A shared trait usually comes from one ancestor
- The idea
- Predict
- See it
- Why
- Check
The idea
“Related species that share a trait each evolved it separately, or got it from another species.”
With each species on its own branch, it seems fair to give each one its own origin. Every extra origin is another rare event to explain.
The short version
The mistake: Related species that share a trait each evolved it separately, or got it from another species.
What's actually true: When related species share a trait, the simplest history is usually that it arose once in their common ancestor.
- A trait that relatives inherited from a common ancestor is homologous.
- Parsimony means picking the history with the fewest gains and losses. Each separate origin of the same complex trait or DNA change is unlikely, so one origin beats four.
- Separate origins do happen. Bird and bat wings evolved separately, which is convergent evolution. The arm bones inside them are still inherited.
On the exam: When relatives share a trait, assume one origin in their common ancestor unless evidence says otherwise.