Name: ______Block ______Date ______

Similarities and differences: understanding homology and analogy

In everyday life, people look like one another for different reasons. Two sisters, for example, might look alike because they both inherited brown eyes and black hair from their father. On the other hand, two people attending an Elvis impersonators' convention may look alike because they are both wearing rhinestone studded suits and long sideburns. The similarity between the sisters is inherited, but the similarity between the Elvis impersonators is not.

Biological similarity: It works the same way in biology. Some traits shared by two living things were inherited from their ancestor, and some similarities evolved in other ways. These are called homologies and analogies.

Homology: traits inherited by two different organisms from acommon ancestor

Analogy: similarity due toconvergent evolution (often occurs when both lineages face similar environmental challenges and selective pressures)

The tale of the limb
Take a look at the diagrams of front limbs presented below. Each is from a different animal. In each grouping you see, you should select the limb that doesn't belong, and take note of the similarities that the others share. Cross out the limb that does not fit with the group. Explain why you removed that limb in the box beneath the group.
Explain why you made this choice: / Explain why you made this choice: / Explain why you made this choice:
The Tetrapod limb
The four limbs all below all belong totetrapods— animals with four legs. Notice how these tetrapod limbs are similar to one another: 1) They are all built from many individual bones. 2) They are all spin-offs of the same basic bone layout: one long bone attached to two other long bones. Here you can see the same bones labeled in these different limbs: Even though these limbs are similar to one another, the animals they belong to are quite different from one another. What animals possess these homologous limbs?
/ / List the animals your think these limbs belong to:
1 ______
2______
3______
4 ______
Inheriting homologies
Whales, lizards, humans, and birds all have the same basic limb layout. But how did such different animals wind up with the same sort of limb? The answer is that they inherited it from a common ancestor, just as cousins might inherit the same trait from their grandfather.
/ This family tree shows several generations starting with a pair of grandparents. In the same way, an evolutionary "family" tree shows relationships over much longer periods of time.
/ This evolutionary tree shows the relationships between different tetrapod lineages, all of which evolved from a single common ancestor. This 350 million year old animal, the first tetrapod, had limbs with one long bone (the humerus) attached to two other long bones (the radius and ulna). Its descendants, including whales, lizards, humans, and birds, as well as many others, inherited the tetrapod limb from this ancestor. Structures inherited from a common ancestor are called homologous structures, or homologies.
Homologies are everywhere
Once you know what a homology is, you can find them anywhere.
/ / The leaves of an oak (left) and the leaves of a gingko (right) are homologous — they were both inherited from a common ancestor with leaves.
/ / The wing of a dragonfly (left) and the wing of a butterfly (right) are homologous — they were both inherited from an ancient flying insect. Since all life shares a common ancestor, we find homologies all over the tree of life
Beyond the obvious
Not all homologies are obvious. If two homologous structures have been adapted for different roles, they may not look very much alike. For example, the chomping front teeth of a beaver look quite different than the tusks of an elephant.
/ / The beaver uses its teeth for chewing through tree trunks, and the elephant uses its tusks for a number of tasks including digging, peeling bark from trees, and fighting. But if you examine these two structures closely, you will see that each is a modification of the basic incisor tooth structure. Over time, evolution adapted each of these animals' incisors to perform different functions. They are homologous structures, inherited from a common ancestor with incisor teeth.
Not all similarity is homology
In the game you played at the beginning of this module, you decided that some legs were "not like the others." The matching limbs in the game have bones, and you eliminated the limbs that don't have bones. Since the octopus, sea star and grasshopper limbs don't have bones, you concluded that they are probably not homologous to tetrapod limbs.
/ Homologies are inherited from common ancestors. The octopus limb could only be homologous to the lizard limb if they both inherited the limb from a common ancestor. This tree shows how the octopus is related to tetrapods, and the points in their evolutionary histories when their limbs evolved.
/ Tetrapod and octopus limbs evolved independently after their point of common ancestry, so they were not inherited from a common ancestor. Therefore, they are not homologous. The same is true of the grasshopper leg and the sea star arm. Simple observation tells us that these limbs are probably not homologous to the tetrapod limb, because they have such different structure. Similar structures that evolved independently are called analogous structures, or analogies.
Analogies
Elvis impersonators have many similarities, such as long black sideburns and rhinestone-studded suits, that are not inherited. In the same way, biological similarities are not always due to inheritance. For example, look at the skulls below. They belong to extinct animals, and both of them have saber teeth — long, ferocious canines. Would you guess that these saber teeth are homologous — inherited from a common ancestor with extra-long saber teeth?
/ / These two skulls certainly look like the animals could have inherited their saber teeth 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 toThylacosmilus, amarsupial mammal. The other belongs toSmilodon, the saber-toothed cat, which is aplacental mammal. Marsupial and placental mammals are very different, and diverged from each other a long time ago on the evolutionary tree.
/ Thylacosmilusis more closely related to other marsupials such as kangaroos and koalas than it is toSmilodon.Smilodonis more closely related to other placentals such as housecats and elephants than it is toThylacosmilus. Saberteeth is not a common trait in the marsupials closely related toThylacosmilus, or the placentals closely related toSmilodon. 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 saber teeth inSmilodonandThylacosmilusevolved 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 saber teeth from normal length teeth.
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. /
/ / Explain how flowers would change in shape and color over time:
______
______
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!
Top of Form
Do you think that sharks' and dolphins' similarities (body shape, fin, and flippers) are homologies or analogies?______
Bottom of Form
/ 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?