Building a Phylogeny: Homology, Shared Derived Characters and Parsimony
A tree is built from characters that are shared because of common ancestry, not because of similar conditions, and only the derived states group taxa together. Parsimony picks the tree that needs the fewest changes to explain the data.
What a learner can do afterwards
- Separates homology from convergence using a stated character and its distribution.
- Counts the changes a candidate tree requires and compares two trees on that count.
- Explains why shared ancestral states do not define a group.
1 · Read
Picture building a family tree from traits. First sort shared blood from apart by looks. Mammal forelimb bones share build from shared ancestors, while bird and bug wings look alike yet evolved apart. Only blood carries reliable news about kinship, so resemblance alone never votes.
Now judge rival drawings by counting. Tally the trait edits each candidate tree forces you to assume and prefer the smallest total. Fresh states shared by some, yet not all, mark true subgroups. Old states shared by everyone fit any grouping and prove nothing. Fewest edits wins.
Next, test whether a group is complete. A valid clan holds an ancestor plus all its kids, with nothing cut and nothing smuggled in. Hair marks mammals because it is fresh at that level. Backbones cannot mark mammals because far more than mammals own them. Always ask if a trait is fresh for the team you propose.
Try the candy jar trick to feel the logic. Sort sweets by colour, shape, and filling, then argue which traits inform and which mislead. Sweetness sits in every piece and groups nothing, mirroring old shared states. Comparing rival trees on edit counts turns sorting into an argument about evidence and thrift.
You keep blood shared traits, tally edits, and crown the smallest total.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.