Analogies document

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Classwork Analogies: Write Notes and Answer Questions IN JOURNAL
Elvis impersonators have many similarities, such as long black sideburns and rhinestone-studded suits, that are
not inherited. In the same way, biological imilarities are not always due to inheritance.
For example, look at the skulls below. They belong to extinct animals, and both of them have saberteeth —
long, ferocious canines. Would you guess that these saberteeth are homologous — inherited from a common
ancestor with extra-long saberteeth?
These two skulls certainly look like the animals could have inherited their saberteeth from a common ancestor.
And the presence of canine teeth themselves is definitely a homology, in that the common ancestor of these
two had canine teeth. However, despite their similarities, the unusual length of these teeth is NOT homologous.
One skull belongs to Thylacosmilus, a marsupial mammal. The other belongs to Smilodon, the saber-toothed
cat, which is a placental mammal. Marsupial and placental mammals are very different, and diverged from each
other a long time ago on the evolutionary tree.
Thylacosmilus is more closely related to other marsupials such as kangaroos and koalas than it is to Smilodon.
Smilodon is more closely related to other placentals such as housecats and elephants than it is to
Thylacosmilus. Saberteeth is not a common trait in the marsupials closely related to Thylacosmilus, or the
placentals closely related to Smilodon. Their common ancestor certainly had canine teeth, but they were
probably not adapted into fierce “sabers.”
As they weren’t inherited from a common ancestor, the saberteeth in Smilodon and Thylacosmilus evolved
independently from one another. That means that one lineage on one part of the tree of life evolved saberteeth
from normal length teeth, and a different lineage somewhere else on the tree also evolved saberteeth from
normal length teeth.
[see family tree details on top of next page!]
These saberteeth are analogous structures.
1. a) What is the difference between a marsupial and a placental?
b) What is Smilodon?
c) What is Thylacosmilus?
d) In the Pliocene Epoch, the Isthmus of Central America connected
North and South America, allowing the Smilodons of North America to
migrate south and the Thylacosmilus to migrate north. The
Thylacosmilus soon went extinct. Discuss with your neighbor on why
the Thylacosmilus may have been less successful than the Smilodon.
Record your ideas in your journal.
How do analogies evolve?
Often, two species face a similar problem or challenge. Evolution may then shape both of them in similar ways
— resulting in analogous structures. For example, imagine two flower species that are not closely related, but
both happen to be pollinated by the same species of bird. A flower that gets pollinated produces many seeds
and has a big advantage over flowers that don't get pollinated.
If the bird prefers long red flowers, the flowers' shapes and colors may evolve in ways that make them more
attractive to the bird. The two flowers might end up looking the same, even though they started out looking
quite different from one another.
2. Describe how two unrelated flowers could evolve to have a similar appearance?
Homology or analogy?
You have probably noticed that dolphins and sharks both have a streamlined body shape with a triangular fin on
the back and two side fins. However, the two animals also have many differences.
Sharks
Dolphins
skeleton made of cartilage
skeleton made of bone<>
use gills to get oxygen from the
water in which they swim
go to the surface and breathe
atmospheric air in through their
blowholes
don't nurse their young
do nurse their young
don't have hair
do have hair — they are born with
hair around their "noses"
They may share the same basic shape, but underneath their skins, sharks and dolphins are very different!
3) Do you think that sharks' and dolphins' similarities (body shape,
fin, and flippers) are homologies or analogies?(See next page for some
hints!)
Hint 1: Having a streamlined body shape, fin, and flippers allows aquatic animals to swim faster. Both sharks and dolphins swim after prey.
Hint 2: This tree shows the where sharks and dolphins are positioned on the tree of life. They are not very closely related to one another.
Is it likely that sharks and dolphins inherited their body shapes, fins, and flippers from the same common ancestor?
Analogy: Squirrels and Sugar Gliders
Beyond being cute and cuddly, flying squirrels and sugar gliders have many striking
similarities: big eyes, a white belly, and a thin piece of skin stretched between their arms
and legs, a trait which helps them "glide" and remain stable when leaping from high places.
Flying squirrel
Sugar glider
But sugar gliders and flying squirrels also have some key differences. Most importantly, they
reproduce and bear their babies in fundamentally different ways:


Flying squirrels are placental mammals. Placental mammals spend a long time
developing inside the mother's body being nourished by a placenta before they are
born.
Sugar gliders are marsupial mammals, like kangaroos. Marsupial mammals may only
spend a short time developing inside the mother's body and are very tiny when born.
After birth, a baby marsupial crawls into its mother's pouch and is nourished by her
milk as it continues to grow and develop.
Flying squirrels and sugar gliders are only distantly related. So why do they look so similar
then? Their gliding "wings" and big eyes are analogous structures. Natural selection
independently adapted both lineages for similar lifestyles: leaping from treetops (hence, the
gliding "wings") and foraging at night (hence, the big eyes).
4) Considering all of the evidence, are the "wings" (actually flaps of skin stretched
between the legs) of sugar gliders and flying squirrels homologous or analogous
structures? Explain why you would conclude this.
Homology: Leave It To the Plants
In the following photos of plants, the leaves are quite different from the "normal" leaves we envision.
Each leaf has a very different shape and function, yet all are homologous structures, derived from a
common ancestral form. The pitcher plant and Venus' flytrap use leaves to trap and digest insects.
The bright red leaves of the poinsettia look like flower petals. The cactus leaves are modified into
small spines, which reduce water loss and can protect the cactus from plant-eaters.
Analogy: Of Shrimp and Snails
Barnacles and limpets (shown below) have many superficial similarities: both are small
creatures with conical shells and can be found in the ocean on rocky shores.
But the similarities end there. Inside their shells, they are very different:
Barnacles have the body layout of a shrimp. Limpets have the body layout of a snail.
independently —
What do these differences mean? They
suggest that barnacles and limpets are not
very closely related, and that their
common ancestor (probably a worm-like
animal) did not have a conical shell. Their
similar shells evolved because both lineages
faced a similar challenge: a rocky, waveswept environment teeming with predators
ready to pry a small creature off a rock for
lunch — and conical, hard-to-get-a-grip-on
armor offers a lot of protection in such an
environment. These lineages evolved their shells
meaning that the shells of these two lineages are analogies.
5) Sidetrip:
See more examples of homology and examples of analogy.
a) How are a venus fly trap and a pitcher plant homologous?
b) How are barnacles and limpets alike? How can these similarities be explained?
6) Read about the some more examples of analogies (BELOW). For each analogy, why would
such different organisms evolve to be so similar?
Analogy: Desert-dwellers
Both of these two plants have thick, water-filled branches and sharp spines. You might guess that they are closely related — but they are not.
In fact, one "cactus" is more closely related to a common weed, and the other is more closely related to a carnation. Their similarities are
analogies — independently evolved adaptations that aid survival in the desert: succulent, waxy stems help store water, and spines provide
shade and protect the plant from herbivores.
Analogy: Jaws versus Flipper
Although their differences are certainly substantial (e.g., sharks are cold-blooded and dolphins are warm-blooded), sharks and dolphins have
some undeniable similarities: side fins, a dorsal fin, and a torpedo-shaped body. These similarities are analogies — traits the two lineages
evolved independently as adaptations for moving swiftly in the water. Sharks were gliding through the oceans long before dolphins descended
from land-dwelling mammals. Dolphins evolved flippers, a dorsal fin, and a torpedo-shaped body as natural selection shaped them for the life
of an ocean predator.
Analogy: Kings of the Anthill
Anteaters live in Latin America and South America, grow to two meters (about six feet) long, and give birth to live young. Echidnas live in
Australia, grow to half a meter (about one and a half feet) long, and, as the only close, living relative of the duck-billed platypus, lay eggs.
But if you get up close and personal with them, you will find that anteaters and echidnas have some striking similarities: both are toothless,
with a pointy snout, a long, sticky tongue, and sharp, curved claws. All of these traits are analogies — testaments to the selective power of
the anthill. These two rather distantly related lineages independently evolved snout, tongue, and claw traits that allow them to more
efficiently pillage and plunder an anthill.
Analogy: When is a thumb a thumb?
Through careful study, biologist can also identify analogies. For example, panda bears have a "thumb" on their hands. They use this "thumb"
to hold onto bamboo as they eat. Is the panda's "thumb" homologous or analogous to the thumb on your own hand?
Studying the anatomy of panda hands and human hands shows that these "thumbs" must be analogous. When you look at the bones of each,
you see that the thumbs are not very similar at all! The human thumb has joints and is made of many bones. The panda thumb is just one
bone sticking out of the side of the hand.
Furthermore, the panda thumb is the sixth "finger" on
its hand! If you watched the hand of a baby panda grow,
you would see that the "thumb" develops from a wrist
bone. The panda thumb and the human thumb don't
grow from the same bones. This is more evidence that
they are analogous structures. In fact, the panda thumb
is homologous to a wrist bone in humans, and the
human thumb is homologous to the first finger in
pandas!
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