Count trait changes on branches
- The idea
- Predict
- See it
- Why
- Check
The idea
“Each species with the trait gets its own change, or they all share one.”
A data table lists traits species by species, so most people count that way. The changes sit on branches.
The short version
The mistake: Each species with the trait gets its own change, or they all share one.
What's actually true: A change happens on one branch, and every species descended from it inherits it unless it's lost later.
- A change on one branch passes to every species descended from it, unless a later change reverses it.
- If the species with a trait form a clade, an ancestor and all its descendants, one change explains them. If one nested among them lacks it, you need at least two.
- Parsimony favors the tree with the fewest total changes, so a miscount can make the wrong tree win.
On the exam: Mark each change on a branch, then count the marks, not the species.