The mammalian blastocyst hatches before implantating into the endometrial lining of the womb. Once implanted the embryo will continue its development through the next stages of gastrulation, neurulation, and organogenesis. Gastrulation is the formation of the three germ layers that will form all of the different parts of the body. Neurulation forms the nervous system, and organogenesis is the development of all the various tissues and organs of the body.
A newly developing human is typically referred to as an embryo until the ninth week after conception, when it is then referred to as a fetus.
In other multicellular organisms, the word "embryo" can be used more
broadly to any early developmental or life cycle stage prior to birth or hatching.
Etymology
First attested in English in the mid-14c., the word embryon derives from Medieval Latinembryo, itself from Greekἔμβρυον (embruon), lit. "young one", which is the neuter of ἔμβρυος (embruos), lit. "growing in", from ἐν (en), "in" and βρύω (bruō), "swell, be full"; the proper Latinized form of the Greek term would be embryum.
This section
is about is a summary of embryonic development in all types of animals,
including humans. For information specific to human embryonic
development, see Human embryonic development.
Embryos (and one tadpole) of the wrinkled frog (Rana rugosa)Mouse and snake embryos
In animals, fertilization begins the process of embryonic development
with the creation of a zygote, a single cell resulting from the fusion
of gametes (e.g. egg and sperm).
The development of a zygote into a multicellular embryo proceeds
through a series of recognizable stages, often divided into cleavage,
blastula, gastrulation, and organogenesis.
Cleavage is the period of rapid mitotic cell divisions that occur
after fertilization. During cleavage, the overall size of the embryo
does not change, but the size of individual cells decrease rapidly as
they divide to increase the total number of cells. Cleavage results in a blastula.
Depending on the species, a blastula or blastocyst stage embryo can appear as a ball of cells on top of yolk, or as a hollow sphere of cells surrounding a middle cavity.
The embryo's cells continue to divide and increase in number, while
molecules within the cells such as RNAs and proteins actively promote
key developmental processes such as gene expression, cell fate
specification, and polarity. Before implanting into the uterine wall the embryo is sometimes known as the pre-implantation embryo or pre-implantation conceptus. Sometimes this is called the pre-embryo a term employed to differentiate from an embryo proper in relation to embryonic stem cell discourses.
Gastrulation is the next phase of embryonic development, and
involves the development of two or more layers of cells (germinal
layers). Animals that form two layers (such as Cnidaria) are called diploblastic, and those that form three (most other animals, from flatworms
to humans) are called triploblastic. During gastrulation of
triploblastic animals, the three germinal layers that form are called
the ectoderm, mesoderm, and endoderm. All tissues and organs of a mature animal can trace their origin back to one of these layers. For example, the ectoderm will give rise to the skin epidermis and the nervous system, the mesoderm will give rise to the vascular system, muscles, bone, and connective tissues, and the endoderm will give rise to organs of the digestive system and epithelium of the digestive system and respiratory system.
Many visible changes in embryonic structure happen throughout
gastrulation as the cells that make up the different germ layers migrate
and cause the previously round embryo to fold or invaginate into a
cup-like appearance.
Past gastrulation, an embryo continues to develop into a mature
multicellular organism by forming structures necessary for life outside
of the womb or egg. As the name suggests, organogenesis is the stage of
embryonic development when organs form. During organogenesis, molecular
and cellular interactions prompt certain populations of cells from the
different germ layers to differentiate into organ-specific cell types.
For example, in neurogenesis, a subpopulation of cells from the
ectoderm segregate from other cells and further specialize to become the
brain, spinal cord, or peripheral nerves.
The embryonic period varies from species to species. In human
development, the term fetus is used instead of embryo after the ninth
week after conception, whereas in zebrafish, embryonic development is considered finished when a bone called the cleithrum becomes visible.
In animals that hatch from an egg, such as birds, a young animal is
typically no longer referred to as an embryo once it has hatched. In viviparous
animals (animals whose offspring spend at least some time developing
within a parent's body), the offspring is typically referred to as an
embryo while inside of the parent, and is no longer considered an embryo
after birth or exit from the parent. However, the extent of development
and growth accomplished while inside of an egg or parent varies
significantly from species to species, so much so that the processes
that take place after hatching or birth in one species may take place
well before those events in another. Therefore, according to one
textbook, it is common for scientists to interpret the scope of embryology broadly as the study of the development of animals.
Flowering plants (angiosperms) create embryos after the fertilization of a haploid ovule by pollen. The DNA from the ovule and pollen combine to form a diploid, single-cell zygote that will develop into an embryo. The zygote, which will divide multiple times as it progresses throughout embryonic development, is one part of a seed. Other seed components include the endosperm,
which is tissue rich in nutrients that will help support the growing
plant embryo, and the seed coat, which is a protective outer covering.
The first cell division of a zygote is asymmetric, resulting in an embryo with one small cell (the apical cell) and one large cell (the basal cell).
The small, apical cell will eventually give rise to most of the
structures of the mature plant, such as the stem, leaves, and roots.
The larger basal cell will give rise to the suspensor, which connects
the embryo to the endosperm so that nutrients can pass between them.
The plant embryo cells continue to divide and progress through
developmental stages named for their general appearance: globular,
heart, and torpedo. In the globular stage, three basic tissue types
(dermal, ground, and vascular) can be recognized. The dermal tissue will give rise to the epidermis or outer covering of a plant, ground tissue will give rise to inner plant material that functions in photosynthesis, resource storage, and physical support, and vascular tissue will give rise to connective tissue like the xylem and phloem that transport fluid, nutrients, and minerals throughout the plant. In heart stage, one or two cotyledons (embryonic leaves) will form. Meristems (centers of stem cell
activity) develop during the torpedo stage, and will eventually produce
many of the mature tissues of the adult plant throughout its life. At the end of embryonic growth, the seed will usually go dormant until germination. Once the embryo begins to germinate (grow out from the seed) and forms its first true leaf, it is called a seedling or plantlet.
Plants that produce spores instead of seeds, like bryophytes and ferns, also produce embryos. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated.
The inner wall of the archegonium lies in close contact with the "foot"
of the developing embryo; this "foot" consists of a bulbous mass of
cells at the base of the embryo which may receive nutrition from its
parent gametophyte. The structure and development of the rest of the embryo varies by group of plants.
Since all land plants create embryos, they are collectively referred to as embryophytes
(or by their scientific name, Embryophyta). This, along with other
characteristics, distinguishes land plants from other types of plants,
such as algae, which do not produce embryos.
Creating and/or manipulating embryos via assisted reproductive technology (ART) is used for addressing fertility concerns in humans and other animals, and for selective breeding in agricultural species. Between the years 1987 and 2015, ART techniques including in vitro fertilization (IVF) were responsible for an estimated one million human births in the United States alone. Other clinical technologies include preimplantation genetic diagnosis (PGD), which can identify certain serious genetic abnormalities, such as aneuploidy, prior to selecting embryos for use in IVF. Some have proposed (or even attempted - see He Jiankui affair) genetic editing of human embryos via CRISPR-Cas9 as a potential avenue for preventing disease; however, this has been met with widespread condemnation from the scientific community.
ART techniques are also used to improve the profitability of
agricultural animal species such as cows and pigs by enabling selective
breeding for desired traits and/or to increase numbers of offspring.
For example, when allowed to breed naturally, cows typically produce
one calf per year, whereas IVF increases offspring yield to 9–12 calves
per year. IVF and other ART techniques, including cloning via interspecies somatic cell nuclear transfer (iSCNT), are also used in attempts to increase the numbers of endangered or vulnerable species, such as Northern white rhinos, cheetahs, and sturgeons.
Cryoconservation of plant and animal biodiversity
Cryoconservation of genetic resources
involves collecting and storing the reproductive materials, such as
embryos, seeds, or gametes, from animal or plant species at low
temperatures in order to preserve them for future use. Some large-scale animal species cryoconservation efforts include "frozen zoos" in various places around the world, including in the UK's Frozen Ark, the Breeding Centre for Endangered Arabian Wildlife (BCEAW) in the United Arab Emirates, and the San Diego Zoo Institute for Conservation in the United States.
As of 2018, there were approximately 1,700 seed banks used to store and
protect plant biodiversity, particularly in the event of mass
extinction or other global emergencies. The Svalbard Global Seed Vault
in Norway maintains the largest collection of plant reproductive
tissue, with more than a million samples stored at −18 °C (0 °F).
Fossilized animal embryos are known from the Precambrian, and are found in great numbers during the Cambrian period. Even fossilized dinosaur embryos have been discovered.
Lamarck argued, as part of his theory of heredity, that a blacksmith's sons inherit the strong muscles he acquires from his work.
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 called the inheritance of acquired characteristics or more recently soft inheritance. The idea is named after the French zoologistJean-Baptiste Lamarck (1744–1829), who incorporated the classical era theory of soft inheritance into his theory of evolution as a supplement to his concept of orthogenesis, a drive towards complexity.
Introductory textbooks contrast Lamarckism with Charles Darwin's theory of evolution by natural selection. However, Darwin's book On the Origin of Species gave credence to the idea of heritable effects of use and disuse, as Lamarck had done, and his own concept of pangenesis similarly implied soft inheritance.
Many researchers from the 1860s onwards attempted to find
evidence for Lamarckian inheritance, but these have all been explained
away, either by other mechanisms such as genetic contamination or as fraud. August Weismann's
experiment, considered definitive in its time, is now considered to
have failed to disprove Lamarckism, as it did not address use and
disuse. Later, Mendelian genetics supplanted the notion of inheritance of acquired traits, eventually leading to the development of the modern synthesis, and the general abandonment of Lamarckism in biology. Despite this, interest in Lamarckism has continued.
In the 21st century, experimental results in the fields of epigenetics, genetics, and somatic hypermutation demonstrated the possibility of transgenerational epigenetic inheritance of traits acquired by the previous generation. These proved a limited validity of Lamarckism. The inheritance of the hologenome,
consisting of the genomes of all an organism's symbiotic microbes as
well as its own genome, is also somewhat Lamarckian in effect, though
entirely Darwinian in its mechanisms.
Early history
Origins
Jean-Baptiste Lamarck repeated the ancient folk wisdom of the inheritance of acquired characteristics.
The inheritance of acquired characteristics was proposed in ancient
times and remained a current idea for many centuries. The historian of
science Conway Zirkle wrote in 1935 that:
Lamarck was neither the first nor
the most distinguished biologist to believe in the inheritance of
acquired characters. He merely endorsed a belief which had been
generally accepted for at least 2,200 years before his time and used it
to explain how evolution could have taken place. The inheritance of
acquired characters had been accepted previously by Hippocrates, Aristotle, Galen, Roger Bacon, Jerome Cardan, Levinus Lemnius, John Ray, Michael Adanson, Jo. Fried. Blumenbach and Erasmus Darwin among others.
Zirkle noted that Hippocrates described pangenesis,
the theory that what is inherited derives from the whole body of the
parent, whereas Aristotle thought it impossible; but that all the same,
Aristotle implicitly agreed to the inheritance of acquired
characteristics, giving the example of the inheritance of a scar, or of
blindness, though noting that children do not always resemble their
parents. Zirkle recorded that Pliny the Elder
thought much the same. Zirkle pointed out that stories involving the
idea of inheritance of acquired characteristics appear numerous times in
ancient mythology and the Bible and persisted through to Rudyard Kipling's Just So Stories. The idea is mentioned in 18th century sources such as Diderot's D'Alembert's Dream. Erasmus Darwin's Zoonomia (c. 1795) suggested that warm-bloodedanimals
develop from "one living filament... with the power of acquiring new
parts" in response to stimuli, with each round of "improvements" being
inherited by successive generations.
Charles 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 On the Origin of Species
proposed natural selection as the main mechanism for development of
species, but (like Lamarck) gave credence to the idea of heritable
effects of use and disuse as a supplementary mechanism. Darwin subsequently set out his concept of pangenesis in the final chapter of his book The Variation of Animals and Plants Under Domestication
(1868), which gave numerous examples to demonstrate what he thought was
the inheritance of acquired characteristics. Pangenesis, which he
emphasised was a hypothesis, was based on the idea that somatic cells would, in response to environmental stimulation (use and disuse), throw off 'gemmules' or 'pangenes' which travelled around the body, though not necessarily in the bloodstream.
These pangenes were microscopic particles that supposedly contained
information about the characteristics of their parent cell, and Darwin
believed that they eventually accumulated in the germ cells where they could pass on to the next generation the newly acquired characteristics of the parents.
Darwin's half-cousin, Francis Galton, carried out experiments on rabbits, with Darwin's cooperation, in which he transfused the blood
of one variety of rabbit into another variety in the expectation that
its offspring would show some characteristics of the first. They did
not, and Galton declared that he had disproved Darwin's hypothesis of
pangenesis, but Darwin objected, in a letter to the scientific journal Nature,
that he had done nothing of the sort, since he had never mentioned
blood in his writings. He pointed out that he regarded pangenesis as
occurring in protozoa and plants, which have no blood, as well as in animals.
Lamarck's evolutionary framework
Lamarck's two-factor theory involves 1) a complexifying force that drives animal body plans towards higher levels (orthogenesis) creating a ladder of phyla,
and 2) an adaptive force that causes animals with a given body plan to
adapt to circumstances (use and disuse, inheritance of acquired
characteristics), creating a diversity of species and genera. Lamarckism is the name now widely used for the adaptive force.
Between 1800 and 1830, Lamarck proposed a systematic theoretical
framework for understanding evolution. He saw evolution as comprising
four laws:
"Life by its own force, tends to increase the volume of all
organs which possess the force of life, and the force of life extends
the dimensions of those parts up to an extent that those parts bring to
themselves;"
"The production of a new organ in an animal body, results from a new
requirement arising. and which continues to make itself felt, and a new
movement which that requirement gives birth to, and its
upkeep/maintenance;"
"The development of the organs, and their ability, are constantly a result of the use of those organs."
"All that has been acquired, traced, or changed, in the physiology
of individuals, during their life, is conserved through the genesis,
reproduction, and transmitted to new individuals who are related to
those who have undergone those changes."
Lamarck's discussion of heredity
In
1830, in an aside from his evolutionary framework, Lamarck briefly
mentioned two traditional ideas in his discussion of heredity, in his
day considered to be generally true. The first was the idea of use
versus disuse; he theorized that individuals lose characteristics they
do not require, or use, and develop characteristics that are useful. The
second was to argue that the acquired traits were heritable. He gave as
an imagined illustration the idea that when giraffes
stretch their necks to reach leaves high in trees, they would
strengthen and gradually lengthen their necks. These giraffes would then
have offspring with slightly longer necks. In the same way, he argued, a
blacksmith,
through his work, strengthens the muscles in his arms, and thus his
sons would have similar muscular development when they mature. Lamarck
stated the following two laws:
Première Loi: Dans tout animal qui n' a point dépassé le
terme de ses développemens, l' emploi plus fréquent et soutenu d' un
organe quelconque, fortifie peu à peu cet organe, le développe, l'
agrandit, et lui donne une puissance proportionnée à la durée de cet
emploi; tandis que le défaut constant d' usage de tel organe,
l'affoiblit insensiblement, le détériore, diminue progressivement ses
facultés, et finit par le faire disparoître.
Deuxième Loi: Tout ce que la nature a fait acquérir ou perdre aux
individus par l' influence des circonstances où leur race se trouve
depuis long-temps exposée, et, par conséquent, par l' influence de l'
emploi prédominant de tel organe, ou par celle d' un défaut constant d'
usage de telle partie; elle le conserve par la génération aux nouveaux
individus qui en proviennent, pourvu que les changemens acquis soient
communs aux deux sexes, ou à ceux qui ont produit ces nouveaux
individus.
English translation:
First Law [Use and Disuse]: In every animal which has not passed
the limit of its development, a more frequent and continuous use of any
organ gradually strengthens, develops and enlarges that organ, and
gives it a power proportional to the length of time it has been so used;
while the permanent disuse of any organ imperceptibly weakens and
deteriorates it, and progressively diminishes its functional capacity,
until it finally disappears.
Second Law [Soft Inheritance]: All the acquisitions or losses
wrought by nature on individuals, through the influence of the
environment in which their race has long been placed, and hence through
the influence of the predominant use or permanent disuse of any organ;
all these are preserved by reproduction to the new individuals which
arise, provided that the acquired modifications are common to both
sexes, or at least to the individuals which produce the young.
In essence, a change in the environment brings about change in "needs" (besoins),
resulting in change in behaviour, causing change in organ usage and
development, bringing change in form over time—and thus the gradual transmutation of the species. As the evolutionary biologists and historians of science Conway Zirkle, Michael Ghiselin, and Stephen Jay Gould have pointed out, these ideas were not original to Lamarck.
Weismann's experiment
August Weismann's germ plasm theory. The hereditary material, the germ plasm, is confined to the gonads and the gametes. Somatic cells (of the body) develop afresh in each generation from the germ plasm, creating an invisible "Weismann barrier" to Lamarckian influence from the soma to the next generation.
August Weismann's germ plasm theory held that germline cells in the gonads
contain information that passes from one generation to the next,
unaffected by experience, and independent of the somatic (body) cells.
This implied what came to be known as the Weismann barrier, as it would make Lamarckian inheritance from changes to the body difficult or impossible.
Weismann conducted the experiment of removing the tails of 68 white mice,
and those of their offspring over five generations, and reporting that
no mice were born in consequence without a tail or even with a shorter
tail. In 1889, he stated that "901 young were produced by five
generations of artificially mutilated parents, and yet there was not a
single example of a rudimentary tail or of any other abnormality in this
organ." The experiment, and the theory behind it, were thought at the time to be a refutation of Lamarckism.
The experiment's effectiveness in refuting Lamarck's hypothesis is doubtful, as it did not address the use and disuse of characteristics in response to the environment. The biologist Peter Gauthier noted in 1990 that:
Can Weismann's experiment be
considered a case of disuse? Lamarck proposed that when an organ was not
used, it slowly, and very gradually atrophied. In time, over the course
of many generations, it would gradually disappear as it was inherited
in its modified form in each successive generation. Cutting the tails
off mice does not seem to meet the qualifications of disuse, but rather
falls in a category of accidental misuse... Lamarck's hypothesis has
never been proven experimentally and there is no known mechanism to
support the idea that somatic change, however acquired, can in some way
induce a change in the germplasm. On the other hand it is difficult to
disprove Lamarck's idea experimentally, and it seems that Weismann's
experiment fails to provide the evidence to deny the Lamarckian
hypothesis, since it lacks a key factor, namely the willful exertion of
the animal in overcoming environmental obstacles.
Ghiselin also considered the Weismann tail-chopping experiment to
have no bearing on the Lamarckian hypothesis, writing in 1994 that:
The acquired characteristics that figured in Lamarck's
thinking were changes that resulted from an individual's own drives and
actions, not from the actions of external agents. Lamarck was not
concerned with wounds, injuries or mutilations, and nothing that Lamarck
had set forth was tested or "disproven" by the Weismann tail-chopping
experiment.
The historian of science Rasmus Winther stated that Weismann had
nuanced views about the role of the environment on the germ plasm.
Indeed, like Darwin, he consistently insisted that a variable
environment was necessary to cause variation in the hereditary material.
Textbook Lamarckism
The long neck of the giraffe
is often used as an example in popular explanations of Lamarckism.
However, this was only a small part of his theory of evolution towards
"perfection"; it was a hypothetical illustration; and he used it to
discuss his theory of heredity, not evolution.
The identification of Lamarckism with the inheritance of acquired
characteristics is regarded by evolutionary biologists including
Ghiselin and Gould as a falsified artifact of the subsequent history of
evolutionary thought, repeated in textbooks without analysis, and
wrongly contrasted with a falsified picture of Darwin's thinking.
Ghiselin notes that "Darwin accepted the inheritance of acquired
characteristics, just as Lamarck did, and Darwin even thought that there
was some experimental evidence to support it."
Gould wrote that in the late 19th century, evolutionists "re-read
Lamarck, cast aside the guts of it ... and elevated one aspect of the
mechanics—inheritance of acquired characters—to a central focus it never
had for Lamarck himself."
He argued that "the restriction of 'Lamarckism' to this relatively
small and non-distinctive corner of Lamarck's thought must be labelled
as more than a misnomer, and truly a discredit to the memory of a man
and his much more comprehensive system."
The period of the history of evolutionary thought between Darwin's death in the 1880s, and the foundation of population genetics in the 1920s and the beginnings of the modern evolutionary synthesis in the 1930s, is called the eclipse of Darwinism
by some historians of science. During that time many scientists and
philosophers accepted the reality of evolution but doubted whether
natural selection was the main evolutionary mechanism.
Among the most popular alternatives were theories involving the
inheritance of characteristics acquired during an organism's lifetime.
Scientists who felt that such Lamarckian mechanisms were the key to
evolution were called neo-Lamarckians. They included the British botanistGeorge Henslow
(1835–1925), who studied the effects of environmental stress on the
growth of plants, in the belief that such environmentally-induced
variation might explain much of plant evolution, and the American entomologist Alpheus Spring Packard Jr., who studied blind animals living in caves and wrote a book in 1901 about Lamarck and his work. Also included were paleontologists like Edward Drinker Cope and Alpheus Hyatt,
who observed that the fossil record showed orderly, almost linear,
patterns of development that they felt were better explained by
Lamarckian mechanisms than by natural selection. Some people, including
Cope and the Darwin critic Samuel Butler,
felt that inheritance of acquired characteristics would let organisms
shape their own evolution, since organisms that acquired new habits
would change the use patterns of their organs, which would kick-start
Lamarckian evolution. They considered this philosophically superior to
Darwin's mechanism of random variation acted on by selective pressures.
Lamarckism also appealed to those, like the philosopher Herbert Spencer and the German anatomist Ernst Haeckel, who saw evolution as an inherently progressive process. The German zoologistTheodor Eimer combined Larmarckism with ideas about orthogenesis, the idea that evolution is directed towards a goal.
With the development of the modern synthesis
of the theory of evolution, and a lack of evidence for a mechanism for
acquiring and passing on new characteristics, or even their
heritability, Lamarckism largely fell from favour. Unlike neo-Darwinism,
neo-Lamarckism is a loose grouping of largely heterodox theories and
mechanisms that emerged after Lamarck's time, rather than a coherent
body of theoretical work.
Neo-Lamarckian versions of evolution were widespread in the late 19th
century. The idea that living things could to some degree choose the
characteristics that would be inherited allowed them to be in charge of
their own destiny as opposed to the Darwinian view, which placed them at
the mercy of the environment. Such ideas were more popular than natural
selection in the late 19th century as it made it possible for
biological evolution to fit into a framework of a divine or naturally
willed plan, thus the neo-Lamarckian view of evolution was often
advocated by proponents of orthogenesis. According to the historian of science Peter J. Bowler, writing in 2003:
One of the most emotionally
compelling arguments used by the neo-Lamarckians of the late nineteenth
century was the claim that Darwinism was a mechanistic theory which
reduced living things to puppets driven by heredity. The selection
theory made life into a game of Russian roulette, where life or death
was predetermined by the genes one inherited. The individual could do
nothing to mitigate bad heredity. Lamarckism, in contrast, allowed the
individual to choose a new habit when faced with an environmental
challenge and shape the whole future course of evolution.
Scientists from the 1860s onwards conducted numerous experiments
that purported to show Lamarckian inheritance. Some examples are
described in the table.
Early 20th century
Paul Kammerer claimed in the 1920s to have found evidence for Lamarckian inheritance in midwife toads, in a case celebrated by the journalist Arthur Koestler, but the results are thought to be either fraudulent or at best misinterpreted.
A century after Lamarck, scientists and philosophers continued to
seek mechanisms and evidence for the inheritance of acquired
characteristics. Experiments were sometimes reported as successful, but
from the beginning these were either criticised on scientific grounds or
shown to be fakes. For instance, in 1906, the philosopher Eugenio Rignano argued for a version that he called "centro-epigenesis", but it was rejected by most scientists. Some of the experimental approaches are described in the table.
Early 20th century experiments attempting to demonstrate Lamarckian inheritance
Criticised by William Bateson; Tower claimed all results lost in fire; William E. Castle visited laboratory, found fire suspicious, doubted claim that steam leak had killed all beetles, concluded faked data.
The British anthropologist Frederic Wood Jones and the South African paleontologist Robert Broom supported a neo-Lamarckian view of human evolution. The German anthropologist Hermann Klaatsch relied on a neo-Lamarckian model of evolution to try and explain the origin of bipedalism.
Neo-Lamarckism remained influential in biology until the 1940s when the
role of natural selection was reasserted in evolution as part of the
modern evolutionary synthesis.
Herbert Graham Cannon, a British zoologist, defended Lamarckism in his 1959 book Lamarck and Modern Genetics. In the 1960s, "biochemical Lamarckism" was advocated by the embryologist Paul Wintrebert.
In 1987, Ryuichi Matsuda coined the term "pan-environmentalism" for his evolutionary theory which he saw as a fusion of Darwinism with neo-Lamarckism. He held that heterochrony is a main mechanism for evolutionary change and that novelty in evolution can be generated by genetic assimilation.His views were criticized by Arthur M. Shapiro
for providing no solid evidence for his theory. Shapiro noted that
"Matsuda himself accepts too much at face value and is prone to
wish-fulfilling interpretation."
A form of Lamarckism was revived in the Soviet Union of the 1930s when Trofim Lysenko promoted the ideologically driven research programme, Lysenkoism; this suited the ideological opposition of Joseph Stalin to genetics.
Lysenkoism influenced Soviet agricultural policy which in turn was
later blamed for the numerous massive crop failures experienced within
Soviet states.
Critique
George Gaylord Simpson in his book Tempo and Mode in Evolution (1944) claimed that experiments in heredity have failed to corroborate any Lamarckian process.
Simpson noted that neo-Lamarckism "stresses a factor that Lamarck
rejected: inheritance of direct effects of the environment" and
neo-Lamarckism is closer to Darwin's pangenesis than Lamarck's views.
Simpson wrote, "the inheritance of acquired characters, failed to meet
the tests of observation and has been almost universally discarded by
biologists."
What Lamarck really did was to accept the hypothesis that
acquired characters were heritable, a notion which had been held almost
universally for well over two thousand years and which his
contemporaries accepted as a matter of course, and to assume that the
results of such inheritance were cumulative from generation to
generation, thus producing, in time, new species. His individual
contribution to biological theory consisted in his application to the
problem of the origin of species of the view that acquired characters
were inherited and in showing that evolution could be inferred logically
from the accepted biological hypotheses. He would doubtless have been
greatly astonished to learn that a belief in the inheritance of acquired
characters is now labeled "Lamarckian," although he would almost
certainly have felt flattered if evolution itself had been so
designated.
Peter Medawar
wrote regarding Lamarckism, "very few professional biologists believe
that anything of the kind occurs—or can occur—but the notion persists
for a variety of nonscientific reasons." Medawar stated there is no
known mechanism by which an adaptation acquired in an individual's
lifetime can be imprinted on the genome and Lamarckian inheritance is
not valid unless it excludes the possibility of natural selection, but
this has not been demonstrated in any experiment.
A host of experiments have been designed to test
Lamarckianism. All that have been verified have proved negative. On the
other hand, tens of thousands of experiments— reported in the journals
and carefully checked and rechecked by geneticists throughout the world—
have established the correctness of the gene-mutation theory beyond all
reasonable doubt... In spite of the rapidly increasing evidence for
natural selection, Lamarck has never ceased to have loyal followers....
There is indeed a strong emotional appeal in the thought that every
little effort an animal puts forth is somehow transmitted to his
progeny.
According to Ernst Mayr, any Lamarckian theory involving the inheritance of acquired characters has been refuted as "DNA
does not directly participate in the making of the phenotype and that
the phenotype, in turn, does not control the composition of the DNA."
Peter J. Bowler has written that although many early scientists took
Lamarckism seriously, it was discredited by genetics in the early
twentieth century.
Mechanisms resembling Lamarckism
Studies in the field of epigenetics, genetics and somatic hypermutation have highlighted the possible inheritance of traits acquired by the previous generation. However, the characterization of these findings as Lamarckism has been disputed.
Transgenerational epigenetic inheritance
DNA molecule with epigenetic marks, created by methylation, enabling a neo-Lamarckian pattern of inheritance for some generations
Epigenetic inheritance has been argued by scientists including Eva Jablonka and Marion J. Lamb to be Lamarckian. Epigenetics is based on hereditary elements other than genes that pass into the germ cells. These include methylation patterns in DNA and chromatin marks on histone proteins, both involved in gene regulation. These marks are responsive to environmental stimuli, differentially affect gene expression, and are adaptive, with phenotypic
effects that persist for some generations. The mechanism may also
enable the inheritance of behavioral traits, for example in chickens, rats
and human populations that have experienced starvation, DNA methylation
resulting in altered gene function in both the starved population and
their offspring. Methylation similarly mediates epigenetic inheritance in plants such as rice. Small RNA molecules, too, may mediate inherited resistance to infection. Handel and Ramagopalan commented that "epigenetics allows the peaceful co-existence of Darwinian and Lamarckian evolution."
Joseph Springer and Dennis Holley commented in 2013 that:
Lamarck and his ideas were ridiculed and discredited. In a
strange twist of fate, Lamarck may have the last laugh. Epigenetics, an
emerging field of genetics, has shown that Lamarck may have been at
least partially correct all along. It seems that reversible and
heritable changes can occur without a change in DNA sequence (genotype)
and that such changes may be induced spontaneously or in response to
environmental factors—Lamarck's "acquired traits." Determining which
observed phenotypes are genetically inherited and which are
environmentally induced remains an important and ongoing part of the
study of genetics, developmental biology, and medicine.
The prokaryoticCRISPR system and Piwi-interacting RNA could be classified as Lamarckian, within a Darwinian framework.However, the significance of epigenetics in evolution is uncertain. Critics such as the evolutionary biologist Jerry Coyne point out that epigenetic inheritance lasts for only a few generations, so it is not a stable basis for evolutionary change.
The evolutionary biologist T. Ryan Gregory
contends that epigenetic inheritance should not be considered
Lamarckian. According to Gregory, Lamarck did not claim that the
environment directly affected living things. Instead, Lamarck "argued
that the environment created needs to which organisms responded by using
some features more and others less, that this resulted in those
features being accentuated or attenuated, and that this difference was
then inherited by offspring." Gregory has stated that Lamarckian
evolution in epigenetics is more like Darwin's point of view than
Lamarck's.
In 2007, David Haig
wrote that research into epigenetic processes does allow a Lamarckian
element in evolution but the processes do not challenge the main tenets
of the modern evolutionary synthesis as modern Lamarckians have claimed.
Haig argued for the primacy of DNA and evolution of epigenetic switches
by natural selection.
Haig has written that there is a "visceral attraction" to Lamarckian
evolution from the public and some scientists, as it posits the world
with a meaning, in which organisms can shape their own evolutionary
destiny.
Thomas Dickens and Qazi Rahman (2012) have argued that epigenetic
mechanisms such as DNA methylation and histone modification are
genetically inherited under the control of natural selection and do not
challenge the modern synthesis. They dispute the claims of Jablonka and
Lamb on Lamarckian epigenetic processes.
In 2015, Khursheed Iqbal and colleagues discovered that although
"endocrine disruptors exert direct epigenetic effects in the exposed
fetal germ cells, these are corrected by reprogramming events in the
next generation."
Also in 2015, Adam Weiss argued that bringing back Lamarck in the
context of epigenetics is misleading, commenting, "We should remember
[Lamarck] for the good he contributed to science, not for things that
resemble his theory only superficially. Indeed, thinking of CRISPR and
other phenomena as Lamarckian only obscures the simple and elegant way
evolution really works."
Somatic hypermutation and reverse transcription to germline
In the 1970s, the Australian immunologist Edward J. Steele developed a neo-Lamarckian theory of somatic hypermutation within the immune system and coupled it to the reverse transcription of RNA derived from body cells to the DNA of germline
cells. This reverse transcription process supposedly enabled
characteristics or bodily changes acquired during a lifetime to be
written back into the DNA and passed on to subsequent generations.
The mechanism was meant to explain why homologous DNA sequences from the VDJ gene
regions of parent mice were found in their germ cells and seemed to
persist in the offspring for a few generations. The mechanism involved
the somatic selection and clonal amplification of newly acquired antibodygene sequences generated via somatic hypermutation in B-cells. The messenger RNA products of these somatically novel genes were captured by retrovirusesendogenous
to the B-cells and were then transported through the bloodstream where
they could breach the Weismann or soma-germ barrier and reverse
transcribe the newly acquired genes into the cells of the germ line, in
the manner of Darwin's pangenes.
The historian of biology Peter J. Bowler
noted in 1989 that other scientists had been unable to reproduce his
results, and described the scientific consensus at the time:
There is no feedback of information
from the proteins to the DNA, and hence no route by which
characteristics acquired in the body can be passed on through the genes.
The work of Ted Steele (1979) provoked a flurry of interest in the
possibility that there might, after all, be ways in which this reverse
flow of information could take place. ... [His] mechanism did not, in
fact, violate the principles of molecular biology, but most biologists
were suspicious of Steele's claims, and attempts to reproduce his
results have failed.
Bowler commented that "[Steele's] work was bitterly criticized at the
time by biologists who doubted his experimental results and rejected
his hypothetical mechanism as implausible."
The hologenome theory of evolution, while Darwinian, has Lamarckian aspects. An individual animal or plant lives in symbiosis with many microorganisms, and together they have a "hologenome" consisting of all their genomes. The hologenome can vary like any other genome by mutation, sexual recombination, and chromosome rearrangement,
but in addition it can vary when populations of microorganisms increase
or decrease (resembling Lamarckian use and disuse), and when it gains
new kinds of microorganism (resembling Lamarckian inheritance of
acquired characteristics). These changes are then passed on to
offspring.
The mechanism is largely uncontroversial, and natural selection does
sometimes occur at whole system (hologenome) level, but it is not clear
that this is always the case.
The Baldwin effect, named after the psychologist James Mark Baldwin
by George Gaylord Simpson in 1953, proposes that the ability to learn
new behaviours can improve an animal's reproductive success, and hence
the course of natural selection on its genetic makeup. Simpson stated
that the mechanism was "not inconsistent with the modern synthesis" of
evolutionary theory,
though he doubted that it occurred very often or could be proven to
occur. He noted that the Baldwin effect provided a reconciliation
between the neo-Darwinian and neo-Lamarckian approaches, something that
the modern synthesis
had seemed to render unnecessary. In particular, the effect allows
animals to adapt to a new stress in the environment through behavioural
changes, followed by genetic change. This somewhat resembles Lamarckism
but without requiring animals to inherit characteristics acquired by
their parents. The Baldwin effect is broadly accepted by Darwinists.
In sociocultural evolution
Within the field of cultural evolution, Lamarckism has been applied as a mechanism for dual inheritance theory.
Gould viewed culture as a Lamarckian process whereby older generations
transmitted adaptive information to offspring via the concept of learning. In the history of technology,
components of Lamarckism have been used to link cultural development to
human evolution by considering technology as extensions of human
anatomy.
This image describes the final stage in mitosis, telophase.Fluorescence
micrograph of a human cell in telophase showing chromosomes (DNA) in
blue, microtubules in green and kinetochores in pink
The phosphorylation
of the protein targets of M-Cdks (Mitotic Cyclin-dependent Kinases)
drives spindle assembly, chromosome condensation and nuclear envelope
breakdown in early mitosis. The dephosphorylation of these same
substrates drives spindle disassembly, chromosome decondensation and the
reformation of daughter nuclei in telophase. Establishing a degree of
dephosphorylation permissive to telophase events requires both the
inactivation of Cdks and the activation of phosphatases.
Cdk inactivation is primarily the result of the destruction of its associated cyclin. Cyclins are targeted for proteolytic degradation by the anaphase promoting complex (APC), also known as the cyclosome, a ubiquitin-ligase. The active, CDC20-bound APC (APC/CCDC20) targets mitotic cyclins for degradation starting in anaphase.
Experimental addition of non-degradable M-cyclin to cells induces cell
cycle arrest in a post-anaphase/pre-telophase-like state with condensed
chromosomes segregated to cell poles, an intact mitotic spindle, and no
reformation of the nuclear envelope. This has been shown in frog (Xenopus) eggs, fruit flies (Drosophilla melanogaster), budding (Saccharomyces cerevisiae) and fission (Schizosaccharomyces pombe) yeast, and in multiple human cell lines.
The requirement for phosphatase activation can be seen in budding
yeast, which do not have redundant phosphatases for mitotic exit and
rely on the phosphatase cdc14. Blocking cdc14 activation in these cells results in the same phenotypic arrest as does blocking M-cyclin degradation.
Historically, it has been thought that anaphase and telophase are events that occur passively after satisfaction of the spindle-assembly checkpoint (SAC) that defines the metaphase-anaphase transition.
However, the existence of differential phases to cdc14 activity between
anaphase and telophase is suggestive of additional, unexplored late-mitotic checkpoints.
Cdc14 is activated by its release into the nucleus, from sequestration
in the nucleolus, and subsequent export into the cytoplasm. The Cdc-14
Early Anaphase Release pathway, which stabilizes the spindle, also
releases cdc14 from the nucleolus but restricts it to the nucleus.
Complete release and maintained activation of cdc14 is achieved by the
separate Mitotic Exit Network (MEN) pathway to a sufficient degree (to
trigger the spindle disassembly and nuclear envelope assembly) only
after late anaphase.
Cdc14-mediated dephosphorylation activates downstream regulatory
processes unique to telophase. For example, the dephosphorylation of CDH1 allows the APC/C to bind CDH1. APC/CCDH1 targets CDC20 for proteolysis, resulting in a cellular switch from APC/CCDC20 to APC/CCDH1 activity. The ubiquitination of mitotic cyclins continues along with that of APC/CCDH1-specific targets such as the yeast mitotic spindle component, Ase1, and cdc5, the degradation of which is required for the return of cells to the G1 phase.
Additional mechanisms driving telophase
A shift in the whole-cell phosphoprotein profile is only the broadest of many regulatory mechanisms contributing to the onset of individual telophase events.
The anaphase-mediated distancing of chromosomes from the metaphase plate may trigger spatial cues for the onset of telophase.
An important regulator and effector of telophase is cdc48 (homologous to yeast cdc48 is human p97, both structurally and functionally), a protein that mechanically employs its ATPase
activity to alter target protein conformation. Cdc48 is necessary for
spindle disassembly, nuclear envelope assembly, and chromosome
decondensation. Cdc48 modifies proteins structurally involved in these
processes and also some ubiquitinated proteins which are thus targeted
to the proteasome.
Mitotic spindle disassembly
Stages of late M phase in a vertebrate cell
The breaking of the mitotic spindle, common to the completion of
mitosis in all eukaryotes, is the event most often used to define the
anaphase-B to telophase transition, although the initiation of nuclear reassembly tends to precede that of spindle disassembly.
Spindle disassembly is an irreversible process which must effect
not the ultimate degradation, but the reorganization of constituent
microtubules; microtubules are detached from kinetochores and spindle pole bodies and return to their interphase states.
Spindle depolymerization during telophase occurs from the plus end and is, in this way, a reversal of spindle assembly.
Subsequent microtubule array assembly is, unlike that of the polarized
spindle, interpolar. This is especially apparent in animal cells which
must immediately, following mitotic spindle disassembly, establish the
antiparallel bundle of microtubules known as the central spindle in order to regulate cytokinesis. The ATPase p97 is required for the establishment of the relatively stable and long interphase microtubule arrays following disassembly of the highly dynamic and relatively short mitotic ones.
While spindle assembly has been well studied and characterized as
a process where tentative structures are edified by the SAC, the
molecular basis of spindle disassembly is not understood in comparable
detail. The late-mitotic dephosphorylation cascade
of M-Cdk substrates by the MEN is broadly held to be responsible for
spindle disassembly. The phosphorylation states of microtubule
stabilizing and destabilizing factors, as well as microtubule nucleators
are key regulators of their activities.
For example, NuMA is a minus-end crosslinking protein and Cdk substrate
whose dissociation from the microtubule is effected by its
dephosphorylation during telophase.
A general model for spindle disassembly in yeast is that the
three functionally overlapping subprocesses of spindle disengagement,
destabilization, and depolymerization are primarily effected by APC/CCDH1,
microtubule-stabilizer-specific kinases, and plus-end directed
microtubule depolymerases, respectively. These effectors are known to be
highly conserved between yeast and higher eukaryotes. The APC/CCDH1 targets crosslinking microtubule-associated proteins (NuMA, Ase1, Cin1 and more). AuroraB (yeast IpI1) phosphorylates the spindle-associated stabilizing protein EB1 (yeast Bim1), which then dissociates from microtubules, and the destabilizer She1, which then associates with microtubules. Kinesin8
(yeast Kip3), an ATP-dependent depolymerase, accelerate microtubule
depolymerization at the plus end. It was shown the concurrent disruption
of these mechanisms, but not of any one, results in dramatic spindle
hyperstability during telophase, suggesting functional overlap despite
the diversity of the mechanisms.
Nuclear envelope reassembly
The main components of the nuclear envelope are a double membrane, nuclear pore complexes, and a nuclear lamina
internal to the inner nuclear membrane. These components are dismantled
during prophase and prometaphase and reconstructed during telophase,
when the nuclear envelope reforms on the surface of separated sister
chromatids. The nuclear membrane is fragmented and partly absorbed by the endoplasmic reticulum during prometaphase and the targeting of inner nuclear membrane protein-containing ER vesicles
to the chromatin occurs during telophase in a reversal of this process.
Membrane-forming vesicles aggregate directly to the surface of
chromatin, where they fuse laterally into a continuous membrane.
Ran-GTP
is required for early nuclear envelope assembly at the surface of the
chromosomes: it releases envelope components sequestered by importin β
during early mitosis. Ran-GTP localizes near chromosomes throughout
mitosis, but does not trigger the dissociation of nuclear envelope
proteins from importin β until M-Cdk targets are dephosphorylated in
telophase.
These envelope components include several nuclear pore components, the
most studied of which is the nuclear pore scaffold protein ELYS, which can recognize DNA regions rich in A:T base pairs (in vitro), and may therefore bind directly to the DNA. However, experiments in Xenopus egg extracts have concluded that ELYS fails to associate with bare DNA and will only directly bind histone dimers and nucleosomes.
After binding to chromatin, ELYS recruits other components of the
nuclear pore scaffold and nuclear pore trans-membrane proteins. The
nuclear pore complex is assembled and integrated in the nuclear envelope
in an organized manner, consecutively adding Nup107-160, POM121, and FG Nups.
It is debated whether the mechanism of nuclear membrane
reassembly involves initial nuclear pore assembly and subsequent
recruitment of membrane vesicles around the pores or if the nuclear
envelope forms primarily from extended ER cisternae, preceding nuclear
pore assembly:
In cells where the nuclear membrane fragments into non-ER
vesicles during mitosis, a Ran-GTP–dependent pathway can direct these
discrete vesicle populations to chromatin where they fuse to reform the
nuclear envelope.
In cells where the nuclear membrane is absorbed into the endoplasmic
reticulum during mitosis, reassembly involves the lateral expansion
around the chromatin with stabilization of the expanding membrane over
the surface of the chromatin.
Studies claiming this mechanism is a prerequisite to nuclear pore
formation have found that bare-chromatin-associated Nup107–160 complexes
are present in single units instead of as assembled pre-pores.
The envelope smoothens and expands following its enclosure of the
whole chromatid set. This probably occurs due to the nuclear pores'
import of lamin, which can be retained within a continuous membrane. The nuclear envelopes of Xenopus
egg extracts failed to smoothen when nuclear import of lamin was
inhibited, remaining wrinkled and closely bound to condensed
chromosomes.
However, in the case of ER lateral expansion, nuclear import is
initiated before completion of the nuclear envelope reassembly, leading
to a temporary intra-nuclear protein gradient between the distal and
medial aspects of the forming nucleus.
Lamin subunits disassembled in prophase are inactivated and
sequestered during mitosis. Lamina reassembly is triggered by lamin
dephosphorylation (and additionally by methyl-esterification of COOH residues on lamin-B). Lamin-B can target chromatin as early as mid-anaphase. During telophase, when nuclear import is reestablished, lamin-A
enters the reforming nucleus but continues to slowly assemble into the
peripheral lamina over several hours in throughout the G1 phase.
Xenopus egg extracts and human cancer cell lines have been the primary models used for studying nuclear envelope reassembly.
Yeast lack lamins; their nuclear envelope remains intact throughout mitosis and nuclear division happens during cytokinesis.
Chromosome decondensation
Chromosome
decondensation (also known as relaxation or decompaction) into expanded
chromatin is necessary for the cell's resumption of interphase
processes, and occurs in parallel to nuclear envelope assembly during
telophase in many eukaryotes. MEN-mediated Cdk dephosphorylation is necessary for chromosome decondensation.
In vertebrates, chromosome decondensation is initiated only after nuclear import
is reestablished. If lamin transport through nuclear pores is
prevented, chromosomes remain condensed following cytokinesis, and cells
fail to reenter the next S phase.
In mammals, DNA licensing for S phase (the association of chromatin to
the multiple protein factors necessary for its replication) also occurs
coincidentally with the maturation of the nuclear envelope during late
telophase. This can be attributed to and provides evidence for the nuclear import
machinery's reestablishment of interphase nuclear and cytoplasmic
protein localizations during telophase.
