Name_____________________________ Date _________________ Block A B C D
Understanding Evolution: Homology and Analogy:
INSTRUCTIONS: 1) Read and highlight these articles
2) Answer the ELEVEN questions (questions are at the bottoms of most pages).
Answer all questions in
your Science Journal.
For more information and help, go to http://evolution.berkeley.edu/evolibrary/ then select “What is the Evidence for
Evolution” then select homology and analogy then select Similarities and differences: Understanding homology and
analogy
Similarities and differences: understanding homology and analogy
by the Understanding Evolution team
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 a common ancestor
analogy
similarity due to convergent evolution, not common ancestry
In this module, you'll learn more about homologies and analogies and why they are important. We'll start by playing a game to get you
thinking about similarities and differences.
1) In the image, which pair represents an analogy and which a homology? (ANSWER IN
JOURNAL)
The tetrapod limb
These four limbs all belong to tetrapods — animals with four legs.
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:
2) Define homology: (ANSWER IN JOURNAL)
3) Define analogy: (ANSWER IN JOURNAL)
4) 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? (ANSWER IN JOURNAL)
5) What is a tetrapod? (ANSWER IN JOURNAL)
6) What are the six bones found in all tetrapod legs? (ANSWER IN JOURNAL)
7) Identify (generally) these limbs (to what animal do they belong)? (ANSWER IN JOURNAL)
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 descendents, 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.
8) What did the common ancestor of all modern tetrapods look like? (ANSWER IN JOURNAL)
9). Side Trip: “not just anatomy.” - How are a bird and a crocodile homologous? (ANSWER IN
JOURNAL)
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.
10) Similar structures that evolved independently are called _________________________.
(ANSWER IN JOURNAL)
11) Give 3 examples of Analogies and explain why they are NOT HOMOLOGIES. (ANSWER IN
JOURNAL)