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.

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.
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 use gills to get oxygen from the water in which they swim don't nurse their young don't have hair skeleton made of bone<> go to the surface and breathe atmospheric air in through their blowholes do nurse their young 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:
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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).

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 .

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!