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Outgroups

Outgroups. Outgroups are the most common method for rooting trees Outgroup criteria 1. “Outside” the group of study Closely related enough to be informative. Outgroup Rule: Character states shared by the outgroup and some of the ingroup are likely plesiomorphic.

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Outgroups

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  1. Outgroups • Outgroups are the most common method for rooting trees • Outgroup criteria 1. “Outside” the group of study • Closely related enough to be informative

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

  3. Build the tree with cladistics Characters Taxa 1 2 3 4 5 Outgroup 0 0 0 0 0 A 1 0 0 0 1 B 1 1 0 1 0 C 1 0 1 1 0

  4. Fig. 26-UN5 Draw the tree using cladistics… 1 2 3 4 5 6

  5. Fig. 26-UN10a

  6. If character conflict occurs, pick the correct tree based on the principle of parsimony 10 variable binary characters, minimum number of steps = 10 12 changes Character 3 evolves twice Character 2 evolves once and is lost 11 changes Character 10 evolves once and is lost

  7. Problems in tree building • Character conflict

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

  9. Misleading Characters • Homoplasy = “false homology”

  10. Which lineages are endotherms? The most parsimonious explanation is that endothermy arose twice: Convergence

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

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

  13. 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?

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