A species gets only the changes on its own path
- The idea
- Predict
- See it
- Why
- Check
The idea
“If an ancestor gained a trait, or a close relative has it, this species has it too.”
Relatives share so much that it seems fair to give a species whatever its neighbors have. It's also easy to assume a trait an ancestor gained is still there.
The short version
The mistake: If an ancestor gained a trait, or a close relative has it, this species has it too.
What's actually true: A species has the changes on its own path from root to tip, losses included, and nothing from branches off that path.
- A tick marks a change on one branch. Every species descended from that branch inherits it.
- To find a species' traits, walk from the root to its tip. Add each trait gained on the way and remove each one lost.
- Changes on other branches don't count, even on a close relative. Feathers arose on the bird line after it split from the snakes' line.
On the exam: Walk from the root to the tip. Count only the changes on that path, losses included.