Before Homologous Structures Lab Reading

Similarities and differences: understanding homology

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 similarities between living things were inherited from their ancestor, and some similarities evolved in other ways. These are called homologies and analogies.

homology
similarity inherited from a common ancestor

analogy
similarity due to convergent evolution (similar environments lead to similar adaptations).

In this lab, we are going to learn more about these two kinds of similarity and why they are important for the study of evolution.

The tetrapod limb

Tetrapods are animals with four limbs (this includes everything from birds to lizards!!). Notice how these tetrapod limbs are similar to one another:

·  They are all built from many individual bones.

·  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:

Inheriting homologies

So, whales, lizards, 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.

The 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 descendents, including whales, lizards, and birds, as well as many others, inherited the tetrapod limb from this ancestor.

Similar structures inherited from a common ancestor are called homologous structures, or homologies.

Recognizing Homologies

Biologists use a few criteria to help them decide whether a shared trait (such as having four limbs) is likely to be a homology:

Same basic structure
The same bones (though differently shaped) support the limbs of mice and crocodiles. In the illustration of arms to the right, homologous bones are colored alike /
Same relationship
to other features
The limb bones are connected to the skeleton in similar ways in different tetrapods. The joint between the femur and the pelvis has a ball-and-socket structure which is typical of tetrapods, as shown in the crocodile to the left /
Same development
The limbs of all tetrapods develop from limb buds in similar ways /

These criteria help biologists identify homologous structures that are likely to be indicators of shared ancestry. So, when trying to determine whether chicken wings and human arms are homologous be sure to ask the following: Are the bones similar in shape. Are the bones connected with the same type of joints and in the same order to one another?

Homologies are everywhere

Once you know what a homology is, you can find them anywhere. Since all life shares a common ancestor, we find homologies all over the tree of life.

The wing of a dragonfly (left) and the wing of a butterfly (right) are homologous — they were both inherited from an ancient flying insect.

Beyond the obvious BUT 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 Not all similar structures are homologous structures. For example, birds and bats both have wings, while mice and crocodiles do not. Does that mean that birds and bats are more closely related to one another than to mice and crocodiles? No. When we examine bird wings and bat wings closely, we see that there are some major differences.

Bat wings consist of flaps of skin stretched between the bones of the fingers and arm. Bird wings consist of feathers extending all along the arm. These structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor with wings. In the case of birds and bats, their wings are called analogies—because their similarity is most likely due to similar environmental pressures.
HOMOLOGOUS STRUCTURES LAB Name: ______

Purpose: To determine whether humans and chickens are related by comparing the anatomy of their limb structure.

Hypothesis: (To be written after reading procedure and before lab reading)

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Procedure

1.  Read the BEFORE LAB reading carefully with your group.

2.  When your teacher tells you to begin, carefully pull the skin and flesh off of your chicken wing. DO NOT BREAK THE BONES AND BE SURE TO PUT ALL OF THE GARBAGE IN THE TRAY.

3.  When your wing is clean—compare your bones to the structures of the bones in your diagrams and complete your sketch on the lab sheet.

4.  When you have made our comparisons, raise your hand so that your teacher can give you permission to clean up.

5.  As a group, complete the analysis questions and conclusion below.

Analysis Questions

1.  How do homologous structures provide evidence for evolution? ______

2.  What other types of evidence should scientists examine to determine whether humans and chickens are related? ______

3.  Why might some animals have similar body structures without a common ancestor to explain it? ______

Lab Conclusion

Write at least 5-7 sentences explaining why you should accept or reject your original hypothesis. Are chicken wings and human arms homologous structures? Why or why not? Be sure to cite evidence from your lab and draw a diagram if it is necessary to clarify your response.

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