BI 151
Biological Reasoning
Using Homology and Analogy to Test Hypotheses
When you see a structural similarity between two species, there are two testable explanations for it: the
species are similar because they inherited the similarity from a common ancestor, or because the similarity is
necessary for a particular function. (Well, the similarity could be solely due to chance, but in most cases this
will be awfully unlikely). How can we distinguish between these possibilities?
If you know the ancestry of the species, or at least if you are basing your explanation of the similarity on a
hypothesis about their ancestry, you can look at their most recent common ancestor.
1. What must be true of the most recent common ancestor if the similarity is due to homology?
Often, though, we will be using similarities in living organisms to test hypotheses about their ancestry. The
argument would be: "these two organisms have a similarity which is due to homology; therefore they must
have a common ancestor." So, suppose you're testing the hypothesis we looked at earlier:
Note that this tree shows geese and whales as having a common ancestor. Since you're testing the
hypothesis, you look at a goose and a whale, and note a structural similarity: both have forelimbs that are
flattened and broad (wings and flippers). You then say to a classmate, "I think that similarity is due to
homology, so it supports this hypothesis."
2. When asked to explain how you know the similarity in this case is due to homology, is it all right to say,
"because they got it from a common ancestor?" Explain why or why not.
Obviously, if homology is to be used as evidence to test a hypothesis about ancestry, you can't use the
hypothesis as an argument for how you know something is homology -- that would be a circular argument. We
must have another way to determine if structures are homologous. We need a way that is not based on a
hypothesis about ancestry, an independent test for homology. What we have to do is consider what we know
about the organisms themselves. Remember that an analogy is
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a similarity in structures which serve similar functions
a similarity in structure that is necessary in order for the function to be possible
What we want to do is look at the organisms themselves and use them to test our hypothesis. Under the
hypothesis-testing "Program", we've stated our hypothesis and drawn a model -- the tree -- so now we should
make predictions.
3. If the tree above is a good representation of the relationships among the organisms shown, should we find
any homologies between geese and whales?
Since whales and geese have a common ancestor in the tree shown, they should have some homologies.
4. Does the hypothesis shown in the tree predict what specific homologies they will have? Explain.
It's not really possible to predict from the hypothesis what features two organisms might have retained from a
common ancestor and what features would have changed over time, so it isn't really possible to predict specific
homologies, just that we should expect SOME homologies. Consider another example:
5. Does the hypothesis shown in the tree predict that there will be any homologies between cattle and whales?
Explain.
Note that cattle and whales aren't related, according to the hypothesis. Therefore, the hypothesis shown in the
tree predicts that they won't have any homologies. If we found a homology between cattle and whales, this
would falsify the hypothesis shown in the tree.
6. What about analogies? Does the tree above predict that there will be analogies between ducks and geese?
How about between cattle and whales? Explain.
Actually, if you think about it, there could be analogies between almost any two organisms. Analogy doesn't
depend on them being related -- there just would have to be some organ or limb that had a similar function in
both, and therefore had to have some sort of similar structure. That could be the case if they're not related,
like cattle and whales in this hypothesis, or if they are related, like ducks and geese in this hypothesis. The
hypothesis itself doesn't make any predictions about analogies -- it's just about relationships, and doesn't say
anything about the functions of organs in the various organisms.
When we're testing hypotheses about ancestry, since an analogy can be found both in organisms that are
related and in ones that aren't related, analogies are always inconclusive. They just can't tell us whether
two organisms are related or not.