1. differences among species that allowed each species to adapt to its environment. Thus, the fingers of the human hand, the flipper of the seal, and the wings of birds and bats were seen as marvelous contrivances, fashioned by the Creator, to allow these animals to adapt to their "conditions of existence.“ The structure that allow the organism to adapt was created first and then they are place into such environment where the appropriate organ can/will be used. They have this trait because it is needed in the particular type of environment..
2. adaptations were secondary, and that the "unity of type" (what Owen called "homologies") was critical. The human hand, the seal's flipper, and the wings of bats and birds are each modifications of the same basic plan. There is an original form from which every homologous structures are derived.
Darwin presented a theory that would explain the 2 theories. Unity of type=can be explain by descent from common ancestor Conditions of existence=explained by natural selection
The theory can be understood in two ways.
It appears, then, that the ancestor of all bilaterian organisms had sensory organs based on Pax6, a heart based on tinman, and a head based on Otx,Ems, and tll. It also had something else: an anterior-posterior polarity based on the expression of Hox genes. The analysis of Hox genes has given us critical clues as to how morphological changes could occur through alterations of development. So we return to our analysis of Hox genes.
1. This means that the enormous variation of morphological form in the animal kingdom is underlain by a common set of instructions.
In order to further understand how differences emerged, 4 critical ways was presented that might lead to evolutionary change.
Thus, the wing differences between dipterans (two-winged insects such as flies) and lepidopterans (butterflies and moths) can be attributed to the different ways in which potential target genes in the imaginal discs respond to the Ubx protein.
In Chapter 16, we saw that changes in Hox gene expression are correlated with the change in morphology from the fish fin to the tetrapod limb
Crustaceans are characterized by a pre-gnathal head (similar to the insect acron), gnathal (jawed) head segments, six thoracic segments, genital segments, abdominal segments, and a telson
In the arthropod lineage that gave rise to the insects, each gene would take on different (but sometimes overlapping) functions
The fossil record suggests that the earliest crustaceans lacked maxillipeds and had uniform thoracic segments. This would mean that the presence of maxillipeds is a derived characteristic that evolved in several crustacean lineages.
As shown in Chapter 11, the expression patterns of Hox genes in vertebrates determines the type of vertebral structure formed. Thoracic vertebrae, for instance, have ribs, while cervical (neck) vertebrae and lumbar vertebrae do not. The type of vertebra produced is specified by the Hox genes expressed in the somite.
Snakes evolved from lizards, and they appear to have lost their legs in a two-step process. Both paleontological and embryological evidence supports the view that snakes first lost their forelimbs and later lost their hindlimbs
In the presence of hoxc6 + hoxc8 there will be no formation of forelimbs
In most vertebrates, the forelimb forms just anterior to the most anterior expression domain of Hoxc-6
As we have learned from the previous discussions that AER is important in the establishment of the limb.
Distal-less is found throughout the animal kingdom, and it is expressed in those tissues that stick out from the body axis, notably limbs and antennae.
In this respect, the homology is similar to that of a human forearm and a seal flipper. The parts-the proteins-are homologous, and the structures they make up-the pathways-are homologous.
1. This does not mean that the Drosophila blastoderm is homologous to the human macrophage. It merely means that there is a very ancient pathway that predates the deuterostome-protostome split, and that this pathway can be used in different systems.
It was once thought that the only way to promote evolution was to add a step to the end of embryonic development, but we now know that even early stages can be altered to produce evolutionary novelties. The reason why changes in development can occur is that the embryo, like the adult organism, is composed of modules
Modules- there is autonomy between and among mudules - there is an autonomy between cells, tissues,organs
Example of heterochrony-heterochrony can "return" an organism to a larval state, free from the specialized adaptations of the adult. Heterochrony can also give larval characteristics to an adult organism, as in the small size and webbed feet of arboreal salamanders (Figure 22.17) or the fetal growth rate of human newborn brain tissue
In the very young (4- to 5-mm) whale embryo, the nose is in the usual mammalian position. However, the enormous growth of the maxilla and premaxilla (upper jaw) pushes over the frontal bone and forces the nose to the top of the skull (Figure 22.18). This new position of the nose (blowhole) allows the whale to have a large and highly specialized jaw apparatus and to breathe while parallel to the water's surface
The Hox genes, TGF-β family genes, MyoD family genes, and globin genes each probably started as a single gene that duplicated several times. After the duplication, mutations caused the divergences that gave the members of each family new functions. At the tissue level, one sees duplication and divergence in the somites that give rise to the cervical, thoracic, and lumbar vertebrae.
Wings- A structure originally used for walking has been recruited into a structure suitable for flying.
Developmental correlation- The modular nature of development also expects that modules will aggregate to form larger modules.
The dramatic changes in bone arrangement from agnathans to jawed fishes, from jawed fishes to amphibians, and from reptiles to mammals were coordinated with changes in jaw structure, jaw musculature, tooth deposition and shape, and the structure of the cranial vault and ear
Inserted barriers of gold foil into the prechondrogenic hindlimb buds of a 3.5-day chick embryo. This barrier separated the regions of tibia formation and fibula formation. The results of these experiments were twofold. First, the tibia is shortened, and the fibula bows and retains its connection to the fibulare (the distal portion of the tibia). Such relationships between the tibia and fibula are not usually seen in birds, but they are characteristic of reptiles (Figure 22.24). Second, the musculature of the hindlimb undergoes changes in parallel with the bones. Three of the muscles that attach to these bones now show characteristic reptilian patterns of insertion.
It seems, therefore, that experimental manipulations that alter the development of one part of the mesodermal limb-forming field also alter the development of other mesodermal components. This was crucial in the evolution of the bird hindlimb from the reptile hindlimb
Another example of developmental correlation involves the ability of one tissue to interact with another. In development, things have to fit together if the organism is to survive.
Separation of function---Such separation of functions can cause reproductive isolation and the separation of species when the receptor and ligand are proteins on the sperm and egg. While most proteins of closely related marine species are very similar, the proteins responsible for fertilization are often extremely different
restraints on phenotype production 1.PC- physical properties that restricts or limit the occurrence/emergence of certain structure - elasticity and tensile strength of tissue is also PC
Only in the presence of these inducing substances or organs or structures that can generate the induced structure..No inducer= no final organ
Stress, however, in the form of environmental factors such as temperature, can overpower the buffering systems of development and alter the phenotype. Moreover, the altered phenotype then becomes subject to natural selection, and if selected, will evebtually appear without the stress that originally induced it.(genetic assimilation)
If Heat shock,however, causes other proteins in the cell to become unstable, and Hsp90 is diverted from its normal function (of stabilizing the signal transduction proteins) to the more general function of stabilizing any of the cell's now partially denatured peptides (Jakob et al. 1995; Nathan et al. 1997). Since Hsp90 was known to be involved with inherently unstable proteins and could be diverted by stress, it was possible that Hsp90 might be involved in buffering developmental pathways against environmental contingencies.
The population genetics model contained some major assumptions that have now been called into question. 1.Evolution changes gradually. PE- evolutionary changes tended to be rapid not gradual, *new findings in paleonology and molecular biology”mutations in regulatory genes can create large changes in morphology in a relatively short time” 2. The idea that accumulations of small mutations result in changes leading to new species. -small changes in genetic material can lead to large result in macroevolution level.. 3. One genotype can permit several phenotype to form(POLYPHENISM) same gene can produce different phenotypes depending on the other genes present(gene interaction) 4. Proven wrong by::adult organisms may have dissimilar structures but the genes instructing the formation of these genes are extremely simillar.
Devbio n devgen---how the fittest came about,what are the
Developmental mechanisms of evolutionary change
Advanced Developmental Biology
A. "Unity of Type" and "Conditions of
B. Hox Genes: Descent with Modification
C. Homologous Pathways of Development
D. Modularity: The Prerequisite for
Evolution through Development
E. Developmental Correlation
F. Developmental Constraints
G. A New Evolutionary Synthesis
1. Differences among species that allowed
each species to adapt to its
environment ("conditions of existence.“)
2. Adaptations were secondary, and that
the "unity of type ("homologies") was
*”Evolution consists of modifying
embryonic organisms, not adult ones”
• Unity of type= descent from common
• Conditions of existence= natural
• *common descent- embryonic
• *modification- showing how
development was altered to produce
structures that enabled animals to adapt
to particular condition
1. To find the underlying unities that link
disparate groups of animals,
2. To detect those differences in
development that enable species to
adapt to particular environments.
• Urbilaterian ancestor/ PDA
– Hypothetical ancestor
– Had neither endoskeleton(deu.) or hard
“Paleontology without fossil”
• to find homologous genes that are
performing the same functions in both
a deuterostome (usually a chick or a
mouse) and a protostome (generally
an arthropod such as Drosophila).
• Pax6- plays a role in forming eyes in
both vertebrates and invertebrates
• Tinman/Nkx2 5- involved in heart
formation in deuto. and proto.
• tailless (tll) ,orthodenticle (otd) and
empty spiracles (ems)/(Otx-1, Otx-
2,Emx-1,Emx-2)- genes encoding for
transcription factors involved in head
• Hox genes- basis of anterior-posterior
axis specification throughout the animal
• If the underlying Hox gene expression is
uniform, how did the differences among
the phyla emerge?
– Arose from differences in how the Hox genes
are regulated and what target genes the
Hox-encoded proteins regulate.
1. Changes in the Hox protein-responsive
elements of downstream genes
2. Changes in Hox gene transcription
patterns within a portion of the body
3. Changes in Hox gene transcription
patterns between portions of the body
4. Changes in the number of Hox genes
-Changes in the genes hox proteins
Ultrabithorax gene (Ubx)-expressed in the
imaginal disc of the third thoracic
segment(wing or haltere are dericed)
Drosophila- Ubx downregulate several
genes in the imaginal disc
Butterfly- Genes regulated in Drosophila
are not regulated in butterfly
• Distal-less (Dll) gene is critical for
providing the proximal-distal axis of the
• Distal-less expression occurs in the
cephalic and thoracic limb-forming discs,
but it is excluded in the abdomen by the
abdA and Ubx proteins.
• Dll (in thorax and cephalic)= limb and
wing(thorax) and jaw (head) formation
• Dll inhibited by abdA and Ubx (in
abdomen) = no legs will form
A. Origins of maxillipeds in
• Antp,Ubx, and abdA are
expressed in the thorax=
mirror thoracic segments
• if a thoracic segment does
not express Ubx and abdA, it
converts its anterior
locomotor limb into a
feeding appendage called a
B.Why snakes don’t have legs
Thoracic vertebrae= have ribs
Cervical and lumbar= no ribs
*the type of vertebra produced is specified
By the hox genes expressed in the somites
• the forelimb forms just anterior to the most
anterior expression domain of Hoxc-6
• hoxc6 + hoxc8= thoracic vertebrae forms ribs
• During early python development, Hoxc-6 is
not expressed in the absence of Hoxc-8, so
the forelimbs do not form. Rather, the
combination of Hoxc-6 and Hoxc-8 is
expressed for most of the length of the
organism, telling the vertebrae to form ribs
throughout most of the body
• The hindlimb buds do begin to form in
pythons, but they do not make
anything more than a femur. This
appears to be due to the lack of sonic
hedgehog expression by the limb bud
• Sonic hedgehog is needed both for the
polarity of the limb and for
maintenance of the apical ectodermal
ridge (AER). Python hindlimb buds lack
• Invertebrates – single hox gene
complex per haploid genome.
– sponges- have one or two genes in the
– Insects have numerous genes in the
• Early vertebrate- at least 4 hox gene
• How does a new cell type form?
-involves the duplication and divergence of
Dll gene originally has only one copy in
Amphioxus but have about 5-6 closely
related copies in vertebrates.
The Dll homologues have found new
functions in modern vertebrates;
a. Expressed in mesoderm
b. Expressed in forebrain
• Although it remains unproven, it is possible that the
new type of Distal-less gene could have caused the
migratory ectodermal cells of amphioxus to evolve
into neural crest cells.
• Homologous transduction pathways
• They are composed of homologous
proteins arranged in a homologous
• Homologous pathways form the basic
infrastructure of development. The
targets of these pathways may differ,
however, among organisms.
1. Dorsal-Cactus Pathway
Drosophila- specify dorsal-ventral polarity
Mammal- activate inflammatory protein
The pathways (one in Drosophila, one in
humans) are homologous; the organs
they form are not.
2. RTK pathway
Mammal- epidermal cell division
C.elegans- vulval differentiation and
• When homologous pathways made of
homologous parts are used for the same
function in both protostomes and
demonstrates that in both vertebrates and
invertebrates, chordin/Short-gastrulation (Sog)
inhibits the lateralizing effects of
BMP4/Decapentaplegic (Dpp), thereby allowing
the ectoderm protected by chordin/Sog to
become the neurogenic ectoderm.
*High chordin/Sog= low BMP4/Dpp= ectoderm develop to
*Low chordin/Sog= high BMP4/Dpp= ectoderm develop to
• How can the development of an
embryo change when development is
so finely tuned and complex?
• How can such change occur without
destroying the entire organism?
• Organisms are constructed of units that are coherent within
themselves and yet part of a larger unit. Thus, cells are parts
of tissues, which are parts of organs, which are parts of
systems, and so on.
• In development, such modules include
a. morphogenetic fields (for example, those described for the
limb or eye)
b. pathways (such as those mentioned above), imaginal discs,
c. cell lineages (such as the inner cell mass or trophoblast),
d. insect parasegments, and
e. vertebrate organ rudiments.
Modular units allow certain parts of the body to
change without interfering with the functions of other parts.
– Not all part of the embryo is connected to one
a. Heterochrony - shift in the relative timing of
two developmental processes from one
generation to the next. In other words, one
module can change its time of expression
relative to the other modules of the embryo.
1. gene mutations in the ability to induce or
respond to the hormones initiating
2. heterochronic expression of certain genes.
b. Allometry- growth of different
part at different rates
Whale skull vs human skull
2. Duplication and Divergence
a) duplication part of this process allows
the formation of redundant structures,
b) divergence part allows these structures
to assume new roles.
1. Hox genes
2. TGF-β family genes,
3. MyoD family genes, and
4. Globin genes
5. Duplication and divergence in the somites
that give rise to the cervical, thoracic, and
– No one structure is destined for any particular
– A pencil can be used for writing, but it can also
be used as a toothpick, a dagger, a hole-
puncher, or a drumstick.
1. Engrailed gene
• Segmentation in drosophila
• Specification of neurons
• Provide anterior-posterior axis
2. Enolase or alcohol dehydrogenase
• Enzyme in liver
• Structural crystaline protein in lens
*In other words, preexisting units can be co-
opted (recruited) for new functions.
A. Correlated Progression
– changes in one part of the embryo
induce changes in another.
Skeletal cartilage informs the placement of
muscles, and muscles induce the
placement of nerve axons. In such cases,
if one structure changes, it will induce
other structures to change with it.
B. Coevolution of ligand and receptor
• Ligands have to fit with receptors, and
they have to be expressed at the right
place and at the right time.
• Changes in the ligand must be
accommodated by complementary
changes in the receptor if the receptor is
• If a mutation in a gene encoding ligand (or
receptor) produces too great a change, it
will not bind to its complementary
receptor (or ligand), and development will
stop. When duplications of ligand and
receptor genes occur, they can diverge and
acquire new functions.
1. Physical constraints
– The laws of diffusion, hydraulics, and
physical support allow only certain
mechanisms of development to occur.
Structural parameters and fluid dynamics
forbid the existence of 5-foot-tall
2. Morphogenetic Constraints
– when organisms depart from their
normal development, they do so in only a
limited number of ways.
– If a longer limb is favorable in a given
environment, the humerus may become
elongated, but one never sees two
smaller humeri joined together in
tandem, although one could imagine the
selective advantages that such an
arrangement might have. This
observation indicates a construction
scheme that has certain rules.
3. Phyletic Constraints
– historical restrictions based on the
genetics of an organism's development.
Inductive Interactions generate structure
a) Notochord is vestigial in adult vertebrae
but functional in the specification of
the neural tube
b) Pronephros of chick is the source of
uretic bud that induuces the formation
of functional kidney
• Canalization (Buffer systems of
development) - development appears
to be buffered so that slight
abnormalities of genotype or slight
perturbations of the environment will
not lead to the formation of abnormal
• Not all mutations produce mutant
• Protein that binds to a set of signal
transduction molecules that are inherently
• Provides a way to resist fluctuation due to
slight mutation or environmental change
• Responsible for allowing mutations to
accumulate by keeping them from being
expressed until the environment changes
*transient decrease in Hsp9(damage) would
uncover pre-existing genetic interaction that
would produce morphological variations
• “evolution within a species could be
explained: Diversity within a population
arose from the random production of
mutations, and the environment acted to
select the most fit phenotypes. “
• Those animals capable of reproducing
would transmit the genes that gave them
Vs Punctuated Equilibrium
2. Extrapolation of microevolution to
3. Specificity of phenotype from
4. Lack of genetic similarity in disparate
Based on gene differences in adults
competing for reproductive success
Dev.Bio. And Dev.Gen
Has more concern on the “arrival” of the
fittest than the survival of the fittest.
1. Evolution is caused by the inheritance of changes in
development. Modifications of embryonic or larval
development can create new phenotypes that can then be
2. Darwin's concept of "descent with modification" explained
both homologies and adaptations. The similarities of
structure were due to common ancestry (homology), while
the modifications were due to natural selection (adaptation
to the environmental circumstances).
3. The Urbilaterian ancestor can be extrapolated by looking at
the developmental genes common to invertebrates and
vertebrates and which perform similar functions. These
include the Hox genes that specify body segments, the
tinman gene that regulates heart development, the Pax6 gene
that specifies those regions able to form eyes, and the genes
that instruct head and tail formation.
4. Changes in the targets of Hox genes can alter
what the Hox genes specify. The Ubx protein, for
instance, specifies halteres in flies and hindwings
5. Changes of Hox gene expression within a region
can alter the structures formed by that region.
For instance, changes in the expression of Ubx
and abdA in insects regulate the production of
prolegs in the abdominal segments of the larvae.
6. Changes in Hox gene expression between body
regions can alter the structures formed by that
region. In crustaceans, different Hox expression
patterns enable the body to have or to lack
maxillipeds on its thoracic segments.
7. Changes in Hox gene expression are correlated
with the limbless phenotypes in snakes.
8. Changes in Hox gene number may allow Hox
genes to take on new functions. Large changes
the numbers of Hox genes correlate with major
transitions in evolution.
9. Duplications of genes may also enable these
genes to become expressed in new places. The
formation of new cell types may result from
duplicated genes whose regulation has diverged.
10. In addition to structures being homologous,
developmental pathways can be homologous. Here,
one has homologous proteins organized in homologous
ways. These pathways can be used for different
developmental phenomena in different organisms and
within the same organism.
11. Deep homology results when the homologous pathway
is utilized for the same function in greatly diverged
organisms. The instructions for forming the central
nervous system and for forming limbs are possible
examples of deep homology.
12. Modularity allows for parts of the embryo to change
without affecting other parts.
13. The dissociation of one module from another is shown
by heterochrony (changing in the timing of the
development of one region with respect to another)
and by allometry (when different parts of the organism
grow at different rates).
14. Allometry can create new structures (such as the
pocket gopher cheek pouch) by crossing a threshold.
15. Duplication and divergence are important mechanisms
of evolution. On the gene level, the Hox genes, the
Distal-less genes, the MyoD genes, and many other gene
families started as single genes. The diverged members
can assume different functions.
16. Co-option (recruitment) of existing genes and pathways
for new functions is a fundamental mechanism for
creating new phenotypes. One such recruitment is the
limb development pathway being used to form eyespots
in butterfly wings.
17. Developmental modules can include several tissue
types such that correlated progression occurs. here, a
change in one portion of the module causes changes in
the other portions. When skeletal bones change, the
nerves and muscles serving them also change.
18. Tissue interactions have to be conserved, and if one
component changes, the other must. If a ligand changes,
its receptor must change. Reproductive isolation may
result from changes in sperm or egg proteins.
19. Developmental constraints prevent certain phenotypes
from occurring. Such restraints may be physical (no
rotating limbs), morphogenetic (no middle finger
smaller than its neighbors), or phyletic (no neural tube
without a notochord).
20. The Hsp90 protein enables cells to
accumulate genes that would otherwise give
abnormal phenotypes. When the organisms
are stressed during development, these
phenotypes can emerge.
21. The merging of the population genetics
model of evolution with the developmental
genetics model of evolution is creating a new
evolutionary synthesis that can account for
macroevolutionary as well as