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Studying homologies and analogies
How do scientists figure out if a trait is a homology or an analogy? Biologists use
a few criteria to help them decide whether a shared morphological character (such as
the presence of four limbs) is likely to be a homology. Here's an example comparing mice and crocodiles:
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Same basic structure
The same bones (though differently shaped) support the limbs of mice and crocodiles. |
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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, as shown in this crocodile. |
Same development
The limbs of all tetrapods develop from limb buds in similar ways.
Of course, these criteria don't always apply — for example, two organisms might
share a homologous gene, but the gene doesn't really "develop." However, these criteria
are nonetheless useful. By studying the anatomy of a trait in living organisms and in
fossils and by observing how the trait grows and changes, biologists can usually find
out if a structure in two organisms is analogous or homologous.
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