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20
Genes, Development,
and Evolution
20 Genes, Development, and Evolution
20.1 How Can Small Genetic Changes
Result in Large Changes in
Phenotype?
20.2 How Can Mutations with Large
Effects Change Only One Part of
the Body?
20.3 How Can Developmental Changes
Result in Differences among
Species?
20 Genes, Development, and Evolution
20.4 How Can the Environment
Modulate Development?
20.5 How Do Developmental Genes
Constrain Evolution?
20 Genes, Development, and Evolution
Major evolutionary change can result from
subtle changes in distribution of signaling
molecules and sensitivity of noncoding
regions of the DNA that control gene
expression.
This realization has produced the new field
of evolutionary developmental biology.
Opening Question: How are gene
expression patterns involved in the
shaping of the diverse beaks of birds?
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
Study of the relationship between
development and evolution has given
rise to the new field of evolutionary
developmental biology, or evo-devo.
Principles:
• Organisms share similar mechanisms
for development that include a “toolkit”
of signaling molecules that control
gene expression.
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
• Signaling molecules can act
independently in different tissues and
regions, enabling modular
evolutionary change.
• Developmental differences can arise
from changes in the timing of
signaling molecule action, location of
the action, or quantity of the action.
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
• Differences among species can arise
from alterations in expression of
developmental genes.
• Developmental changes can arise
from environmental influences.
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
When developmental biologists began
to describe differentiation and
development at the molecular level,
they found common regulatory genes
and pathways in many organisms.
Example: eye development in fruit flies
and mice.
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
Although insect and vertebrate eyes
are radically different, research in the
1990s showed that the genes for eye
development are remarkably similar.
Gene sequences for eye development
are highly conserved in many species.
These genes are homologous—they
evolved from a gene in a common
ancestor.
Figure 20.1 DNA Sequence Similarity in Eye Development Genes
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
The Hox gene cluster is another
example of homology.
They code for transcription factors that
provide positional information and
control pattern formation in body
segments.
They share a homologous sequence
called the homeobox.
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
Hox genes may have arisen via gene
duplication—mutated copies can take
on new functions while the other
maintains original functions.
Duplication and divergence of Hox
genes is suggested by the increase in
the number of Hox genes in different
animal groups—from two clusters in
jellyfish to four clusters with many
genes in vertebrates.
Figure 20.2 Regulatory Genes Show Similar Expression Patterns
20.1 How Can Small Genetic Changes Result in Large Changes
in Phenotype?
Over the millions of years elapsed
since cnidarians, insects, and
vertebrates last shared a common
ancestor, the Hox genes have been
conserved.
This leads to one of the principles of
evo-devo: the shared genetic “toolkit”
of developmental mechanisms.
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
Developing embryos are made up of
modules—functional entities
consisting of genes, signaling
pathways, and the physical structures
that result.
Developmental genes can be controlled
separately in the modules, so
structures in different modules can
change independently in both
developmental and evolutionary time.
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
Mechanisms called genetic switches
control how the genetic toolkit is used.
They involve promoters and
transcription factors.
Multiple switches control each gene;
elements of the genetic toolkit can be
involved in multiple developmental
processes while still allowing
individual modules to develop
independently.
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
In an embryo, genetic switches
integrate positional information and
determine developmental pathways
for each module.
In Drosophila, the pattern and function
of each segment depends on the
unique Hox gene or combination of
Hox genes that are expressed in that
segment.
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
Example: Wing development
In segment 2, Hox gene antennapedia
(Antp) results in wing development.
In segment 3, Antp is repressed by
ultrabithorax (Ubx), and halteres
develop instead of wings.
If Ubx is mutated, a second set of
wings can develop.
Figure 20.3 Segments Differentiate under Control of Genetic Switches
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
Modularity allows for differences in
expression of structural genes
Heterometry: differences in the
amount of gene expression.
Beak size and shape in Galápagos
finches is regulated by the relative
amounts of proteins produced by two
regulatory genes.
Figure 20.4 Heterometry and the Beaks of the Finches
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
Heterochrony: changes in timing of
gene expression.
Example—neck bones of the giraffe:
Bone growth results from proliferation
of cells called chondrocytes; growth
stops when the cells receive signals
for apoptosis.
In giraffes, this signaling is delayed in
the neck vertebrae, and they grow
longer.
Figure 20.5 Heterochrony in the Development of a Longer Neck
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
Heterotopy: spatial differences in gene
expression.
All bird embryos have webbing
between the toes, which is retained in
ducks but not in chickens.
Loss of webbing is controlled by BMP4,
a protein that instructs cells in the
webbing to undergo apoptosis.
20.2 How Can Mutations with Large Effects Change Only One
Part of the Body?
Cells in both ducks and chickens
express BMP4, but in ducks, a gene
called Gremlin, which encodes a
BMP4 inhibitor, is expressed in
webbing cells.
In chickens, Gremlin is not expressed,
and BMP4 signals apoptosis of the
webbing cells.
Experimental application of Gremlin to
chicken feet results in a webbed foot.
Figure 20.6 Changes in Gremlin Expression Correlate with Changes in Hindlimb Structure
Figure 20.7 Changing the Form of an Appendage
20.3 How Can Developmental Changes Result in Differences
among Species?
Small differences in expression of
developmental genes can produce
dramatic differences between species.
Mammal species have different
numbers of vertebrae in different parts
of the vertebral column.
This results from the spatial patterns
of Hox gene expression.
Figure 20.8 Changes in Gene Expression and Evolution of the Spine
20.3 How Can Developmental Changes Result in differences
among Species?
Heterotypy: changes in the signaling
molecule itself, rather than where,
how, and when it is expressed.
Example: number of legs in arthropods.
All arthropods express Distalless (Dll),
which controls leg development. The
Hox gene Ubx is also expressed in
abdominal segments but has different
effects on Dll in different species.
20.3 How Can Developmental Changes Result in differences
among Species?
In centipedes, Ubx protein activates
expression of the Dll gene in
abdominal segments, resulting in the
formation of legs.
In insects, a change in the Ubx gene
results in a modified Ubx protein that
represses Dll expression in abdominal
segments, and no legs are formed.
20.3 How Can Developmental Changes Result in Differences
among Species?
This change in Ubx occurred in the
ancestor of insects at the same time
that abdominal legs were lost.
Figure 20.9 A Mutation in a Hox Gene Changed the Number of Legs in Insects
20.3 How Can Developmental Changes Result in Differences
among Species?
Heterotypy in corn:
The wild relative of corn, teosinte, has
kernels encased in tough shells that
develop under the control of Tga1.
A mutation in Tga1 results in a protein
differing in only one amino acid,
resulting in the kernels breaking free.
Figure 20.10 A Result of Heterotypy
20.4 How Can the Environment Modulate Development?
Environmental signals can produce
developmental changes and influence
morphology.
Developmental plasticity or phenotypic
plasticity: a single genotype has the
capacity to produce two or more different
phenotypes.
An organism can modify its development
in response to environmental conditions.
20.4 How Can the Environment Modulate Development?
In some reptiles, sex is determined by
the temperature at which the eggs are
incubated.
Higher temperatures produce only
females, and lower temperatures
produce only males.
Figure 20.11 Hot Females, Cool Males
20.4 How Can the Environment Modulate Development?
Development of sex organs is
controlled by sex steroid hormones,
which are synthesized from
cholesterol. Both males and females
produce testosterone.
In-Text Art, Ch. 20, p. 420
20.4 How Can the Environment Modulate Development?
Incubation temperature controls the
expression of the enzyme aromatase.
If aromatase is expressed, estrogen
dominates and females develop. If
aromatase is not expressed,
testosterone dominates and males
develop.
Incubation temperature may affect
reproductive success.
Figure 20.12 Temperature-Dependent Sex Determination Can Be Associated with Sex-Specific
Fitness Differences
20.4 How Can the Environment Modulate Development?
In species with short life spans,
individuals may encounter different
but predictable environments.
The moth Nemoria arizonaria has two
generations per year.
Larvae that hatch in the spring feed on
oak flowers and resemble those
flowers.
20.4 How Can the Environment Modulate Development?
The second generation feeds on oak
leaves and the larvae resemble small
oak twigs.
A chemical in oak leaves induces them
to develop in the twig-like form.
The ability to avoid predation by
phenotypic plasticity increases
evolutionary fitness.
Figure 20.13 Spring and Summer Forms of a Caterpillar
20.4 How Can the Environment Modulate Development?
Sunlight provides predictive information
about seasonal changes and can
initiate developmental changes:
• Many insects use day length to enter
or exit a period of developmental
arrest called diapause.
• Deer, moose, and elk use day length
to time the development and dropping
of antlers.
20.4 How Can the Environment Modulate Development?
• Many organisms use day length to
optimize timing of reproduction or
migration.
• Many plants initiate flowering in
response to length of night (absence
of light).
• In some plant species developmental
changes are induced by certain
wavelengths of light.
20.4 How Can the Environment Modulate Development?
Development includes changes in body
form and function that occur
throughout the life of the organism.
Developmental processes are
optimized to adjust for environmental
conditions.
Light is an important signal for plant
development. If grown in dim light,
cells in the stem are stimulated to
elongate.
Figure 20.14 Light Seekers
20.5 How Do Developmental Genes Constrain Evolution?
Evolution of form has not been a result
of radically new genes but has
resulted from modifications of existing
genes.
Developmental genes constrain
evolution in two ways:
• Nearly all evolutionary innovations are
modifications of existing structures.
• Genes that control development are
highly conserved.
20.5 How Do Developmental Genes Constrain Evolution?
Example: Wings arose as modifications
of existing structures.
Wings evolved independently in insects
and vertebrates—three times in
vertebrates.
Vertebrate wings are modified
forelimbs.
Figure 20.15 Wings Evolved Three Times in Vertebrates
20.5 How Do Developmental Genes Constrain Evolution?
Organisms may also lose structures.
Ancestors of snakes lost their forelimbs
as a result of changes in expression
of Hox genes.
Then hindlimbs were lost by the loss of
expression of the Sonic hedgehog
gene in limb bud tissue.
20.5 How Do Developmental Genes Constrain Evolution?
Parallel evolution:
Highly conserved developmental genes
make it likely that similar traits will
evolve repeatedly.
Example: three-spined stickleback fish
Marine populations return to freshwater
to breed; freshwater populations
never go into saltwater environments.
20.5 How Do Developmental Genes Constrain Evolution?
Freshwater populations have arisen
many times from adjacent marine
populations.
Marine populations have pelvic spines
and bony plates that protect them
from predation.
These protections are greatly reduced
in freshwater populations.
Figure 20.16 Parallel Phenotypic Evolution in Sticklebacks
20.5 How Do Developmental Genes Constrain Evolution?
One gene, Pitx1, is not expressed in
freshwater sticklebacks, and spines
do not develop.
This same change in regulatory gene
expression has evolved independently
in several populations.
20.5 How Do Developmental Genes Constrain Evolution?
The selective mechanism in fresh
waters may be that with reduced
predation pressure, fish that invest
less energy in unnecessary protective
structures are more successful.
20 Answer to Opening Question
Timing and level of expression of
BMP4 and calmodulin proteins, under
the control of transcription factors and
their promoters, enhancers, and
repressors (the genetic toolkit), result
in modifications of birds’ beaks.
The same proteins regulate the size
and shape of chicken and duck beaks
in the same way as the Galápagos
finches.