A trait starts at the ancestor of the species that share it
- The idea
- Predict
- See it
- Why
- Check
The idea
“A common trait goes back to the root. A new one starts at the newest split.”
A common trait looks ancient, and a new-looking one seems to belong at the newest split. Neither guess checks which species have it.
The short version
The mistake: A common trait goes back to the root. A new one starts at the newest split.
What's actually true: A trait that arose once and was never lost first appeared in the most recent common ancestor of exactly the species that have it.
- A trait that arose once and was never lost is in every descendant of the ancestor where it arose. Those species form a clade.
- To place it, trace the species that have it back to their most recent common ancestor.
- If a trait might have been lost or gained more than once, pick the placement with the fewest changes. That is parsimony.
On the exam: Circle the species with the trait. If they form a clade, their most recent common ancestor is where it began.