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Lec 21 - Phylogeny 2 and Evolutionary Ecology.docx

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Biology 1001A
Beth Mac Dougall- Shackleton

Amr Alsabbagh 22/11/12 Lecture 21 – Phylogeny 2 and Evolutionary Ecology In order to correctly arrange the sequence of branching events on a phylogenetic tree, and understand the patterns of relatedness, we need to reconstruct or make conclusions about the characters states of the common ancestors from which modern-day species are descended. This is difficult because common ancestors are long extinct. Reading and Building Phylogenies Recap  “Related to” is NOT synonymous to “descended from” o i.e. you are related to your cousins, but you are NOT descended from them  “more complex” (in terms of morphological or developmental complexity) is NOT synonymous with “more evolved” since all species today have been evolving for the same amount of time.  Relatedness is NOT equated with similarity o Not all similarities are homologies  Shared traits could be due to convergent evolution o Not all homologies are synapomorphies  According to cladistics, we only pay attention to synapomorphies (shared, derived traits). Is this trait ancestral or derived?  Trait is present in outgroup and ALL of ingroup o Most likely ancestral (strong evidence) o The trait MAY have evolved multiple times over the course of evolutionary history (convergent evolution), but the probability is HIGHLY unlikely  Present in outgroup and SOME of ingroup o Most likely ancestral o For some reason, the other ingroup lineages may have lost this trait over the course of evolutionary history.  Absent in outgroup, but present in SOME of ingroup o Most likely derived o More likely the trait was derived independently in each of the lineages that have the trait  Present in the outgroup, but NONE of ingroup o We cannot tell in this case. o There are two essentially equally likely possibilities:  The common ancestor did not have the trait (the trait is derived, it happened in the lineage leading to the outgroup)  The trait is ancestral, but in the evolutionary step leading to the ingroup, the traits was lost.  Absent in outgroup, but present in ALL of ingroup o We cannot tell The reasoning relies on parsimony  The simples explanation is usually the best  “Ockham’s razor”  Whichever tree requires the smallest number of evolutionary changes (gains or losses of a trait), is probably correct.  Parsimony often used to evaluate potential trees. Amr Alsabbagh 22/11/12 How many potential phylogenetic trees are there?  With increasing # of taxa (t), the number of candidate trees increases hugely (there is a formula, we do not need to know) o 2 taxa, 1 possible tree o 3 taxa, 2 possible trees o 4 taxa, 15 possible trees o 10 taxa, 34.5 million possible trees Using parsimony to evaluate candidate trees  Outgroup: Shark  Ingroup: Chicken, Bat, Chipmunk  First, identify which traits are synapomorphies o Outgroup  Shark: N
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