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Similarities and differences: understanding homology and analogy (High School level) :

Using homologies to reconstruct relationships

After scientists have studied many traits in different species and figured out which are probably homologous, the traits can be used to build an evolutionary tree. Only homologous traits are evidence of shared ancestry. Homologous (but not analogous) traits can be used to reconstruct the evolutionary relationships between different species. For example, humans, chimpanzees, and gorillas all have thumbs that are very similar anatomically and are homologous. The panda's thumb, however, is analogous to these primate thumbs. This (along with lots of other evidence) suggests that humans, chimpanzees, and gorillas are more closely related to one another than any of them are to pandas.

The separate origins of thumb-like structures

Scientists don't always start with anatomy in determining if two structures are homologous. Sometimes a phylogeny is used to help determine if a trait is homologous or analogous; however, this phylogeny is based on other traits, not the trait in question. This ensures that scientists' reasoning is not circular. Many different pieces of evidence feed into such arguments, and scientists go with the conclusion most consistent with the evidence.

Homology & Analogy
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