Studying homologies and analogiesHow 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:
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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Crocodile hip image courtesy of Dave Smith, UCMP; Alligator limb bud image courtesy of the U.S. Geological Survey/Florida Integrated Science Center, photo by James Basto; Mouse limb bud photo courtesy of the Embryo Images: Normal & Abnormal Mammalian Development website
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