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
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
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
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
Whichever tree requires the smallest number of evolutionary changes (gains or losses of a trait), is
Parsimony often used to evaluate potential trees. Amr Alsabbagh
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
Ingroup: Chicken, Bat, Chipmunk
First, identify which traits are synapomorphies