Download PowerPoint Presentation to accompany Life: The Science of

Development and
Evolutionary Change
21
Development and Evolutionary Change
• Introduction
• Evolution and Development
• Regulatory Genes and Modularity: Modifying
Morphology
• Plant Development and Evolution
• Environmental Influences on Developmental
Patterns
• Learning: A Modification of Development
21
Introduction
• Fish that can change their sex in response to their
social environment, such as anemonefish,
demonstrate that an organism’s development is
not determined entirely by its genes.
• The phenotypes of adult organisms are the result
of complex interactions between basic genes,
gene products, and the environment.
21
Evolution and Development
• Charles Darwin’s idea of characterizing evolution
as “descent with modification” led to the
recognition that the results of evolution could be
visualized as a “tree of life.”
• He explained similarities among organisms by
their descent from a common ancestor.
• Differences among organisms were explained as
the result of natural selection, which adapted
them to different environments.
21
Evolution and Development
• Darwin recognized and showed that similarities
among embryos could be used to infer the
relationships among groups of organisms.
• Using similarities in larval forms as a basis,
Darwin was able to conclude that barnacles are
crustaceans.
Figure 21.1 Similarities In Early Developmental Stages Can Be Used to Infer Relationships
21
Evolution and Development
• Late in the twentieth century, the fields of genetics
and embryology came together to form the new
discipline of evolutionary developmental biology.
• Evolutionary developmental biologists investigate
how the course of evolution has been influenced by
heritable changes in the development of organisms.
• Many of the genes regulating development are
highly conserved, meaning their sequences have
changed very little throughout the evolution of
multicellular organisms.
21
Evolution and Development
• Many of the genes that regulate the development
of very different animal species are remarkably
similar.
• For example, many of the same genes are
involved in the development of the compound
eyes of fruit flies and the camera-like eyes of
house mice.
• The genes involved in eye development in these
two species are so similar that the fruit fly cell that
normally develops into part of a leg will form an
eye when a mouse Pax6 gene is expressed in it.
Figure 21.2 The Mouse Pax6 Gene Causes Eye Development in Drosophila
21
Evolution and Development
• The same set of homeobox genes provides the
positional information along the anterior–posterior
axis of the body in both human and insect
embryos.
• For example, the Drosophila gap genes ems, tll,
and otd, as well as the homologous genes of
vertebrates are expressed in the anterior regions
of the brain.
Figure 21.3 Genes Show Similar Expression Patterns
21
Evolution and Development
• Mutations of genes involved in development can
result in abnormal differentiation during
development.
• The bithorax mutation in insects, for example,
results in the development of two sets of
forewings instead of one pair.
• When the expression of certain vertebrate Hox
genes is altered, vertebrae that normally develop
into lumbar vertebrae instead develop instead into
thoracic vertebrae.
Figure 21.4 Altering Homeobox Genes Changes Morphology
21
Evolution and Development
• The enormous amount of variation of
morphological forms found in animals is underlain
by a common set of instructions that have been
conserved in thousands of species.
• The vast differences in morphological form that
result from similar genetic instructions means that
these instructions alone cannot be entirely
responsible for an organism’s morphology.
21
Regulatory Genes and Modularity:
Modifying Morphology
• Developing embryos exhibit modularity—they are
made up of self-contained units that can be
changed independently of the other units, or
modules, that compose the organism.
• There are two ways in which changes in genes that
regulate development can lead to important
morphological changes:
 Mutations in genes that regulate developmental
processes
 Changes in the time or place of expression of
developmental regulatory genes
• The modular nature of most organisms makes both
of these pathways of evolution easier.
21
Regulatory Genes and Modularity:
Modifying Morphology
• Insects provide examples of how mutations in
genes that regulate segmentation can lead to the
evolution of morphological changes.
• For example, the homeotic gene Ultrabithorax
(Ubx), which is found in all organisms.
• The insect Ubx gene has a mutation not found in
other arthropods.
• The Ubx protein produced from this mutated gene
is expressed in the abdomen of insects, where it
represses the expression of the distal-less (dll)
gene, which is essential for leg formation.
• As a result of the Ubx repression of the dll gene,
insects do not form legs on their abdomens.
Figure 21.5 A Mutation Changed the Number of Legs in Insects
21
Regulatory Genes and Modularity:
Modifying Morphology
• The evolution of webbed feet in ducks provides an
example of an altered spatial expression pattern
of a regulatory gene.
• A gene encoding a protein called bone
morphogenetic protein 4 (BMP4) is expressed in
the spaces between the developing bones of the
toes and instructs the cells in those spaces to
undergo apoptosis, destroying the webbing
between the toes.
• Ducks express a BMP inhibitor protein called
Gremlin in their webbing cells.
• This protein prevents the BMP4 protein from
signaling for cell death in the webbing, resulting in
a webbed foot.
Figure 21.6 Changes in gremlin Expression Correlate with Changes in Hindlimb Structure
Figure 21.7 Changing the Form of an Appendage
21
Regulatory Genes and Modularity:
Modifying Morphology
• Modularity allows the relative timing of two different
developmental processes to shift independently of
one another.
• This process is known as heterochrony and has
been widely studied in salamanders.
• Two species of Bolitoglossa illustrate heterochrony.
• The webbing between the feet of most salamander
species disappears as the animals mature.
• If expression of genes that dissolve the webbing is
slowed, the digits don’t grow, and “juvenile” webbed
feet result.
• These feet can act like suction cups, opening an
arboreal way of life.
Figure 21.8 Heterochrony Created an Arboreal Salamander
21
Regulatory Genes and Modularity:
Modifying Morphology
• Modularity also allows structural changes to
evolve via gene duplication.
• If a gene is duplicated, the new copy can evolve a
new function without disrupting the organism as
long the other copy is performing its original
function.
21
Plant Development and Evolution
• Rapid progress has been made during the past
decade in identifying the genes that regulate
growth and cell differentiation in plants.
• The sequencing of the complete genome of the
thale cress, Arabidopsis thaliana, has provided
much of this information.
• About 1,500 of the nearly 26,000 Arabidopsis
genes code for transcription factors.
• Over half of the known families of transcription
factors are found in all eukaryotes, but many
others are found only in plants.
21
Plant Development and Evolution
• Plants and animals share many regulatory genes, but
plants differ from animals in important ways:
 Plant cells do not move relative to one another.
Changes in the shape of a developing plant result
from cell proliferation and elongation.
 Future reproductive cells are not set aside early
during plant development. Plants produce clusters
of undifferentiated, actively dividing cells called
meristems throughout their lives.
 Plants have tremendous developmental
plasticity. Plants can change their development in
response to environmental conditions.
21
Plant Development and Evolution
• Members of the MADS box and homeobox
families of genes encode transcription factors that
regulate developmental processes in both plants
and animals.
• Plants and animals share many of the genes that
regulate their development, even though they
have been evolving separately for a long time.
• This is in part due to their modular construction
which allows different parts of their bodies to
change independently of one another.
21
Plant Development and Evolution
• Plants have greater developmental plasticity than
animals do because plasticity is especially
valuable for a sessile organism.
• The combination of repeated production of
meristems and developmental plasticity
compensates for being sessile.
21
Environmental Influences
on Developmental Patterns
• The idea that the environment plays an important
role in the development of organisms was
downplayed until recently.
• Developmental biologists tended to study small
organisms that develop rapidly and do not change
dramatically under controlled conditions.
• It is now known that the development of many
organisms is very sensitive to environmental
conditions.
• A single genotype may encode a range of
phenotypes under different environmental conditions.
21
Environmental Influences
on Developmental Patterns
• Signals from the environment can be divided into
two major types:
 Environmental signals that are accurate
predictors of future conditions. It is expected that
the developmental processes of organisms
respond adaptively to these signals.
 Environmental signals that are poorly correlated
with future conditions. Organisms are unlikely to
respond to these signals.
21
Environmental Influences
on Developmental Patterns
• Developing organisms respond to signals such as
day length, temperature, and precipitation in such a
way that the adults they become are adapted to the
predicted conditions.
• The West African butterfly Bicyclus anynana has a
dry-season form and a wet-season form with
different wing coloration.
• The temperatures experienced during pupation
determine which form of adult butterfly will be
produced. Temperature influences the expression of
the distal-less gene.
Figure 21.9 Development of Eyespots in Bicyclus anynana Responds to Temperature
21
Environmental Influences
on Developmental Patterns
• The moth Nemoria arizonaria provides another
example of developmental plasticity in response to
seasonal changes.
• The spring larvae of this moth feed on and
resemble oak flowers; the summer larvae feed on
oak leaves and resemble small oak branches.
• Spring caterpillars have been experimentally
converted to summer caterpillars by feeding them
oak leaves.
• A chemical in the oak leaves probably induces
them to develop into the twiglike summer form.
Figure 21.10 The Spring and Summer Forms of a Caterpillar Differ
21
Environmental Influences
on Developmental Patterns
• Some organisms need help from another species
to complete their development.
• For example, house mice raised in microbe-free
environments do not have the bacteria that
normally colonize their gut.
• These gut bacteria induce gene expression in the
mouse intestine, which is essential for normal
capillary development.
21
Environmental Influences
on Developmental Patterns
• Many changes to an organism’s environment are
not as certain as signals such as day length or
wet- and dry-seasons.
• Despite this uncertainty, if the changes have
occurred frequently during the evolution of a
species, developmental plasticity may allow
individuals to respond to them.
• The presence or absence of active predators is an
example of one of these uncertain environmental
signals.
21
Environmental Influences
on Developmental Patterns
• Water fleas (Daphnia), for example, increase the
size of the “helmets” on the top of their heads when
they encounter the predatory larvae of the fly
Chaoborus.
• Helmet induction occurs if the Daphnia are exposed
to water in which the fly larvae have been swimming.
• Offspring that are developing in the abdomens of
mothers with induced large helmets are born with
large helmets.
• The tradeoff for this defensive advantage is that
Daphnia with large helmets produce fewer eggs.
Figure 21.11 Predator-Induced Developmental Plasticity in Daphnia
21
Environmental Influences
on Developmental Patterns
• Tadpoles of the spadefoot toad can respond
developmentally if their pond begins to dry up
while they are growing.
• Some of the tadpoles respond to crowding in a
shrinking pond by developing a wider mouth and
powerful jaw muscles.
• These tadpoles complete their development
rapidly before the pond dries up by eating other
tadpoles.
21
Environmental Influences
on Developmental Patterns
• Plants respond developmentally to light
availability.
• In low light conditions, plant cells elongate so that
the plants become spindly and are more likely to
reach a patch of brighter light than if they were to
remain compact.
• Because they have meristems, plants can
continue to respond to light as long as they grow.
Figure 21.12 Light Seekers
21
Environmental Influences
on Developmental Patterns
• Organisms generally ignore environmental signals
that are poorly correlated with future conditions.
• Plants, for example, produce seeds that will
germinate in future years with different and
unknowable conditions.
• Plant seed sizes remain relatively constant in
spite of changing environmental conditions.
• Seed size is adjusted to the average conditions
encountered by plants over many generations.
Figure 21.13 Seed Production
21
Environmental Influences
on Developmental Patterns
• Organisms cannot be expected to have evolved
appropriate responses to environmental signals
that they have not encountered before.
• This is an important problem because human
societies have changed the environment in so
many ways.
• One way humans change the environment is
through the release of new chemical compounds.
• Understanding how chemicals affect development
is important because it may help in the
development of less harmful substitutes.
21
Learning: A Modification of Development
• Learning allows an individual to adjust its behavior
to the physical, biological, and social environment
in which it matures.
• Learning is especially important in species with
complex social structures, in which individuals
must learn the identities and characteristics of
their associates and adjust their behavior
accordingly.
• The field of evolutionary developmental biology is
generating many new insights with which to
understand the evolution of life.