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Lecture 21.docx

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Biology 1001A

Lecture 21 Phylogeny II  Outgroup analysis provides clues to which traits derived, which ancestral  o evolution does not always proceed toward greater complexity (4 legs -> 6 legs)  Reading and Building phylogenies  o "related to" =/= "descended from" o  related to cousins, related to siblings, modern species are all related to one another does not descended from one another o more complex =/= "more evolved" o relatedness =/= similarity o  not all similarities are homologies  not all homologies are synapomorphies  Is this trait ancestral or derived? or neither (cannot tell)  o present in outgroup and ALL of ingroup o  it is ancestral  don't know for sure, but it is very likely that it is ancestral o Present in outgroup and SOME of ingroup? o  it is ancestral  more likely its ancestral, but it might be lost in some lineages o Absent in outgroup, present in SOME of ingroup o  it is derived  a trait is not found in the outgroup, only some of ingroup have it then its derived, not ancestral o Present in outgroup, but NONE of ingroup? o  We cannot tell, it can go either way o Absent in outgroup, present in ALL of ingroup? o  we cannot tell  This reasoning relies on parsimony  o the simplest explanation is usually the best o whichever tree requires the smallest number of evolutionary changes (gains or losses of a trait), is probably correct o Parsimony often used to evaluate potential trees  How many potential phylogenetic trees are there?  o with increasing number of taxa (t), number of candidate trees increases hugely o 2 taxa 1 possibilities o 3 taxa 2 possibilities o 4 taxa 15 possibilities o 10 taxa 34.5 million possibilities  Using parsimony to evaluate candidate trees  o Outgroup o  shark o Ingroup o  chicken  bat  chipmunk  Which tree is more likely to be correct?  First identify which traits are synapomorphies  o outgroup
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