Introduction
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Many fish species are able to change their sex in response to their social environments.
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Anemonefish live in groups of five or so individuals within a single sea anemone.
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These groups have only one female, who is also the group’s leader.
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If the female is removed from the group, the largest male in the group changes its sex and becomes a female.
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Fish that can change their sex in response to their social environment demonstrate that an organism’s
development is not determined entirely by its genes.
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The phenotypes of adult organisms are the result of complex interactions between basic genes, gene products,
and the environment.
Evolution and Development
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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.”
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Similarities among organisms were explained by their descent from a common ancestor.
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Differences among organisms were the result of natural selection, which adapted them to different
environments.
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Darwin recognized and showed that similarities among embryos could be used to infer the relationships
among groups of organisms.
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Using similarities in larval forms as a basis, Darwin was able to conclude that barnacles are crustaceans. (See
Figure 21.1.)
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Late in the twentieth century, the fields of genetics and embryology came together to form the new discipline
of evolutionary developmental biology.
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Evolutionary developmental biologists investigate how the course of evolution has been influenced by
heritable changes in the development of organisms.
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These scientists have discovered that many of the genes regulating development are highly conserved,
meaning their sequences have changed very little throughout the evolution of multicellular organisms.
Development uses the same sets of genes throughout the animal kingdom
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Many of the genes that regulate the development of very different animal species are remarkably similar.
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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.
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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. (See Figure 21.2.)
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The same set of homeobox genes provides the positional information along the anterior–posterior axis of the
body in both human and insect embryos. (See Figure 21.3.)
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Mutations of genes involved in development can result in abnormal differentiation during development.
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The bithorax mutation in insects, for example, results in the development of two sets of forewings instead of
one pair. (See Figure 21.4a.)
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When the expression of certain vertebrate Hox genes is altered, vertebrae that normally develop into lumbar
vertebrae develop instead into thoracic vertebrae. (See Figure 21.4b.)
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The enormous amount of variation of morphological forms found in the animal kingdom is underlain by a
common set of instructions that has been conserved in thousands of species.
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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.
Regulatory Genes and Modularity: Modifying Morphology
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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.
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For example, the heart of a developing embryo can change independently of changes to its limbs because the
genes that govern the formation of each do not affect one another.
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Two ways in which changes in the genes that regulate development can lead to evolutionarily important
morphological changes have been elucidated:
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Mutations in genes that regulate developmental processes.
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Changes in the time or place of expression of developmental regulatory genes.
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The modular nature of most organisms makes both of these pathways of evolution easier.
Mutations can result in new phenotypes
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Insects provide examples of how mutations in genes that regulate developmental processes, specifically in
genes that regulate segmentation, can lead to the evolution of morphological changes.
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Consider, for example, the homeotic gene Ultrabithorax (Ubx), which is found in all organisms.
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The insect Ubx gene has a mutation not found in other arthropods. (See Figure 21.5.)
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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.
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As a result of the Ubx repression of the dll gene, insects do not form legs on their abdomens.
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The evolution of webbed feet in ducks provides an example of how an evolutionary change may result from
the altered spatial expression pattern of a regulatory gene.
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A gene encoding a protein called bone morphogenetic protein 4 (BMP4) is expressed in the spaces between
the developing bones of the toes.
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This protein instructs the cells in those spaces to undergo apoptosis, destroying the webbing between the
toes.
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Although both chickens and ducks express BMP4 in the webbing between the toes, only ducks express a
BMP inhibitor protein called Gremlin in their webbing cells.
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This protein prevents the BMP4 protein from signaling for cell death in the webbing, resulting in a webbed
foot. (See Figure 21.6 and Animated Tutorial 21.1.)
The timing of a gene’s expression can affect morphology
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Modularity allows the relative timing of two different developmental processes to shift independently of each
other.
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This process is known as heterochrony and has been widely studied in salamanders, where extensive
examples of the phenomenon are seen.
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Two species in the genus Bolitoglossa illustrate how heterochrony can result in new morphology.
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The webbing between the feet of most salamander species disappears as the animals mature, resulting in
“toed” feet that are suited for a terrestrial environment. (See Figure 21.8a.)
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If expression of genes that dissolve the webbing is slowed, the digits don’t grow, and “juvenile” webbed feet
result. (See Figure 21.8b.)
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These feet can act like suction cups, opening an arboreal way of life to the new species that possess them.
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Modularity also allows structural changes to evolve via gene duplication.
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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.
Plant Development and Evolution
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Rapid progress has been made during the past decade in identifying the genes that regulate growth and cell
differentiation in plants.
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The sequencing of the complete genome of the thale cress, Arabidopsis thaliana, has provided much of this
information.
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About 1,500 of the nearly 26,000 Arabidopsis genes code for transcription factors.
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Over half of the known families of transcription factors are found in all eukaryotes, but many others are
found only in plants.
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Plants and animals share many regulatory genes, but plants differ from animals in important ways:
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Plant cells do not move relative to one another.
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Changes in the shape of a developing plant result from cell proliferation and elongation.
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Future reproductive cells are not set aside early during plant development.
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Plants produce clusters of undifferentiated, actively dividing cells called meristems throughout their lives.
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Meristems allow plants to develop and form new organs throughout their lives.
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Plants have tremendous developmental plasticity.
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Plants can change their development in response to environmental conditions.
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For example, leaf meristems allow a plant to replace a part that may have been eaten by an herbivore.
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Members of the MADS box and homeobox families of genes encode transcription factors that regulate
developmental processes in both plants and animals.
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The modular construction of multicellular organisms helps explain why plants and animals can share so many
of the genes that regulate their development even though they have been evolving separately for so long.
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This modular nature allows different parts of their bodies to change independently of one another.
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Plants have greater developmental plasticity than animals do because plasticity is especially valuable for a
sessile organism.
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The combination of repeated production of meristems and developmental plasticity compensates for being
sessile.
Environmental Influences on Developmental Patterns
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The idea that the environment plays an important role in the development of organisms was downplayed until
recently.
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Developmental biologists tended to study small organisms that develop rapidly and do not change
dramatically under controlled conditions.
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It is now known that the development of many organisms is very sensitive to environmental conditions.
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In many cases, a single genotype may encode a range of phenotypes under different environmental
conditions.
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Ultimately, signals from the environment may determine which phenotype is expressed.
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Signals from the environment can be divided into two major types:
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Environmental signals that are accurate predictors of future conditions.
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It is expected that the developmental processes of organisms respond adaptively to these signals.
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Environmental signals that are poorly correlated with future conditions.
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It is expected that organisms will fail to respond to these signals.
Organisms respond to signals that accurately predict the future
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Developing organisms respond to signals such as changes in day length, temperature fluctuations, and wet
and dry seasons in such a way that the adults they become are adapted to the predicted conditions.
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The West African butterfly Bicyclus anynana has a wet-season and a dry-season form.
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The dry-season form matches the dead brown leaves on the dry-season forest floor.
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The wet-season form has conspicuous ventral hindwing eyespots that deceive predators into attacking the
wing rather than the butterfly’s actual eye, increasing its odds of survival.
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The temperatures experienced during pupation determine which form of adult butterfly will be produced.
(See Figure 21.9.)
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The moth Nemoria arizonaria provides another example of developmental plasticity in response to seasonal
changes.
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The spring larvae of this moth feed on and resemble oak flowers, whereas the summer larvae feed on oak
leaves and resemble small oak branches. (See Figure 21.10.)
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Spring caterpillars have been experimentally converted to summer caterpillars by feeding them oak leaves.
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A chemical in the oak leaves probably induces them to develop into the twiglike summer form.
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Some organisms need help from another species to complete their development.
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For example, house mice raised in microbe-free environments do not have the bacteria that normally colonize
their gut.
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These gut bacteria induce gene expression in the mouse intestine, which is essential for normal capillary
development.
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(See Video 21.1.)
Some conditions that accurately predict the future may not always occur
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Many changes to an organism’s environment are not as certain as signals such as day length or wet and dry
seasons.
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Despite this uncertainty, if the changes have occurred frequently during the evolution of a species,
developmental plasticity may allow individuals to respond to them.
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The presence or absence of active predators is an example of one of these uncertain environmental signals.
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Developmental responses to predators have evolved in numerous species.
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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 Chaeoborus. (See Figure 21.11.)
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Helmet induction occurs if the Daphnia are exposed to water in which the fly larvae have been swimming.
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Offspring that are developing in the abdomens of mothers with induced large helmets are born with large
helmets.
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The trade-off for this defensive advantage is that Daphnia with large helmets produce fewer eggs.
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Tadpoles of the spadefoot toad can respond developmentally if their pond begins to dry up while they are
growing.
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Some of the tadpoles respond to crowding in a shrinking pond by developing a wider mouth and powerful
jaw muscles.
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These tadpoles complete their development rapidly before the pond dries up by eating other tadpoles.
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Plants respond developmentally to light availability.
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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. (See Figure 21.12.)
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Because they have meristems, plants can continue to respond to light as long as they grow.
Organisms do not respond to environmental signals that are poorly correlated
with future conditions
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Organisms generally ignore environmental signals that are poorly correlated with future conditions.
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Plants, for example, produce seeds that will germinate in future years with different and unknowable
conditions.
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Plant seed sizes remain relatively constant in spite of changing environmental conditions. (See Figure 21.13.)
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Seed size is adjusted to the average conditions encountered by plants over many generations.
Organisms may lack appropriate responses to new environmental signals
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Organisms cannot be expected to have evolved appropriate responses to environmental signals that they have
not encountered before.
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This is an important current problem because human societies have changed the environment in so many
ways.
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One way humans change the environment is through the release of new chemical compounds.
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Many women who used the drug thalidomide during pregnancy gave birth to deformed infants.
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Deformed frogs are appearing in ponds contaminated by certain pesticides where they are less able to resist
the parasite that causes these deformities.
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Understanding how such chemicals affect development is important because it may help in the development
of less harmful substitutes.
Learning: A Modification of Development
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Learning allows an individual to adjust its behavior to the physical, biological, and social environment in
which it matures.
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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.
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The field of evolutionary developmental biology is generating many new insights with which to understand
the form of the Tree of Life.