Homology or Analogy?
Biological Examples
The remainder of this course will be a series of ways to test your hypotheses about how (or if)
organisms are related. In order to do this, we'll need to determine if particular similarities are due
to homology or analogy. Here's a flowchart showing a method for making these decisions:
Describe
structural
similarity
Describe function in each
organism
Analogy
Yes
Is the similarity
necessary for the
function?
No
Homology
1. Describe the structural similarity you wish to test.


Described a specific organ or structure that is similar in the two. You can't say a
whole organism is analogous or homologous to another; you have to work with
one structure at a time.
Described the way they are similar, not simply name the similar structures
2 Describe the function of the structures in each organism.

Consider each organism separately, and describe the functions in
operationally. That is, it's better not to say "they use them to move around,"
since moving around could be accomplished with feet, wings, jets, or wheels,
each of which works quite differently from the rest. Instead, say exactly what they
do. For example: the wing in the goose is used in flying, to push down on air and
move the body.
3. Is the structural similarity necessary in order to have this function?

For example all wings push down on air to move the body.
4. Analogy. Remember an analogy doesn't provide any evidence that organisms are related,
but it doesn't falsify the hypothesis either. For judging the hypothesis in the tree, this
similarity gives no conclusive evidence.
5. Homology. Homologies can only be concluded if a similarity is not necessary for the
function.
Today we will examine the question whether a similarity is due to homology or analogy? In
order to make this decision without basing it on a hypothesis about their ancestry, use the flow
chart at the top of this page and follow the steps. (Remember that, for the purposes of this course,
if we cannot falsify analogy, then we'll accept it as a tentative explanation.)
Part I. Wings:
Consider the wings of a bird and a wasp.
1. Describe their structural similarity (not just "both are wings" – what is similar about the
structures that we call wings in a bird and a wasp?)
2. What is the function of the wing in the bird and wasp?
3. Is the similarity you described above necessary in order to serve this function? Explain. If
you described multiple similarities, (flat, thin, lightweight, etc...) consider each similarity
separately and determine if it is necessary for the function.
4. What do you conclude? Is the structural similarity you described in question 1 explained by
homology or analogy?
5. Does this evidence support or falsify the hypothesis that birds and wasps have a common
ancestor? Explain.
Part II Leaves:
In this example we will look at three structures: the leaf of a maple tree, the leaf of an oak
tree, and the blade of kelp. There are specimens (please be careful with them) and cross section
slides of leaves and kelp blades.
1. Consider the leaves and blades of the three organisms. Describe their structural similarity.
2. What is the function of the leaves and blades in the oak, maple and kelp?
3. Is the similarity you described above necessary in order to serve this function? Explain. If
you described multiple similarities, (flat, thin, lightweight, etc...) consider each similarity
separately and determine if it is necessary for the function.
4. What do you conclude? Is the structural similarity you described in question 1 explained by
homology or analogy?
5. Does this evidence support or falsify the hypothesis that oaks and kelp have a common
ancestor? Explain.
6. How about oaks and maples? Explain.
Part III. Limbs
There are three organisms for you to look at here an ant, wasp and preying mantis. In this
case we are going to start off with just the wasp and ant and bring the mantis in later. Examine
the front leg of an ant and a wasp.
1 Consider the legs of an ant and a wasp. Describe their structural similarity
2. What is the function of the leg of the ant and wasp?
3. Is the similarity you described above necessary in order to serve this function?
4. What do you conclude? Is the structural similarity explained by homology or analogy?
5. Now look at the front limb of a mantis.
a. What is its function?
b. How is it similar to the front leg of a wasp or ant?
c. Is the similarity you described above necessary in order to serve this function?
d. What do you conclude? Is the structural similarity explained by homology or analogy?
6. When you examine the mantis as well now what can you conclude about the
front limb of a wasp, ant and mantis?