Outgroups

• Outgroups are the most common method for rooting trees

• Outgroup criteria1. “Outside” the group of study2. Closely related enough to be informative

• Outgroup Rule: Character states shared by the outgroup and some of the ingroup are likely plesiomorphic

Build the tree with cladistics

CharactersTaxa 1 2 3 4 5Outgroup 0 0 0 0 0A 1 0 0 0 1B 1 1 0 1 0C 1 0 1 1 0

Fig. 26-UN5Draw the tree using cladistics…

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Fig. 26-UN10a

If character conflict occurs, pick the correct tree based on the principle of parsimony

12 changesCharacter 3 evolves twiceCharacter 2 evolves once and is lost

11 changesCharacter 10 evolves once and is lost

10 variable binary characters, minimum number of steps = 10

Problems in tree building

Character conflict

Good synapomorphies

• Homology = similarity in form or function shared by organisms because the feature was found in their common ancestor

Misleading Characters

• Homoplasy = “false homology”

Which lineages are endotherms?

The most parsimonious explanation is that endothermy arose twice:Convergence

Mammal tree article

Convergence in mammals (easier to ID for marsupials vs. placentals)—similar ecological niches

DNA data (right) has turned conventional mammal taxonomy (left) on its head. Morphological analyses were mislead by convergence.

Mapping character state changes onto a tree

2 independent gains of echolocation (convergence) or 1 gain followed by one loss (character state reversal)?Which do you consider more likely?