Ontogenyand

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, developmentis a consequence of the confluence of genes and prior experience, which in turn alters the subsequent expressionof genes and their interactions with the environment

RousseauPreformationist; adult determined in egg, simply

unfolds

LockeTabula 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)

2. From the more general characters develop the less generaland finally the special characters (e.g., differentiation ofwings and forelimbs from primitive limb buds)

1. During development, general characters appear before special characters; e.g., those of phylum before those of class (e.g. notochord before limbs).

3. Animals of different species become increasingly different as they develop.

4. Young stages do not resemble the adult stages ofancestors, but resemble the young of those ancestors

Eight ways altered ontogeny can producephylogenetic change (deBeer)

1. caenogenesis—addition of characters in youthful stagese.g., amnion of vertbrates

Eight ways altered ontogeny can producephylogenetic change (deBeer)

1. caenogenesis

2. adult variation—new characters expressed in adulthoode.g. species-specific plumage in birds after final molt

Eight ways altered ontogeny can producephylogenetic change (deBeer)

1. caenogenesis

2. adult variation

3. deviation—developmental alterations persist in adultse.g. gill slitsgills in fish,

gill slitsEustacian tubes in mammals

Eight ways altered ontogeny can producephylogenetic change (deBeer)

1. caenogenesis

2. adult variation

3. deviation

4. reduction—developmental loss of a charactere.g. tails of tadpoles, human fetuses

Eight ways altered ontogeny can producephylogenetic change (deBeer)

1. caenogenesis

2. adult variation

3. deviation

4. reduction

5. retardation—delay in the development of a charactere.g. “wisdom teeth” of humans

Eight ways altered ontogeny can producephylogenetic change (deBeer)

1. caenogenesis

2. adult variation

3. deviation

4. reduction

5. retardation

6. neoteny—retention of immature characteristicse.g. vertical face of humans; also plumage of flightlessbirds (e.g. ostrich)

Eight ways altered ontogeny can producephylogenetic 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 producephylogenetic 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 producephylogenetic 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?

analogy?

a character in two species is homologous if thatSAME character is present in a common ancestor

a character in two species is analogous if itserves a similar function but was not present ina 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.

acceleratio

n

retardatio

n

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 ofbrains and parts of brains?

Problem: No fossil record of brains

Similarity of nuclear groups—cell typesPhysical resemblance

Similarity of connnections—afferent and efferentBiochemical similarity—e.g. neurotransmitter used

Functional similarity

• minuteness of detail in similarities important• continuance through species of intermediate classes

Similar ontogenetic development

Related problem:How does one distinguish betweenprimitive and derived characters?

Primitive: of two characters, the one that has evolutionary precedence is the primitive character

Derived: of two characters, the one appearing lateris the derived character

A B B A

Leftmost in each diagram: out-groupCenter in each diagram: sister groupRightmost in each diagram: targetWhich is the primitive condition, A or B?

Out-group analysis

A or B? A or B?

B A B

Left-most in each diagram: out-groupCenter in each diagram: sister groupRightmost in each diagram: target

Out-group analysis

B

A B A

A

Out-group AnalysisApplied to Development

Terminal Addition or Paedomorphosisi.e., is B added in one species, or lost in the other?

some species: ABother species: A (throughout life)

A A AB

Aterminal addition

AB AB A

ABpaedomorphosis

Encephalizationas 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 onbehavior, 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 brainorganization.

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 makesense to talk about behavioral homologies?If so, how does one determine behavioral homology?