Using the Tree to Understand History
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Predicting ancestral characters
Knowing the characters of the organisms at the tips of a phylogeny lets you figure out what the ancestral characters were likely to have been. For example, modern flies and moths have silk glands but beetles and wasps don’t.

Probable point on the phylogeny where silk glands evolved

The simplest explanation is that silk glands evolved only once in the ancestor of flies and moths.

Probable point on the phylogeny where silk glands evolved

That means that when we imagine the most recent common ancestor of flies and moths (the ancestor at the node between them), we should imagine an insect with silk glands.

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Spanish translation of Understanding Evolution For Teachers from the Spanish Society of Evolutionary Biology.