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.
The synthesis was defined differently by its founders, with Ernst Mayr in 1959, G. Ledyard Stebbins in 1966, and Theodosius Dobzhansky
in 1974 offering differing basic postulates, though they all include
natural selection, working on heritable variation supplied by mutation.
Other major figures in the synthesis included E. B. Ford, Bernhard Rensch, Ivan Schmalhausen, and George Gaylord Simpson. An early event in the modern synthesis was R. A. Fisher's 1918 paper on mathematical population genetics, though William Bateson, and separately Udny Yule, had already started to show how Mendelian genetics could work in evolution in 1902.
Darwin's pangenesis theory. Every part of the body emits tiny gemmules which migrate to the gonads
and contribute to the next generation via the fertilised egg. Changes
to the body during an organism's life would be inherited, as in Lamarckism.
Charles Darwin's 1859 book, On the Origin of Species, convinced most biologists that evolution had occurred, but not that natural selection was its primary mechanism. In the 19th and early 20th centuries, variations of Lamarckism (inheritance of acquired characteristics), orthogenesis (progressive evolution), saltationism (evolution by jumps) and mutationism (evolution driven by mutations) were discussed as alternatives. Darwin himself had sympathy for Lamarckism, but Alfred Russel Wallace advocated natural selection and totally rejected Lamarckism. In 1880, Samuel Butler labelled Wallace's view neo-Darwinism.
From the 1880s onwards, biologists grew skeptical of Darwinian evolution. This eclipse of Darwinism (in Julian Huxley's words) grew out of the weaknesses in Darwin's account, with respect to his view of inheritance. Darwin believed in blending inheritance, which implied that any new variation, even if beneficial, would be weakened by 50% at each generation, as the engineer Fleeming Jenkin noted in 1868.
This in turn meant that small variations would not survive long enough
to be selected. Blending would therefore directly oppose natural
selection. In addition, Darwin and others considered Lamarckian
inheritance of acquired characteristics entirely possible, and Darwin's
1868 theory of pangenesis,
with contributions to the next generation (gemmules) flowing from all
parts of the body, actually implied Lamarckism as well as blending.
August Weismann's germ plasm theory. The hereditary material, the germplasm, is confined to the gonads and the gametes. Somatic cells (of the body) develop afresh in each generation from the germplasm.
August Weismann's idea, set out in his 1892 book Das Keimplasma: eine Theorie der Vererbung ("The Germ Plasm: a Theory of Inheritance"), was that the hereditary material, which he called the germ plasm, and the rest of the body (the soma)
had a one-way relationship: the germ-plasm formed the body, but the
body did not influence the germ-plasm, except indirectly in its
participation in a population subject to natural selection. If correct,
this made Darwin's pangenesis wrong, and Lamarckian inheritance
impossible. His experiment on mice, cutting off their tails and showing
that their offspring had normal tails, demonstrated that inheritance was
'hard'. He argued strongly and dogmatically
for Darwinism and against Lamarckism, polarising opinions among other
scientists. This increased anti-Darwinian feeling, contributing to its
eclipse.
While carrying out breeding experiments to clarify the mechanism of inheritance in 1900, Hugo de Vries and Carl Correns independently rediscovered Gregor Mendel's work. News of this reached William Bateson in England, who reported on the paper during a presentation to the Royal Horticultural Society in May 1900. In Mendelian inheritance,
the contributions of each parent retain their integrity, rather than
blending with the contribution of the other parent. In the case of a
cross between two true-breeding varieties such as Mendel's round and
wrinkled peas, the first-generation offspring are all alike, in this
case, all round. Allowing these to cross, the original characteristics
reappear (segregation): about 3/4 of their offspring are round, 1/4
wrinkled. There is a discontinuity between the appearance of the
offspring; de Vries coined the term allele for a variant form of an inherited characteristic.
This reinforced a major division of thought, already present in the
1890s, between gradualists who followed Darwin, and saltationists such
as Bateson.
The two schools were the Mendelians, such as Bateson and de
Vries, who favoured mutationism, evolution driven by mutation, based on
genes whose alleles segregated discretely like Mendel's peas;and the biometric school, led by Karl Pearson and Walter Weldon.
The biometricians argued vigorously against mutationism, saying that
empirical evidence indicated that variation was continuous in most
organisms, not discrete as Mendelism seemed to predict; they wrongly
believed that Mendelism inevitably implied evolution in discontinuous
jumps.
A traditional view is that the biometricians and the Mendelians
rejected natural selection and argued for their separate theories for 20
years, the debate only resolved by the development of population
genetics.
A more recent view is that Bateson, de Vries, Thomas Hunt Morgan and Reginald Punnett
had by 1918 formed a synthesis of Mendelism and mutationism. The
understanding achieved by these geneticists spanned the action of
natural selection on alleles (alternative forms of a gene), the Hardy–Weinberg equilibrium,
the evolution of continuously-varying traits (like height), and the
probability that a new mutation will become fixed. In this view, the
early geneticists accepted natural selection but rejected Darwin's
non-Mendelian ideas about variation and heredity, and the synthesis
began soon after 1900.
The traditional claim that Mendelians rejected the idea of continuous
variation is false; as early as 1902, Bateson and Saunders wrote that
"If there were even so few as, say, four or five pairs of possible
allelomorphs, the various homo- and heterozygous combinations might, on
seriation, give so near an approach to a continuous curve, that the
purity of the elements would be unsuspected". Also in 1902, the statistician Udny Yule
showed mathematically that given multiple factors, Mendel's theory
enabled continuous variation. Yule criticised Bateson's approach as
confrontational, but failed to prevent the Mendelians and the biometricians from falling out.
Castle's hooded rats, 1911
Starting in 1906, William Castle carried out a long study of the effect of selection on coat colour in rats. The piebald or hooded pattern was recessive
to the grey wild type. He crossed hooded rats with both wild and
"Irish" types, and then back-crossed the offspring with pure hooded
rats. The dark stripe on the back was bigger. He then tried selecting
different groups for bigger or smaller stripes for 5 generations and
found that it was possible to change the characteristics considerably
beyond the initial range of variation. This effectively refuted de
Vries's claim that continuous variation was caused by the environment
and could not be inherited. By 1911, Castle noted that the results could
be explained by Darwinian selection on a heritable variation of a
sufficient number of Mendelian genes.
Thomas Hunt Morgan began his career in genetics as a saltationist
and started out trying to demonstrate that mutations could produce new
species in fruit flies. However, the experimental work at his lab with
the fruit fly, Drosophila melanogaster
showed that rather than creating new species in a single step,
mutations increased the supply of genetic variation in the population.
By 1912, after years of work on the genetics of fruit flies, Morgan
showed that these insects had many small Mendelian factors (discovered
as mutant flies) on which Darwinian evolution could work as if the
variation was fully continuous. The way was open for geneticists to
conclude that Mendelism supported Darwinism.
The theoretical biologist and philosopher of biologyJoseph Henry Woodger led the introduction of positivism into biology with his 1929 book Biological Principles. He saw a mature science as being characterised by a framework of hypotheses that could be verified by facts established by experiments. He criticised the traditional natural history style of biology, including the study of evolution, as immature science, since it relied on narrative. Woodger set out to play the role of Robert Boyle's 1661 Sceptical Chymist, intending to convert the subject of biology into a formal, unified science, and ultimately, following the Vienna Circle of logical positivists like Otto Neurath and Rudolf Carnap, to reduce biology to physics and chemistry. His efforts stimulated the biologist J. B. S. Haldane
to push for the axiomatisation of biology, and by influencing thinkers
such as Huxley, helped to bring about the modern synthesis.
The positivist climate made natural history unfashionable, and in
America, research and university-level teaching on evolution declined
almost to nothing by the late 1930s. The Harvard physiologist William John Crozier told his students that evolution was not even a science: "You can't experiment with two million years!"
The tide of opinion turned with the adoption of mathematical modelling and controlled experimentation in population genetics, combining genetics, ecology and evolution in a framework acceptable to positivism.
Elements of the synthesis
Fisher and Haldane's mathematical population genetics, 1918–1930
In the 1920s, a series of papers by J. B. S. Haldane analyzed real-world examples of natural selection, such as the evolution of industrial melanism in peppered moths. and showed that natural selection could work even faster than Fisher had assumed.
Both of these scholars, and others, such as Dobzhansky and Wright,
wanted to raise biology to the standards of the physical sciences by
basing it on mathematical modeling and empirical testing. Natural
selection, once considered unverifiable, was becoming predictable,
measurable, and testable.
De Beer's embryology, 1930
The traditional view is that developmental biology played little part in the modern synthesis, but in his 1930 book Embryos and Ancestors, the evolutionary embryologist Gavin de Beer anticipated evolutionary developmental biology by showing that evolution could occur by heterochrony, such as in the retention of juvenile features in the adult. This, de Beer argued, could cause apparently sudden changes in the fossil record,
since embryos fossilise poorly. As the gaps in the fossil record had
been used as an argument against Darwin's gradualist evolution, de
Beer's explanation supported the Darwinian position.
However, despite de Beer, the modern synthesis largely ignored embryonic
development when explaining the form of organisms, since population
genetics appeared to be an adequate explanation of how such forms
evolved.
The population geneticist Sewall Wright focused on combinations of genes that interacted as complexes, and the effects of inbreeding on small relatively isolated populations, which could be subject to genetic drift. In a 1932 paper, he introduced the concept of an adaptive landscape
in which phenomena such as cross breeding and genetic drift in small
populations could push them away from adaptive peaks, which would in
turn allow natural selection to push them towards new adaptive peaks.
Wright's model would appeal to field naturalists such as Theodosius
Dobzhansky and Ernst Mayr who were becoming aware of the importance of
geographical isolation in real world populations. The work of Fisher, Haldane and Wright helped to found the discipline of theoretical population genetics.
Theodosius Dobzhansky, an immigrant from the Soviet Union to the United States,
who had been a postdoctoral worker in Morgan's fruit fly lab, was one
of the first to apply genetics to natural populations. He worked mostly
with Drosophila pseudoobscura.
He says pointedly: "Russia has a variety of climates from the Arctic to
sub-tropical... Exclusively laboratory workers who neither possess nor
wish to have any knowledge of living beings in nature were and are in a
minority." Not surprisingly, there were other Russian geneticists with similar ideas, though for some time their work was known to only a few in the West. His 1937 work Genetics and the Origin of Species
was a key step in bridging the gap between population geneticists and
field naturalists. It presented the conclusions reached by Fisher,
Haldane, and especially Wright in their highly mathematical papers in a
form that was easily accessible to others.
Further, Dobzhansky asserted the physicality, and hence the biological
reality, of the mechanisms of inheritance: that evolution was based on
material genes, arranged in a string on physical hereditary structures,
the chromosomes, and linked
more or less strongly to each other according to their actual physical
distances on the chromosomes. As with Haldane and Fisher, Dobzhansky's
"evolutionary genetics" was a genuine science, now unifying cell biology, genetics, and both micro and macroevolution.
His work emphasized that real-world populations had far more genetic
variability than the early population geneticists had assumed in their
models and that genetically distinct sub-populations were important.
Dobzhansky argued that natural selection worked to maintain genetic
diversity as well as by driving change. He was influenced by his
exposure in the 1920s to the work of Sergei Chetverikov,
who had looked at the role of recessive genes in maintaining a
reservoir of genetic variability in a population, before his work was
shut down by the rise of Lysenkoism in the Soviet Union.
By 1937, Dobzhansky was able to argue that mutations were the main
source of evolutionary changes and variability, along with chromosome
rearrangements, effects of genes on their neighbours during development,
and polyploidy. Next, genetic drift (he used the term in 1941),
selection, migration, and geographical isolation could change gene
frequencies. Thirdly, mechanisms like ecological or sexual isolation and
hybrid sterility could fix the results of the earlier processes.
E. B. Ford was an experimental naturalist who wanted to test natural selection in nature, virtually inventing the field of ecological genetics.
His work on natural selection in wild populations of butterflies and
moths was the first to show that predictions made by R. A. Fisher were
correct. In 1940, he was the first to describe and define genetic polymorphism, and to predict that human blood group polymorphisms might be maintained in the population by providing some protection against disease. His 1949 book Mendelism and Evolution helped to persuade Dobzhansky to change the emphasis in the third edition of his famous textbook Genetics and the Origin of Species from drift to selection.
Ivan Schmalhausen developed the theory of stabilizing selection,
the idea that selection can preserve a trait at some value, 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. He developed it from J. M. Baldwin's 1902 concept that changes
induced by the environment will ultimately be replaced by hereditary
changes (including the Baldwin effect
on behaviour), following that theory's implications to their Darwinian
conclusion, and bringing him into conflict with Lysenkoism. Schmalhausen
observed that stabilizing selection would remove most variations from
the norm, most mutations being harmful. Dobzhansky called the work "an important missing link in the modern view of evolution".
In 1942, Julian Huxley's serious but popularisingEvolution: The Modern Synthesis
introduced a name for the synthesis and intentionally set out to
promote a "synthetic point of view" on the evolutionary process. He
imagined a wide synthesis of many sciences: genetics, developmental
physiology, ecology, systematics, palaeontology, cytology, and
mathematical analysis of biology, and assumed that evolution would
proceed differently in different groups of organisms according to how
their genetic material was organised and their strategies for
reproduction, leading to progressive but varying evolutionary trends. His vision was of an "evolutionary humanism",
with a system of ethics and a meaningful place for "Man" in the world
grounded in a unified theory of evolution which would demonstrate
progress leading to humanity at its summit. Natural selection was in his
view a "fact of nature capable of verification by observation and
experiment", while the "period of synthesis" of the 1920s and 1930s had
formed a "more unified science", rivalling physics and enabling the "rebirth of Darwinism".
However, the book was not the research text that it appeared to be. In the view of the philosopher of science Michael Ruse, and in Huxley's own opinion, Huxley was "a generalist, a synthesizer of ideas, rather than a specialist".
Ruse observes that Huxley wrote as if he were adding empirical evidence
to the mathematical framework established by Fisher and the population
geneticists, but that this was not so. Huxley avoided mathematics, for
instance not even mentioning Fisher's fundamental theorem of natural selection.
Instead, Huxley used a mass of examples to demonstrate that natural
selection is powerful and that it works on Mendelian genes. The book was
successful in its goal of persuading readers of the reality of
evolution, effectively illustrating topics such as island biogeography, speciation, and competition. Huxley further showed that the appearance of long-term orthogenetic trends – predictable directions for evolution – in the fossil record were readily explained as allometric growth
(since parts are interconnected). All the same, Huxley did not reject
orthogenesis out of hand, but maintained a belief in progress all his
life, with Homo sapiens as the endpoint, and he had since 1912 been influenced by the vitalist philosopher Henri Bergson, though in public he maintained an atheistic position on evolution.
Huxley's belief in progress within evolution and evolutionary humanism
was shared in various forms by Dobzhansky, Mayr, Simpson and Stebbins,
all of them writing about "the future of Mankind". Both Huxley and
Dobzhansky admired the palaeontologist priest Pierre Teilhard de Chardin, Huxley writing the introduction to Teilhard's 1955 book on orthogenesis, The Phenomenon of Man. This vision required evolution to be seen as the central and guiding principle of biology.
Ernst Mayr's key contribution to the synthesis was Systematics and the Origin of Species, published in 1942.
It asserted the importance of and set out to explain population
variation in evolutionary processes including speciation. He analysed in
particular the effects of polytypic species, geographic variation, and isolation by geographic and other means. Mayr emphasized the importance of allopatric speciation, where geographically isolated sub-populations diverge so far that reproductive isolation occurs. He was skeptical of the reality of sympatric speciation
believing that geographical isolation was a prerequisite for building
up intrinsic (reproductive) isolating mechanisms. Mayr also introduced
the biological species concept
that defined a species as a group of interbreeding or potentially
interbreeding populations that were reproductively isolated from all
other populations.Before he left Germany for the United States in 1930, Mayr had been influenced by the work of the German biologist Bernhard Rensch,
who in the 1920s had analyzed the geographic distribution of polytypic
species, paying particular attention to how variations between
populations correlated with factors such as differences in climate.
George Gaylord Simpson was responsible for showing that the modern synthesis was compatible with palaeontology in his 1944 book Tempo and Mode in Evolution.
Simpson's work was crucial because so many palaeontologists had
disagreed, in some cases vigorously, with the idea that natural
selection was the main mechanism of evolution. It showed that the trends
of linear progression (in for example the evolution of the horse) that earlier palaeontologists had used as support for neo-Lamarckism and orthogenesis did not hold up under careful examination. Instead, the fossil record was consistent with the irregular, branching, and non-directional pattern predicted by the modern synthesis.
Society for the Study of Evolution, 1946
During World War II,
Mayr edited a series of bulletins of the Committee on Common Problems
of Genetics, Paleontology, and Systematics, formed in 1943, reporting on
discussions of a "synthetic attack" on the interdisciplinary problems
of evolution. In 1946, the committee became the Society for the Study of
Evolution, with Mayr, Dobzhansky and Sewall Wright the first of the
signatories. Mayr became the editor of its journal, Evolution.
From Mayr and Dobzhansky's point of view, suggests the historian of
science Betty Smocovitis, Darwinism was reborn, evolutionary biology was
legitimised, and genetics and evolution were synthesised into a newly
unified science. Everything fitted into the new framework, except
"heretics" like Richard Goldschmidt who annoyed Mayr and Dobzhansky by insisting on the possibility of speciation by macromutation, creating "hopeful monsters". The result was "bitter controversy".
Speciation via polyploidy: a diploid cell may fail to separate during meiosis, producing diploid gametes, which self-fertilize to produce a fertile tetraploid zygote that cannot interbreed with its parent species.
Stebbins's botany, 1950
The botanist G. Ledyard Stebbins extended the synthesis to encompass botany. He described the important effects on speciation of hybridization and polyploidy in plants in his 1950 book Variation and Evolution in Plants.
These permitted evolution to proceed rapidly at times, polyploidy in
particular evidently being able to create new species effectively
instantaneously.
Definitions by the founders
The
modern synthesis was defined differently by its various founders, with
differing numbers of basic postulates, as shown in the table.
Definitions of the modern synthesis by its founders, as they numbered them
After the synthesis, evolutionary biology continued to develop with major contributions from workers including W. D. Hamilton, George C. Williams, E. O. Wilson, Edward B. Lewis and others.
In 1964, W. D. Hamilton published two papers on "The Genetical Evolution of Social Behaviour". These defined inclusive fitness
as the number of offspring equivalents an individual rears, rescues or
otherwise supports through its behaviour. This was contrasted with
personal reproductive fitness, the number of offspring that the
individual directly begets. Hamilton, and others such as John Maynard Smith,
argued that a gene's success consisted in maximising the number of
copies of itself, either by begetting them or by indirectly encouraging
begetting by related individuals who shared the gene, the theory of kin selection.
In 1975, E. O. Wilson published his controversial book Sociobiology: The New Synthesis, the subtitle alluding to the modern synthesis
as he attempted to bring the study of animal society into the
evolutionary fold. This appeared radically new, although Wilson was
following Darwin, Fisher, Dawkins and others. Critics such as Gerhard Lenski
noted that he was following Huxley, Simpson and Dobzhansky's approach,
which Lenski considered needlessly reductive as far as human society was
concerned. By 2000, the proposed discipline of sociobiology had morphed into the relatively well-accepted discipline of evolutionary psychology.
In 1977, recombinant DNA technology enabled biologists to start to explore the genetic control of development. The growth of evolutionary developmental biology from 1978, when Edward B. Lewis discovered homeotic genes, showed that many so-called toolkit genes
act to regulate development, influencing the expression of other genes.
It also revealed that some of the regulatory genes are extremely
ancient, so that animals as different as insects and mammals share
control mechanisms; for example, the Pax6 gene is involved in forming the eyes of mice and of fruit flies. Such deep homology provided strong evidence for evolution and indicated the paths that evolution had taken.
Later syntheses
In 1982, a historical note on a series of evolutionary biology books
could state without qualification that evolution is the central
organizing principle of biology. Smocovitis commented on this that "What
the architects of the synthesis had worked to construct had by 1982
become a matter of fact", adding in a footnote that "the centrality of
evolution had thus been rendered tacit knowledge, part of the received wisdom of the profession".
By the late 20th century, however, the modern synthesis was
showing its age, and fresh syntheses to remedy its defects and fill in
its gaps were proposed from different directions. These have included
such diverse fields as the study of society, developmental biology, epigenetics, molecular biology, microbiology, genomics, symbiogenesis, and horizontal gene transfer. The physiologist Denis Noble
argues that these additions render neo-Darwinism in the sense of the
early 20th century's modern synthesis "at the least, incomplete as a
theory of evolution", and one that has been falsified by later biological research.
Michael Rose and Todd Oakley note that evolutionary biology, formerly divided and "Balkanized",
has been brought together by genomics. It has in their view discarded
at least five common assumptions from the modern synthesis, namely that
the genome is always a well-organised set of genes; that each gene has a
single function; that species are well adapted biochemically to their
ecological niches; that species are the durable units of evolution, and
all levels from organism to organ, cell and molecule within the species
are characteristic of it; and that the design of every organism and cell
is efficient. They argue that the "new biology" integrates genomics, bioinformatics, and evolutionary genetics into a general-purpose toolkit for a "Postmodern Synthesis".
In 2009, Darwin's 200th anniversary, the Origin of Species' 150th, and the 200th of Lamarck's "early evolutionary synthesis", Philosophie Zoologique, the evolutionary biologist Eugene Koonin stated that while "the edifice of the [early 20th century] Modern Synthesis has crumbled, apparently, beyond repair",
a new 21st-century synthesis could be glimpsed. Three interlocking
revolutions had, he argued, taken place in evolutionary biology:
molecular, microbiological, and genomic. The molecular revolution included the neutral theory, that most mutations are neutral and that negative selection happens more often than the positive form, and that all current life evolved from a single common ancestor. In microbiology, the synthesis has expanded to cover the prokaryotes, using ribosomal RNA to form a tree of life. Finally, genomics brought together the molecular and microbiological syntheses - in particular, horizontal gene transfer between bacteria
shows that prokaryotes can freely share genes. Many of these points had
already been made by other researchers such as Ulrich Kutschera and Karl J. Niklas.
Towards a replacement synthesis
Inputs
to the modern synthesis, with other topics (inverted colours) such as
developmental biology that were not joined with evolutionary biology
until the turn of the 21st century
Biologists, alongside scholars of the history and philosophy of
biology, have continued to debate the need for, and possible nature of, a
replacement synthesis. For example, in 2017 Philippe Huneman and Denis
M. Walsh stated in their book Challenging the Modern Synthesis
that numerous theorists had pointed out that the disciplines of
embryological developmental theory, morphology, and ecology had been
omitted. They noted that all such arguments amounted to a continuing
desire to replace the modern synthesis with one that united "all
biological fields of research related to evolution, adaptation, and
diversity in a single theoretical framework."
They observed further that there are two groups of challenges to the
way the modern synthesis viewed inheritance. The first is that other
modes such as epigenetic inheritance, phenotypic plasticity, the Baldwin effect, and the maternal effect
allow new characteristics to arise and be passed on and for the genes
to catch up with the new adaptations later. The second is that all such
mechanisms are part, not of an inheritance system, but a developmental system:
the fundamental unit is not a discrete selfishly competing gene, but a
collaborating system that works at all levels from genes and cells to
organisms and cultures to guide evolution. The molecular biologist Sean B. Carroll has commented that had Huxley had access to evolutionary developmental biology,
"embryology would have been a cornerstone of his Modern Synthesis, and
so evo-devo is today a key element of a more complete, expanded
evolutionary synthesis."
Historiography
Looking back at the conflicting accounts of the modern synthesis, the historian Betty Smocovitis notes in her 1996 book Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology
that both historians and philosophers of biology have attempted to
grasp its scientific meaning, but have found it "a moving target"; the only thing they agreed on was that it was a historical event. In her words
"by
the late 1980s the notoriety of the evolutionary synthesis was
recognized ... So notorious did 'the synthesis' become, that few serious
historically minded analysts would touch the subject, let alone know
where to begin to sort through the interpretive mess left behind by the
numerous critics and commentators".
Charles Darwin's pangenesis theory postulated that every part of the body emits tiny particles called gemmules which migrate to the gonads
and are transferred to offspring. Gemmules were thought to develop into
their associated body parts as offspring matures. The theory implied
that changes to the body during an organism's life would be inherited,
as proposed in Lamarckism.
Pangenesis was Charles Darwin's hypothetical mechanism for heredity, in which he proposed that each part of the body continually emitted its own type of small organic particles called gemmules that aggregated in the gonads, contributing heritable information to the gametes. He presented this 'provisional hypothesis' in his 1868 work The Variation of Animals and Plants Under Domestication,
intending it to fill what he perceived as a major gap in evolutionary
theory at the time. The etymology of the word comes from the Greek words pan (a prefix meaning "whole", "encompassing") and genesis ("birth") or genos ("origin"). Pangenesis mirrored ideas originally formulated by Hippocrates and other pre-Darwinian scientists, but using new concepts such as cell theory,
explaining cell development as beginning with gemmules which were
specified to be necessary for the occurrence of new growths in an
organism, both in initial development and regeneration. It also accounted for regeneration and the Lamarckian
concept of the inheritance of acquired characteristics, as a body part
altered by the environment would produce altered gemmules. This made
Pangenesis popular among the neo-Lamarckian school of evolutionary
thought. This hypothesis was made effectively obsolete after the 1900 rediscovery among biologists of Gregor Mendel's theory of the particulate nature of inheritance.
Early history
Pangenesis was similar to ideas put forth by Hippocrates, Democritus and other pre-Darwinian scientists in proposing that the whole of parental organisms participate in heredity (thus the prefix pan).
Darwin wrote that Hippocrates' pangenesis was "almost identical with
mine—merely a change of terms—and an application of them to classes of
facts necessarily unknown to the old philosopher."
Zirkle demonstrated that the idea of inheritance of acquired
characteristics had become fully accepted by the 16th century and
remained immensely popular through to the time of Lamarck's work, at
which point it began to draw more criticism due to lack of hard
evidence.
He also stated that pangenesis was the only scientific explanation ever
offered for this concept, developing from Hippocrates' belief that "the
semen was derived from the whole body."
In the 13th century, pangenesis was commonly accepted on the principle
that semen was a refined version of food unused by the body, which
eventually translated to 15th and 16th century widespread use of
pangenetic principles in medical literature, especially in gynecology. Later pre-Darwinian important applications of the idea included hypotheses about the origin of the differentiation of races.
A theory put forth by Pierre Louis Maupertuis
in 1745 called for particles from both parents governing the attributes
of the child, although some historians have called his remarks on the
subject cursory and vague.
In 1749, the French naturalist Georges-Louis Leclerc, Comte de Buffon
developed a hypothetical system of heredity much like Darwin's
pangenesis, wherein 'organic molecules' were transferred to offspring
during reproduction and stored in the body during development. Commenting on Buffon's views, Darwin stated, "If Buffon had assumed
that his organic molecules had been formed by each separate unit
throughout the body, his view and mine would have been very closely
similar."
In 1801, Erasmus Darwin advocated a hypothesis of pangenesis in the third edition of his book Zoonomia. In 1809, Jean-Baptiste Lamarck in his Philosophie Zoologique
put forth evidence for the idea that characteristics acquired during
the lifetime of an organism, from either environmental or behavioural
effects, may be passed on to the offspring. Charles Darwin first had
significant contact with Lamarckism during his time at the University of Edinburgh Medical School in the late 1820s, both through Robert Edmond Grant, whom he assisted in research, and in Erasmus's journals.
Darwin's first known writings on the topic of Lamarckian ideas as they
related to inheritance are found in a notebook he opened in 1837, also
entitled Zoonomia. Historian Jonathan Hodge states that the theory of pangenesis itself first appeared in Darwin's notebooks in 1841.
In 1861, the Irish physician Henry Freke developed a variant of pangenesis in his book Origin of Species by Means of Organic Affinity. Freke proposed that all life was developed from microscopic organic agents which he named granules, which existed as 'distinct species of organizing matter' and would develop into different biological structures.
Four years before the publication of Variation, in his 1864 book Principles of Biology, Herbert Spencer
proposed a theory of "physiological units" similar to Darwin's
gemmules, which likewise were said to be related to specific body parts
and responsible for the transmission of characteristics of those body
parts to offspring. He supported the Lamarckian idea of transmission of acquired characteristics.
Darwin had debated whether to publish a theory of heredity for an
extended period of time due to its highly speculative nature. He
decided to include pangenesis in Variation after sending a 30-page manuscript to his close friend and supporter Thomas Huxley in May 1865, which was met by significant criticism from Huxley that made Darwin even more hesitant.
However, Huxley eventually advised Darwin to publish, writing:
"Somebody rummaging among your papers half a century hence will find
Pangenesis & say 'See this wonderful anticipation of our modern
Theories—and that stupid ass, Huxley, prevented his publishing them'" Darwin's initial version of pangenesis appeared in the first edition of Variation in 1868, and was later reworked for the publication of a second edition in 1875.
Theory
Darwin
Darwin's pangenesis theory attempted to explain the process of sexual reproduction, inheritance of traits, and complex developmental phenomena such as cellular regeneration in a unified mechanistic structure. Longshan Liu wrote that in modern terms, pangenesis deals with issues of "dominance inheritance, graft hybridization, reversion, xenia, telegony,
the inheritance of acquired characters, regeneration and many groups of
facts pertaining to variation, inheritance and development."
Mechanistically, Darwin proposed pangenesis to occur through the
transfer of organic particles which he named 'gemmules.' Gemmules, which
he also sometimes referred to as plastitudes, pangenes, granules,
or germs, were supposed to be shed by the organs of the body and
carried in the bloodstream to the reproductive organs where they
accumulated in the germ cells or gametes. Their accumulation was thought to occur by some sort of a 'mutual affinity.'
Each gemmule was said to be specifically related to a certain body
part- as described, they did not contain information about the entire
organism.
The different types were assumed to be dispersed through the whole
body, and capable of self-replication given 'proper nutriment'. When
passed on to offspring via the reproductive process, gemmules were
thought to be responsible for developing into each part of an organism
and expressing characteristics inherited from both parents.
Darwin thought this to occur in a literal sense: he explained cell
proliferation to progress as gemmules to bind to more developed cells of
their same character and mature. In this sense, the uniqueness of each
individual would be due to their unique mixture of their parents'
gemmules, and therefore characters. Similarity to one parent over the other could be explained by a quantitative superiority of one parent's gemmules.
Yongshen Lu points out that Darwin knew of cells' ability to multiply
by self-division, so it is unclear how Darwin supposed the two
proliferation mechanisms to relate to each other.
He did clarify in a later statement that he had always supposed
gemmules to only bind to and proliferate from developing cells, not
mature ones. Darwin hypothesized that gemmules might be able to survive and multiply outside of the body in a letter to J. D. Hooker in 1870.
Some gemmules were thought to remain dormant for generations,
whereas others were routinely expressed by all offspring. Every child
was built up from selective expression of the mixture of the parents and
grandparents' gemmules coming from either side. Darwin likened this to
gardening: a flowerbed could be sprinkled with seeds "most of which soon
germinate, some lie for a period dormant, whilst others perish." He did not claim gemmules were in the blood, although his theory was often interpreted in this way. Responding to Fleming Jenkin's review of On the Origin of Species,
he argued that pangenesis would permit the preservation of some
favourable variations in a population so that they wouldn't die out
through blending.
Darwin thought that environmental effects that caused altered
characteristics would lead to altered gemmules for the affected body
part. The altered gemmules would then have a chance of being transferred
to offspring, since they were assumed to be produced throughout an
organism's life.
Thus, pangenesis theory allowed for the Lamarckian idea of transmission
of characteristics acquired through use and disuse. Accidental gemmule
development in incorrect parts of the body could explain deformations
and the 'monstrosities' Darwin cited in Variation.
De Vries
Hugo de Vries characterized his own version of pangenesis theory in his 1889 book Intracellular Pangenesis with two propositions, of which he only accepted the first:
I. In the cells there are numberless particles which differ from
each other, and represent the individual cells, organs, functions and
qualities of the whole individual. These particles are much larger than
the chemical molecules and smaller than the smallest known organisms;
yet they are for the most part comparable to the latter, because, like
them, they can divide and multiply through nutrition and growth. They
are transmitted, during cell-division, to the daughter-cells: this is
the ordinary process of heredity.
II. In addition to this, the cells of the organism, at every stage
of development, throw off such particles, which are conducted to the
germ-cells and transmit to them those characters which the respective
cells may have acquired during development.
Other variants
The historian of science Janet Browne points out that while Spencer and Carl von Nägeli
also put forth ideas for systems of inheritance involving gemmules,
their version of gemmules differed from Darwin's in that it contained "a
complete microscopic blueprint for an entire creature." Spencer published his theory of "physiological units" three years prior to Darwin's publication of Variation.
Browne adds that Darwin believed specifically in gemmules from each
body part because they might explain how environmental effects could be
passed on as characteristics to offspring.
Interpretations and applications of pangenesis continued to appear frequently in medical literature up until Weismann's experiments and subsequent publication on germ-plasm theory in 1892.
For instance, an address by Huxley spurred on substantial work by Dr.
James Ross in linking ideas found in Darwin's pangenesis to the germ theory of disease. Ross cites the work of both Darwin and Spencer as key to his application of pangenetic theory.
Collapse
Galton's experiments on rabbits
Darwin's half-cousin Francis Galton
conducted wide-ranging inquiries into heredity which led him to refute
Charles Darwin's hypothetical theory of pangenesis. In consultation with
Darwin, he set out to see if gemmules were transported in the blood. In
a long series of experiments from 1869 to 1871, he transfused the blood
between dissimilar breeds of rabbits, and examined the features of
their offspring. He found no evidence of characters transmitted in the
transfused blood.
Galton was troubled because he began the work in good faith, intending to prove Darwin right, and having praised pangenesis in Hereditary Genius
in 1869. Cautiously, he criticized his cousin's theory, although
qualifying his remarks by saying that Darwin's gemmules, which he called
"pangenes", might be temporary inhabitants of the blood that his
experiments had failed to pick up.
Darwin challenged the validity of Galton's experiment, giving his reasons in an article published in Nature where he wrote:
Now, in the chapter on Pangenesis in my Variation of Animals and Plants under Domestication,
I have not said one word about the blood, or about any fluid proper to
any circulating system. It is, indeed, obvious that the presence of
gemmules in the blood can form no necessary part of my hypothesis; for I
refer in illustration of it to the lowest animals, such as the
Protozoa, which do not possess blood or any vessels; and I refer to
plants in which the fluid, when present in the vessels, cannot be
considered as true blood." He goes on to admit: "Nevertheless, when I
first heard of Mr. Galton's experiments, I did not sufficiently reflect
on the subject, and saw not the difficulty of believing in the presence
of gemmules in the blood.
After the circulation of Galton's results, the perception of
pangenesis quickly changed to severe skepticism if not outright
disbelief.
Weismann
August Weismann's germ plasm
theory. The hereditary material, the germ plasm, is confined to the
gonads. Somatic cells (of the body) develop afresh in each generation
from the germ plasm. The implied Weismann barrier between the germ line and the soma prevents Lamarckian inheritance.
August Weismann's idea, set out in his 1892 book Das Keimplasma: eine Theorie der Vererbung (The Germ Plasm: a Theory of Inheritance), was that the hereditary material, which he called the germ plasm, and the rest of the body (the soma)
had a one-way relationship: the germ-plasm formed the body, but the
body did not influence the germ-plasm, except indirectly in its
participation in a population subject to natural selection. This
distinction is commonly referred to as the Weismann Barrier.
If correct, this made Darwin's pangenesis wrong and Lamarckian
inheritance impossible. His experiment on mice, cutting off their tails
and showing that their offspring had normal tails across multiple
generations, was proposed as a proof of the non-existence of Lamarckian
inheritance, although Peter Gauthier has argued that Weismann's experiment showed only that injury did not affect the germ plasm and neglected to test the effect of Lamarckian use and disuse. Weismann argued strongly and dogmatically for Darwinism and against neo-Lamarckism, polarising opinions among other scientists. This increased anti-Darwinian feeling, contributing to its eclipse.
After pangenesis
Darwin's pangenesis theory was widely criticised, in part for its Lamarckian premise that parents could pass on traits acquired in their lifetime. Conversely, the neo-Lamarckians of the time seized upon pangenesis as evidence to support their case.
Italian Botanist Federico Delpino's objection that gemmules' ability to
self-divide is contrary to their supposedly innate nature gained
considerable traction; however, Darwin was dismissive of this criticism,
remarking that the particulate agents of smallpox and scarlet fever
seem to have such characteristics. Lamarckism fell from favour after August Weismann's
research in the 1880s indicated that changes from use (such as lifting
weights to increase muscle mass) and disuse (such as being lazy and
becoming weak) were not heritable.
However, some scientists continued to voice their support in spite of
Galton's and Weismann's results: notably, in 1900 Karl Pearson wrote
that pangenesis "is no more disproved by the statement that 'gemmules
have not been found in the blood,' than the atomic theory is disproved
by the fact that no atoms have been found in the air." Finally, the rediscovery of Mendel's Laws of Inheritance in 1900 led to pangenesis being fully set aside. Julian Huxley has observed that the later discovery of chromosomes and the research of T. H. Morgan also made pangenesis untenable.
Some of Darwin's pangenesis principles do relate to heritable aspects of phenotypic plasticity,
although the status of gemmules as a distinct class of organic
particles has been firmly rejected. However, starting in the 1950s, many
research groups in revisiting Galton's experiments found that heritable
characteristics could indeed arise in rabbits and chickens following
DNA injection or blood transfusion.
This type of research originated in the Soviet Union in the late 1940s
in the work of Sopikov and others, and was later corroborated by
researchers in Switzerland as it was being further developed by the
Soviet scientists.Notably, this work was supported in the USSR in part due to its conformation with the ideas of Trofim Lysenko, who espoused a version of neo-Lamarckism as part of Lysenkoism. Further research of this heritability of acquired characteristics developed into, in part, the modern field of epigenetics.
Darwin himself had noted that "the existence of free gemmules is a
gratuitous assumption"; by some accounts in modern interpretation,
gemmules may be considered a prescient mix of DNA, RNA, proteins,
prions, and other mobile elements that are heritable in a non-Mendelian
manner at the molecular level. Liu points out that Darwin's ideas about gemmules replicating outside of the body are predictive of in vitro gene replication used, for instance, in PCR.
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