested that the circular eyespots positioned on the
wings are favoured functionally compared to other shapes and sizes.
Gall Size − The Eurosta solidaginis fly lays its eggs on the tip of plants, which then encase the larvae in a protective gall.
The size of this gall is under stabilizing selection, as determined by
predation. These larvae are under threat from parasitic wasps, which lay
a single egg in galls containing the flies. The single wasp offspring
then consumes the fly larvae to survive. Therefore, a larger gall is
favored to allow more places for larvae to hide from the wasp. However,
larger galls attract a different type of predation from birds, as they
can penetrate large galls with their beak. Therefore, the optimal gall
is moderately sized in order to avoid predation from both birds and
wasps.
Birds
Clutch
Size − The number of eggs laid by a female bird (clutch size) is
typically under stabilizing selection. This is because the female must
lay as many eggs as possible to maximize the number of offspring.
However, they can only lay as many eggs as they can support with their
own resources. Laying too many eggs could expend all of the energy of
the mother bird causing her to die and the death of the chicks.
Additionally, once the eggs hatch the mother must be able to obtain
enough resources to keep all of the chicks alive. Therefore, the mother
typically lays a moderate amount of eggs in order to increase offspring
survival and maximize the number of offspring.
Mammals
The Siberian husky experiences stabilizing selection in terms of their leg muscles, allowing them to be strong but light.The
Siberian husky experiences stabilizing selection in terms of their leg
muscles. These dogs have to have enough muscle in order to pull sleds
and move quickly. However, they also must be light enough to stay on top
of the snow. This means that the leg muscles of the husky are most fit
when they are moderately sized, to balance their strength and their
weight.
.jpg)
,_by_Th%C3%A9r%C3%A8se_Schwartze_(1851-1918).jpg)
.jpg)
