Ontogeny

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Ontogeny and Phylogeny
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In biology, epigenesis has at least two distinct
meanings the unfolding development of an
organism, and in particular the development of a
plant or animal from an egg or spore through a
sequence of steps in which cells differentiate
and organs form the theory that plants and
animals develop in this way, in contrast to
theories of preformation. From Wikepedia
More than this! Epigenesis implies that, at every
time point, development is a consequence of the
confluence of genes and prior experience, which
in turn alters the subsequent expression of genes
and their interactions with the environment
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Rousseau Preformationist adult determined in
egg, simply unfolds
Locke Tabula rasa blank slate upon which
experience writes
Preformationism was the answer to the difficult
question of how a complex adult can arise from
the simple gametes the preformed embryo was
complete and a direct result of evolution.
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Closely associated with the preformationist view
is the notion that ontogeny recapitulates
phylogeny. What does this mean? Ernst Haeckel
was principal advocate of the principle that
evolution occurred through changes introduced
into the adult ontogenies were simply the
reflection of prior evolutionary change, which
became compressed in time with
evolution thus, evolution was the cause of
ontogeny served to curtail research into
developmental biology
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Laws of von Baer (1828)
1. During development, general characters appear
before special characters e.g., those of
phylum before those of class (e.g. notochord
before limbs).
2. From the more general characters develop the
less general and finally the special characters
(e.g., differentiation of wings and forelimbs
from primitive limb buds)
3. Animals of different species become
increasingly different as they develop.
4. Young stages do not resemble the adult stages
of ancestors, but resemble the young of those
ancestors
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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)
1. caenogenesisaddition of characters in
youthful stages e.g., amnion of vertbrates
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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

2. adult variationnew characters expressed in
adulthood e.g. species-specific plumage in birds
after final molt
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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

1. adult variation

• deviationdevelopmental alterations persist in
  adults
• e.g. gill slits?gills in fish,
• gill slits?Eustacian tubes in mammals

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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

1. adult variation

3. deviation

• reductiondevelopmental loss of a character
• e.g. tails of tadpoles, human fetuses

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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

1. adult variation

3. deviation
4. reduction

• retardationdelay in the development of a
  character
• e.g. wisdom teeth of humans

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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

1. adult variation

3. deviation
4. reduction
5. retardation

• neotenyretention of immature characteristics
• e.g. vertical face of humans also plumage of
  flightless
• birds (e.g. ostrich)

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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

1. adult variation

3. deviation
4. reduction
5. retardation
6. neoteny
7. accelerationcharacters develop at earlier
stages e.g. development of heart in avian
embryos calluses on the knees of ostriches
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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

1. adult variation

3. deviation
4. reduction
5. retardation
6. neoteny
7. acceleration
8. hypermorphosisprolongation of developmental
period ontogeny gets longer as special
characters are added during evolution.
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Eight ways altered ontogeny can
produce phylogenetic change (deBeer)

1. caenogenesis

1. adult variation

3. deviation
4. reduction
5. retardation
6. neoteny
7. acceleration
8. hypermorphosis
deBeer, G. (1958) Embryos and Ancestors. Oxford
Clarendon Press.
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Eight Principles of de Beer Simplified to Two by
Stephen Jay Gould

1. Acceleration

2. Retardation
Changes in the relative timing of developmental
events more generally known as heterochrony.
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What is meant by
homology?
a character in two species is homologous if
that SAME character is present in a common
ancestor
analogy?
a character in two species is analogous if
it serves a similar function but was not present
in a common ancestor
arises from parallel or convergent evolution
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These structures are homologous as forelimbs, as
are bird wings (not shown). However, bird wings
and bat wings are NOT homologous as wings.
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Evolutionary changes in ontogenetic timing os
capable of producing morphological
evolution i.e., by changing the timing of tissue
induction, the number and quality of cells
induced may be modified
retardation
acceleration
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How can one determine homology of brains and
parts of brains?
Problem No fossil record of brains
Similarity of nuclear groupscell types
Physical resemblance
Biochemical similaritye.g. neurotransmitter used
Similarity of connnectionsafferent and efferent
Functional similarity
Similar ontogenetic development

• minuteness of detail in similarities important

• continuance through species of intermediate
  classes

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Related problem How does one distinguish
between primitive and derived characters?
Primitive of two characters, the one that has
evolutionary precedence is the primitive
character
Derived of two characters, the one appearing
later is the derived character
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Out-group analysis
A
B
B
A
A or B?
A or B?
Leftmost in each diagram out-group Center in
each diagram sister group Rightmost in each
diagram target Which is the primitive condition,
A or B?
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Out-group analysis
B
A
B
B
Left-most in each diagram out-group Center in
each diagram sister group Rightmost in each
diagram target
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Out-group Analysis Applied to Development
some species A?B other species A (throughout
life)
Terminal Addition or Paedomorphosis i.e., is B
added in one species, or lost in the other?
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Encephalization as an Ontogenetic and
Phylogenetic Principle
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How do brains evolve?
Evolution is a consequence of natural
selection. Selection acts on the phenotype,
not the genotype, although transmission is
primarily genetic. Natural selection selects
against characters, not for them.
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How do brains evolve?
What are the sources of selection pressures on
the brain?
Natural selection acts principally on behavior,
not directly on the brain
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How do brains evolve?
Two general mechanisms
Addition of new projections and targets
(invasion hypothesis)
Progressive differentiation of previously
undifferentiated tissue (parcellation hypothesis).
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Effects of periphery on brain organization.
Motor systems Sensory systems
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Finally, why do most long connections in the
nervous system decussate?
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What about Behavior?
If natural selection acts on brains by selection
of behavior, then does it make sense to talk
about behavioral homologies?
If so, how does one determine behavioral
homology?