Genetic assimilation is a process described by
Conrad H. Waddington by which a
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
originally produced in response to an environmental condition, such as exposure to a
teratogen, later becomes genetically encoded via
artificial selection
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant m ...
or
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
. Despite superficial appearances, this does not require the (
Lamarckian
Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. It is also calle ...
)
inheritance of acquired characters
Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. It is also calle ...
, although
epigenetic inheritance
Transgenerational epigenetic inheritance is the transmission of epigenetic markers from one organism to the next (i.e., from parent to child) that affects the traits of offspring without altering the primary structure of DNA (i.e. the sequence of ...
could potentially influence the result.
Waddington stated that genetic assimilation overcomes the barrier to selection imposed by what he called
canalization of developmental pathways; he supposed that the organism's genetics evolved to ensure that development proceeded in a certain way regardless of normal environmental variations.
The classic example of genetic assimilation was a pair of experiments in 1942 and 1953 by Waddington. He exposed ''
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'' fruit fly
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s to
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
, producing an extreme change in their phenotype: they developed a double thorax, resembling the effect of the ''
bithorax
The Bithorax complex (BX-C) is one of two ''Drosophila melanogaster '' homeotic gene complexes, located on the right arm of chromosome 3. It is responsible for the differentiation of the posterior two-thirds (posterior thorax and each abdominal ...
'' gene. This is called a
homeotic
In evolutionary developmental biology, homeosis is the transformation of one organ into another, arising from mutation in or misexpression of certain developmentally critical genes, specifically homeotic genes. In animals, these developmental ge ...
change. Flies which developed
haltere
''Halteres'' (; singular ''halter'' or ''haltere'') (from grc, ἁλτῆρες, weights held in the hands to give an impetus in leaping) are a pair of small club-shaped organs on the body of two orders of flying insects that provide infor ...
s (the modified hindwings of
true flies
Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
, used for balance) with wing-like characteristics were chosen for breeding for 20 generations, by which point the phenotype could be seen without other treatment.
Waddington's explanation has been controversial, and has been accused of being Lamarckian. More recent evidence appears to confirm the existence of genetic assimilation in evolution; in yeast, when a
stop codon
In molecular biology (specifically protein biosynthesis), a stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that signals the termination of the translation process of the current protein. Most codons in me ...
is lost by
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
, the
reading frame
In molecular biology, a reading frame is a way of dividing the nucleic acid sequence, sequence of nucleotides in a nucleic acid (DNA or RNA) molecule into a set of consecutive, non-overlapping triplets. Where these triplets equate to amino acids or ...
is preserved much more often than would be expected.
History
Waddington's experiments
Conrad H. Waddington's classic experiment (1942) induced an extreme environmental reaction in the developing embryos of ''
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
''. In response to
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
vapor, a proportion of embryos developed a radical
phenotypic
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
change, a second
thorax
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...
. At this point in the experiment ''bithorax'' is not innate; it is induced by an unusual environment. Waddington then repeatedly selected ''Drosophila'' for the ''bithorax'' phenotype over some 20 generations. After this time, some ''Drosophila'' developed ''bithorax'' without the ether treatment.
Waddington carried out a similar experiment in 1953, this time inducing the ''cross-veinless''
phenocopy
A phenocopy is a variation in phenotype (generally referring to a single trait) which is caused by environmental conditions (often, but not necessarily, during the organism's development), such that the organism's phenotype matches a phenotype wh ...
in ''Drosophila'' with a heat shock, with 40% of the flies showing the phenotype prior to selection. Again he selected for the phenotype over several generations, applying heat shock each time, and eventually the phenotype appeared even without heat shock.
Waddington's explanation
Waddington called the effect he had seen "genetic assimilation". His explanation was that it was caused by a process he called "
canalization". He compared
embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
to a ball rolling down a slope in what he called an
epigenetic
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
landscape, where each point on the landscape is a possible state of the organism (involving many variables). As a particular pathway becomes entrenched or "canalized", it becomes more stable, likely to occur even in the face of environmental changes. Major perturbations such as ether or heat shock eject the developmental pathway from the
metaphor
A metaphor is a figure of speech that, for rhetorical effect, directly refers to one thing by mentioning another. It may provide (or obscure) clarity or identify hidden similarities between two different ideas. Metaphors are often compared wit ...
ical canal, exploring other parts of the epigenetic landscape. Selection in the presence of that perturbation leads to the evolution of a new canal; after the perturbation is discontinued, developmental trajectories continue to follow the canalized pathway.
A Darwinian explanation
Other evolutionary biologists have agreed that assimilation occurs, but give a different, purely
quantitative genetics
Quantitative genetics deals with phenotypes that vary continuously (such as height or mass)—as opposed to discretely identifiable phenotypes and gene-products (such as eye-colour, or the presence of a particular biochemical).
Both branches u ...
explanation in terms of Darwin's
natural
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are p ...
or
artificial selection
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant m ...
. The phenotype, say ''cross-veinless'', is presumed to be caused by a combination of multiple genes. The phenotype appears when the sum of
gene effects exceeds a
threshold; if that threshold is lowered by a perturbation, say a heat shock, the phenotype is more likely to be seen. Continued selection under perturbing conditions increases the
frequency of the alleles of genes that promote the phenotype until the threshold is breached, and the phenotype appears without requiring the heat shock.
Perturbations can be genetic or epigenetic rather than environmental. For example, ''Drosophila'' fruit flies have a heat shock protein,
Hsp90
Hsp90 (heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, ...
, which protects the development of many structures in the adult fly from heat shock. If the protein is damaged by a mutation, then just as if it were damaged by the environmental effects of drugs, many different phenotypic variants appear; if these are selected for, they quickly establish without further need for the mutant Hsp90.
A mutational explanation
In 2017, L. Fanti and colleagues replicated Waddington's experiments, but included
DNA sequencing
DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. Th ...
, revealing that the wing phenotypes were due to
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
al events, small deletions and the insertions of
transposable element
A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transp ...
s that were mobilised by the heat exposure.
Neo-Darwinism or Lamarckism
![Lamarck Compared to Darwin, Baldwin, Waddington](https://upload.wikimedia.org/wikipedia/commons/5/5d/Lamarck_Compared_to_Darwin%2C_Baldwin%2C_Waddington.svg)
Waddington's theory of genetic assimilation was controversial.
The
evolutionary biologists
Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity of life on Earth. It is also defined as the study of ...
Theodosius Dobzhansky
Theodosius Grigorievich Dobzhansky (russian: Феодо́сий Григо́рьевич Добржа́нский; uk, Теодо́сій Григо́рович Добржа́нський; January 25, 1900 – December 18, 1975) was a prominent ...
and
Ernst Mayr
Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists. He was also a renowned Taxonomy (biology), taxonomist, tropical explorer, ornithologist, Philosophy of biology, philosopher o ...
both thought that Waddington was using genetic assimilation to support so-called
Lamarckian
Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. It is also calle ...
inheritance. They denied that the inheritance of acquired characteristics had taken place, and asserted that Waddington had simply observed the natural selection of genetic variants that already existed in the study population. Waddington himself interpreted his results in a
Neo-Darwinian
Neo-Darwinism is generally used to describe any integration of Charles Darwin's theory of evolution by natural selection with Gregor Mendel's theory of genetics. It mostly refers to evolutionary theory from either 1895 (for the combinations of Dar ...
way, particularly emphasizing that they "could bring little comfort to those who wish to believe that environmental influences tend to produce heritable changes in the direction of adaptation."
The evolutionary developmental biologist Adam S. Wilkins wrote that "
addingtonin his lifetime... was widely perceived primarily as a critic of Neo-Darwinian evolutionary theory. His criticisms ... were focused on what he saw as unrealistic, 'atomistic' models of both gene selection and trait evolution." In particular, according to Wilkins, Waddington felt that the Neo-Darwinians badly neglected the phenomenon of extensive gene interactions and that the 'randomness' of mutational effects, posited in the theory, was false.
Even though Waddington became critical of the
neo-Darwinian synthetic theory of evolution, he still described himself as a Darwinian, and called for an
extended evolutionary synthesis
The extended evolutionary synthesis consists of a set of theoretical concepts argued to be more comprehensive than the earlier modern synthesis of evolutionary biology that took place between 1918 and 1942. The extended evolutionary synthesis was ...
based on his research.
Waddington did not deny the threshold-based conventional genetic interpretation of his experiments, but regarded it "as a told to the children version of what I wished to say" and considered the debate to be about "mode of expression, rather than of substance".
Both genetic assimilation and the related
Baldwin effect
In evolutionary biology, the Baldwin effect, a phenotype-first theory of evolution, describes the effect of learned behaviour on evolution. James Mark Baldwin and others suggested during the eclipse of Darwinism in the late 19th century that an ...
are theories of
phenotypic plasticity
Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...
, where aspects of an organism's physiology and behaviour are affected by the environment. The evolutionary ecologist Erika Crispo states that they differ in that genetic assimilation decreases the level of plasticity (returning to Waddington's original definition of canalization; whereas the Baldwin effect may increase it) but does not change the mean phenotypic value (where the Baldwin effect changes it).
Crispo defines genetic assimilation as a kind of genetic accommodation, "evolution in response to both genetically based and environmentally induced novel traits",
which in turn is in her view central to the Baldwin effect.
Relationship to adaptation
Mathematical modeling suggests that under certain circumstances, natural selection favours the evolution of canalization that is designed to fail under extreme conditions.
If the result of such a failure is favoured by natural selection, genetic assimilation occurs. In the 1960s, Waddington and his colleague the animal geneticist J. M. Rendel argued for the importance of genetic assimilation in natural adaptation, as a means of providing new and potentially beneficial variation to populations under stress, enabling them to evolve rapidly. Their contemporary
George C. Williams argued that genetic assimilation proceeds at the cost of a loss of previously adaptive
developmental plasticity
Developmental plasticity is a general term referring to changes in neural connections during development as a result of environmental interactions as well as neural changes induced by learning. Much like neuroplasticity or brain plasticity, develo ...
, and therefore should be seen as resulting in a net loss rather than gain of complexity, making it in his view uninteresting from the perspective of the constructive process of adaptation. However, the preceding phenotypic plasticity need not be adaptive, but simply represent a breakdown of canalization.
In natural populations
![Chappell Island tiger snake](https://upload.wikimedia.org/wikipedia/commons/c/c5/Chappell_Island_tiger_snake.jpg)
Several instances of genetic assimilation have been documented contributing to natural selection in the wild. For example, populations of the island tiger snakes (''
Notechis scutatus'') have become isolated on islands and have larger heads to cope with large prey animals. Young populations have larger heads by phenotypic plasticity, whereas large heads have become genetically assimilated in older populations.
In another example, patterns of left-right asymmetry or "handedness", when present, can be determined either genetically or plastically. During evolution, genetically determined directional asymmetry, as in the left-oriented human heart, can arise either from a nonheritable (phenotypic) developmental process, or directly by
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
from a symmetric ancestor. An excess of transitions from plastically determined to genetically determined handedness points to the role of genetic assimilation in evolution.
A third example has been seen in
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...
. Evolutionary events in which
stop codon
In molecular biology (specifically protein biosynthesis), a stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that signals the termination of the translation process of the current protein. Most codons in me ...
s are lost preserve the
reading frame
In molecular biology, a reading frame is a way of dividing the nucleic acid sequence, sequence of nucleotides in a nucleic acid (DNA or RNA) molecule into a set of consecutive, non-overlapping triplets. Where these triplets equate to amino acids or ...
much more often than would be expected from
mutation bias Mutation bias refers to a pattern in which some type of mutation occurs more often than expected under uniformity. The types are most often defined by the molecular nature of the mutational change (see examples below), but sometimes they are based ...
. This finding is consistent with the role of the
yeast prion
A fungal prion is a prion that infects fungal hosts. Fungal prions are naturally occurring proteins that can switch between multiple, structurally distinct conformations, at least one of which is self-propagating and transmissible to other prion ...
SI+in epigenetically facilitating stop codon readthrough, followed by genetic assimilation via the permanent loss of the stop codon.
See also
*
Evolutionary developmental biology
Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved.
The field grew from 19th-century beginni ...
*
List of genetics-related topics
Topic, topics, TOPIC, topical, or topicality may refer to:
Topic / Topics
* Topić, a Slavic surname
* ''Topics'' (Aristotle), a work by Aristotle
* Topic (chocolate bar), a brand of confectionery bar
* Topic (DJ), German musician
* Topic ( ...
References
{{Evolution
Evolutionary biology
Extended evolutionary synthesis
Selection