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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
TEKS 12B Compare variations and adaptations of organisms in different ecosystems.
TEKS 12B: Variations and Adaptations
What are variations in organisms?
Have you ever looked closely at a litter of house cats? Or at the faces of people in a mall? No two
individuals, other than identical twins, look precisely like any other individuals of their species.
Individual organisms within a species exhibit variation. If you studied populations of wild plants and
animals, you would see that those organisms vary too.
Some variation is affected by environmental conditions,
such as diet or temperature, during growth and
development. A lot of variation, however, is heritable,
which means that it is at least partially controlled by
genes. Some heritable traits may be controlled by a single
gene that has only two alleles. An example of this is the
presence or absence of bands on the shell of one snail
species. If one allele exhibits simple dominance over the
other allele, all genotypes for the trait produce one of two
phenotypes. In that snail species, individuals either have
colored bands on their shells or they don’t. It is important
to remember, however, that gene expression is not
controlled by genes exclusively—the environment plays a
role as well.
Many traits are controlled by two or more genes and are
called polygenic traits. Each gene of a polygenic trait often
has two or more alleles. As a result, a single polygenic trait
can have many possible genotypes and even more different
phenotypes. Often, those phenotypes are not clearly distinct
from one another. Height in humans is one example of a
polygenic trait. People aren’t simply “short” or “tall.” Height
varies over a wide range from very short to very tall and
everywhere in between. If you graph the number of
individuals of each height in your class, for instance, you
may get a graph with a bell-like shape. A bell-shaped curve
is also called a normal distribution.
Detailed reports of variations among organisms in nature have been provided by studies of many species,
including finches in the Galápagos islands. One Galápagos study focused on medium ground finches
living on the island of Daphne Major. Researchers caught nearly every medium ground finch living on
Daphne Major and measured traits including wing length, leg length, beak length, beak depth, and beak
color. Many of these characteristics appeared in bell-shaped distributions typical of polygenic traits.
These data indicate that there is great variation in heritable traits among Galápagos finches. Other studies
have revealed similar kinds of variation in many plants and animals.
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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
How do variations of organisms in different ecosystems
compare?
Many species have geographic ranges that span several ecosystems with different
environmental conditions. Some species, for example, may have ranges that
encompass both moist and dry areas, or areas with long frost-free seasons and areas where winters are
long and cold. When scientists study these organisms, they find variations in characteristics related to
those environmental differences.
A number of tree species in temperate areas, for example, show variations in seasonal growth patterns
over very short distances. One study looked at individual loblolly pines from the southern part of its range
that had been transplanted to places in the northern part of the range. Transplanted trees from southern
populations often suffered from early frosts in the fall, although trees from local strains of the same
species growing nearby were fine. Other studies involved deciduous trees such as maples, cherries, and
redbuds. Comparisons of individuals from dry areas showed significant differences from individuals from
wet area in rates of photosynthesis and water loss. Many similar kinds of variations have also been found
in a wide range of organisms from different ecosystems.
What are adaptations of organisms?
Adaptations are a specific kind of variation. How do we determine whether a variation is an adaptation or
not? We go back to Charles Darwin’s original insights. Before Darwin, scientists viewed all variations
among individuals in a species as unimportant. To be fair, some variations in nature are unimportant.
Whether a flower is slightly more reddish-orange or slightly more yellowish-orange may not make any
difference to the individual plant that bears those flowers.
But some variations are important. Darwin realized that organisms which exhibit certain variations are
sometimes more likely to survive and reproduce in their particular ecosystem than individuals that exhibit
other variations. Members of predatory species that have sharper teeth or longer claws may catch more
prey. And members of a prey species that are faster or better camouflaged may avoid being caught. Any
heritable variation that increases an organism’s ability to survive and reproduce in its environment is
called an adaptation.
Adaptation is one of the key concepts that links ecology and evolutionary theory. Differences in
adaptations affect organisms’ fitness in the particular ecosystem in which they live. Fitness describes how
well an organism can survive and reproduce in its environment. Individuals with adaptations that are wellsuited to their ecosystem’s environment can survive and reproduce and are said to have high fitness.
Individuals without suitable adaptations to their ecosystem’s environment are said to have low fitness.
Organisms with low fitness tend to either die without reproducing or leave few offspring. This difference
in rates of survival and reproduction is called survival of the fittest. Note that survival in this context
means more than just staying alive. In evolutionary terms, survival means reproducing and passing on
adaptations to the next generation.
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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
How do adaptations of organisms in different
ecosystems compare?
Adaptations can be obvious variations in anatomical structures, such the size and shape
of a bird’s beak. A famous study comparing the adaptations of animals in different
ecosystems involves the Galápagos finches mentioned earlier. As shown in the figure, various finch
species from different islands differ in the sizes and shapes of their beaks. Each type of beak is suited for
the finch’s particular diet. Some species, which have small, thin beaks, feed on exposed insects. Other
species, which have larger, thicker beaks, are common in places where many plants have large seeds,
which are tougher to crack open.
Adaptations can also involve physiological functions, such as the way a plant performs photosynthesis or
the pigments it makes to color its flowers. If the plant that was discussed earlier grows in an ecosystem
where it is pollinated by hummingbirds, for example, having a redder flower might be a useful adaptation,
because hummingbirds are strongly attracted to red flowers. Here are some examples of plant and animal
adaptations in different ecosystems around the world:
Tropical Ecosystems Throughout the year, tropical rain forests experience long days, warm
temperatures, and plentiful rainfall. These ecosystems are densely populated with diverse species of
plants, animals, and all other forms of life. Plants in tropical rain forests compete for available sunlight, so
most have large, broad leaves that maximize their ability to capture light. Because tropical soils tend to be
nutrient-poor, most plant roots are concentrated in the very top layers. Tall trees have buttresses—large
aboveground roots—that support their shallow roots. Plants called epiphytes grow on the branches of
other plants as opposed to in the soil, enabling them to gain access to sunlight while obtaining nutrients
through their hosts. Animals in tropical rain forests tend to be active all year round. Many use camouflage
to hide from predators; some can change color to match their surroundings. Animals that live in the
canopy have adaptations for climbing, jumping, and/or flight.
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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
In the a tropical dry forest, there are distinct rainy and dry seasons. Adaptations that
enable plants to survive the dry season include the seasonal loss of leaves. A plant that
sheds its leaves and stops photosynthesis during a particular season is described as
deciduous. Seasonal leaf loss is advantageous in dry climates because it enables the
plant to seal off the area between the leaf stem and trunk, thereby preventing water
loss. Some plants also have an additional thick waxy layer on their leaves to reduce
water. Others store water in their tissues. Animals in the tropical dry forest may reduce their need for
water by entering long periods of inactivity called estivation. Other animals, including many
birds and primates, move to areas where water is available.
Tropical grasslands and shrublands experience wet and dry seasons as well as strong winds and periodic
fires. Plant adaptations are similar to those of tropical dry forest plants: waxy coverings and seasonal leaf
loss. Many trees have bark that helps prevent fire damage and have the ability to grow quickly following
a disturbance. Some grasses have a high silica content that makes them less appetizing to grazing
herbivores. Also, unlike most plants, grasses grow from their bases, not their tips, so they can continue to
grow after being grazed—a particular advantage in ecosystems that are full of hungry herbivores.
Animals living in tropical grasslands and shrublands tend to migrate during the dry season in search of
water. Some smaller animals may burrow and remain dormant until water is once again available. Many
animals give birth in the rainy season, when water is readily available.
Desert Ecosystems Access to water is the major challenge for organisms living in the desert. Many
plants, including cacti, store water in their tissues. Cactus spines are actually modified leaves. Their
minimal surface area helps cut down on water loss through transpiration. The trunks and stems of cacti
are full of chloroplasts, enabling the plants to carry out photosynthesis without broad leaves. Some desert
plants, called succulents, have thick, fleshy leaves that can hold water. Many desert plants use special
forms of photosynthesis that enable them to open their leaf pores only at night, conserving moisture on
hot, dry days. Many desert animals get all the water they need from the food they eat. To help conserve
water, they have specially-adapted excretory systems that release very concentrated urine. To avoid the
hottest parts of the day, many are nocturnal—active only at night. Large or elongated ears and other
extremities are often full of blood vessels close to the surface. These help the animal lose excess body
heat and regulate their internal temperature.
Temperate Ecosystems Temperate grasslands experience variable seasons and occasional fires.
Plants in the temperate grassland, especially grasses, are resistant to grazing and fire. Many plants
disperse their seeds by wind, which tends to be strong in the open grasslands. The roots of native
grassland plants form entangled mats that help establish and retain deep, rich, fertile topsoil. Grassland
animals are under a constant threat of predation in the open and exposed grassland ecosystem.
Camouflage and burrowing are common adaptations.
Temperate woodlands and shrublands generally experience a range of climate conditions and periodic
fires. Soils here tend to be nutrient-poor. Plants here must be adapted to drought conditions. Woody plants
have waxy leaves that resist water loss. Some plants have thick bark and deep roots that can withstand
fires. Other plants contain compounds that actually promote fires. These plants somehow benefit from
fire! Fire can help seeds to germinate, for example, or clear away dead vegetation, opening up space for
new growth. Animals in these ecosystems tend to be browsers—meaning that they have varied diets of
grasses, shrubs, and other vegetation. In the exposed shrubland, camouflage is common. Many animals
burrow or are nocturnal, enabling them to avoid the hottest parts of the day when they’d need the most
water.
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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
Temperate forests have year-round precipitation and nutrient-rich soils. Plants in these
ecosystems drop their leaves and go into a state of dormancy in the winter. Coniferous
trees, trees that produce seed-bearing cones, have needlelike leaves that minimize
water loss in cold, dry, winter air. Animals must cope with changing weather. Some
hibernate, while others migrate to warmer climates. Hibernation is a deep, sleeplike
state that an animal enters to avoid extended periods of cold. Animals that do not
hibernate or migrate will store up food to survive the winter. Many have camouflage in winter to escape
predation when bare trees leave them exposed to predators.
Cold Ecosystems Boreal forests are ecosystems in which winters are long and summers are short.
Boreal forests occur in the northern hemisphere and tend to have acidic, nutrient-poor soils. Conifers are
the most common trees here. Their conical shape sheds snow, and their wax-covered, needlelike leaves
help prevent water loss. Animals in boreal forests have the challenge of staying warm during the long
winter season. Many have small extremities and extra insulation in the form of downy feathers or fat.
Some migrate to warmer areas in winter.
Tundra ecosystems are so cold that the subsoil is permanently frozen. Plants here tend to hug the ground
to avoid damage from frequent, strong winds. Seed dispersal by wind is a common adaptation. Many
plants also have adaptations that enable them to grow in the nutrient-poor soil of the tundra. Legumes, for
example, have nitrogen-fixing bacteria on their roots enabling them to gain access to this important
nutrient. Most tundra animals migrate south to avoid the winter. Animals that do live in the tundra yearround display adaptations such as natural antifreeze compounds, thick insulating layers of fat and fur, and
small extremities.
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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
Lesson Check
1. Compare Compare variations of organisms in different ecosystems. Suppose a species of plant has
a large geographic range. One ecosystem within the range is moist, and another is dry. What kind of
variations would you expect to find if you compared members of the species in these two
ecosystems?
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Use the illustration below to answer Questions 2a and 2b.
2a. Apply Concepts The figure above shows a cactus growing in a desert. How are the cactus’s spines
and thick, fleshy structure adaptations that help the cactus survive in its environment?
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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
2b. Compare Compare adaptations of organisms in different ecosystems. Would
spiny leaves like those on the prickly-pear cactus be an adaptation for a plant
growing in an ecosystem that has a wet environment? Explain your answer.
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3. Define What is a polygenic trait? Give an example.
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4. Define A heritable trait is one that
A
depends mostly on environmental conditions.
B
is at least partly controlled by genes.
C
is always beneficial to the organism that has it.
D
is always harmful to the organism that has it.
5a. Compare Compare variations of organisms in different ecosystems. Researchers on the island of
Daphne Major caught medium ground finches and measured various traits such as beak length and
wing length. The researchers learned that there is great variation in those traits. All the birds that the
researchers studied were members of the same species. The researchers’ investigation showed that
A
environmental conditions are very different on different Galápagos islands.
B
environmental conditions on Daphne Major are very similar to the conditions on other Galápagos
Islands.
C
most finches in the Galápagos Islands are very much like one another.
D
there is great variation in heritable traits among medium ground finches.
5b. Plan Comparative Investigations Which of the following questions might the scientists in Question
5a have asked themselves before they conducted their investigation?
A
How do the medium ground finches on Daphne Major compare to one another in certain traits
such as beak length?
B
How is beak length an adaptation to environmental conditions on Daphne Major?
C
What are the characteristics of the environment on Daphne Major?
D
How many species of birds live on Daphne Major?
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Name ___________________________ Class ________ Date ___________
TEKS
Biology
Lesson 12B
6. Compare Compare adaptations of organisms living in different ecosystems.
Some plants in a dry forest ecosystem have leaves with a waxy covering. Why
would you not expect this same adaptation to be common in a humid ecosystem?
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