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Ontogeny
and
Phylogeny
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.
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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
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.
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
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
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis—addition of characters in youthful stages
e.g., amnion of vertbrates
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation—new characters expressed in adulthood
e.g. species-specific plumage in birds after final molt
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation
3. deviation—developmental alterations persist in adults
e.g. gill slitsgills in fish,
gill slitsEustacian tubes in mammals
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation
3. deviation
4. reduction—developmental loss of a character
e.g. tails of tadpoles, human fetuses
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation
3. deviation
4. reduction
5. retardation—delay in the development of a character
e.g. “wisdom teeth” of humans
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation
3. deviation
4. reduction
5. retardation
6. neoteny—retention of immature characteristics
e.g. vertical face of humans; also plumage of flightless
birds (e.g. ostrich)
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation
3. deviation
4. reduction
5. retardation
6. neoteny
7. acceleration—characters develop at earlier stages
e.g. development of heart in avian embryos; calluses on
the knees of ostriches
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation
3. deviation
4. reduction
5. retardation
6. neoteny
7. acceleration
8. hypermorphosis—prolongation of developmental period
ontogeny gets longer as special characters are added
during evolution.
Eight ways altered ontogeny can produce
phylogenetic change (deBeer)
1. caenogenesis
2. adult variation
3. deviation
4. reduction
5. retardation
6. neoteny
7. acceleration
8. hypermorphosis
deBeer, G. (1958) Embryos and Ancestors. Oxford:
Clarendon Press.
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.
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
These structures are homologous as forelimbs, as are bird wings (not
shown). However, bird wings and bat wings are NOT homologous as
wings.
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
How can one determine homology of
brains and parts of brains?
Problem: No fossil record of brains
Similarity of nuclear groups—cell types
Physical resemblance
Biochemical similarity—e.g. neurotransmitter used
Similarity of connnections—afferent and efferent
Functional similarity
Similar ontogenetic development
• minuteness of detail in similarities important
• continuance through species of intermediate classes
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
Out-group analysis
A
A or B?
B
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?
A
Out-group analysis
B
A
B
B
A
B
A
Left-most in each diagram: out-group
Center in each diagram: sister group
Rightmost in each diagram: target
A
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?
A
A
A
terminal addition
AB
AB
AB
AB
paedomorphosis
A
Encephalization
as an
Ontogenetic
and
Phylogenetic Principle
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.
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
How do brains evolve?
Two general mechanisms
Addition of new projections and
targets (invasion hypothesis)
Progressive differentiation of
previously undifferentiated tissue
(parcellation hypothesis).
Effects of periphery on brain
organization.
Motor systems
Sensory systems
Finally, why do
most long connections in
the nervous system
decussate?
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?