An old trait or an old absence doesn't mark a clade
- The idea
- Predict
- See it
- Why
- Check
The idea
“If two species both have a trait, or both lack one, they must be each other's closest relatives.”
Any shared trait feels like evidence of kinship. Lizards and crocodiles both lack feathers, but so does every living animal except birds.
The short version
The mistake: If two species both have a trait, or both lack one, they must be each other's closest relatives.
What's actually true: Only a shared derived trait, a state that is new compared with the outgroup, marks a clade.
- A clade is one ancestor and all of its descendants. It is marked by a shared derived trait, a new state that arose in that ancestor.
- A state the outgroup also has, present or absent, is ancestral. It was there before the group split, so it can't show who split from whom.
On the exam: Check the outgroup. A state it shares groups nothing. Only a shared new state marks a clade.