Biology · Introductory biology · Worked example
Find the closest relatives on a tree
A tree of five animals reads, in Newick form, (Shark,(Frog,(Lizard,(Mouse,Human)))). Which animals are the lizard’s closest relatives? Do the frog and the lizard form a clade? Using the characters four limbs, amniotic egg and hair, how many changes does the tree need?
Find the lizard’s most recent common ancestor
The node nearest the lizard joins it to the group (Mouse, Human). That ancestor is shared by no other animal, so the mouse and the human are equally and most closely related to the lizard, not the frog beside it in the list.
Test frog and lizard as a clade
The smallest clade holding both the frog and the lizard also holds the mouse and the human. A group of frog and lizard alone leaves out descendants of their common ancestor, so it is not a clade.
Place each character once
Four limbs arose on the branch below (Frog, …), the amniotic egg on the branch below (Lizard, …), and hair on the branch below (Mouse, Human). Each character changes once.
Judge the tree by parsimony
Three characters can never need fewer than three changes, so no tree fits these characters with fewer: this one is the most parsimonious.
Result
The lizard’s closest relatives are the mouse and the human, which share its most recent common ancestor. Frog and lizard alone are not a clade, because their common ancestor’s descendants include the mouse and the human. The tree needs 3 changes, one per character, the minimum, so it is the most parsimonious.
Your turn
On the same tree, do the mouse and the human form a clade? Which character marks it?
Show the answer and explanation
Yes; hair marks it.
Their most recent common ancestor has no other descendants on this tree, so the two form a clade, and hair arose on the branch leading to it.
Keep exploring
The tree opens in Phylogenetic tree with its characters; rotate a node and see that the relationships stay the same.
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