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.
HOMOLOGOUS TETRAPOD LIMBS
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:
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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
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Same development
The limbs of all tetrapods develop from
limb buds in similar ways
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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?
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2. What other types of evidence should scientists examine to determine whether humans
and chickens are related?
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3. Why might some animals have similar body structures without a common ancestor to
explain it?
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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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