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Chapter 4

BIO342H5 Chapter Notes - Chapter 4: Convergent Evolution, Phylogenetic Tree, Synapomorphy

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BGYB 51 Chapter 4 (pg.111-119)
Estimating Evolutionary Trees
The evolutionary history of a group of species is called it phylogeny and a
phylogenetic tree is a graphical summary of this history. An evolutionary tree
describes the patter, and in some cases the timing, of events that occurred as species
diversified. It also records the sequence in which lineages appeared and documents
which organisms are more closely or distantly related.
4.1. The Logic of Phylogeny Inference
The most closely related taxa should have the most traits in common
Many types of characters could qualify: the sequence of nucleotides in a
particular gene, the presence or absence of specific skeletal elements or flower
parts, or the mode of embryonic or larval development
Synapomorphies Identify Monophyletic Groups
oThe most fundamental principle of phylogeny inference is that only
certain types of homologous characters are useful in estimating
phylogenetic trees
oA Synapomorphy is a homologous trait that is shared among certain
species and is similar because it was modified in a common ancestor
They are shared, derived traits
oAny group that includes an ancestor and all of its descendants is called a
monophyletic group (or clade or lineage)
This means all synapomorphies are homologous traits but not all
homologous traits are synapomorphies
For example, the genetic code helps identify bacteria and
mammals (eukaryotes) as members of the same monophyletic
group (homologous trait) but it does not help us distinguish
bacteria from eukaryotes
Bacteria and mammals each have synapomorphies that
identify them as distinct monophyletic groups
All bacteria have cell walls that contain pepitdoglycan (one
synapomorphy) while all eukaryote cells contain a nuclear
envelope (another Synapomorphy)
oTwo ideas are key to understanding why evolutionary relationships can
be inferred by analyzing synapomorphies
Synapomorphies identify evolutionary branch points (speciation)
Speciation starts when two populations become genetically
During speciation some of the homologous traits of the two
independently developing species undergo changes due to
mutation, selection and drift.
These changes become synapomorphies distinguishing the
two new populations
Synapomorphies are nest
As you move through time and trace a tree from its root to
its tips, each branching event adds one or more shared,
derived traits
The hierarchy described by synapomorphies also describes
the hierarchy of branching events. See figure 4.2.
Read paragraph on this method of drawing phylogenetic trees
Each branch on the tree corresponds to one or more
synapomorphies that distinguish the derived groups
o A phylogenetic tree inferred by clustering
synapomorphies in this way is called a cladogram.
Check out figure 4.3 and its associating paragraph
on page 114.
Problems in Reconstructing Phylogenies
oNot all Similar Traits are Homologous
See figure 4.4 for examples
Some morphological similarities evolve independently in different
lineages due to convergent evolution.
These occur when natural selection favours similar
structures as solutions to problems by similar
This can happen at the molecular level as well
Species can share nucleotide sequences for reasons other
than common ancestry
See Fig. 4.5
oThe example show is an example of DNA reversal
oConvergence and reversal are lumped under the
term homoplasy
oDistinguishing Homology from Homoplasy
The most efficient way to distinguish homology from homoplasy is
to analyze many traits in reconstructing evolutionary relationships
instead of one or just one
See Fig. 4.6 for an example and read page 117 and 118
oResolving conflicts in Data Sets: The Role of Parsimony
Parsimony provides one way to identify which branching pattern
minimizes the confusing effects of homoplasy and most
accurately reflects actual evolutionary history.
Under parsimony, simplex explanations are preferred over more
complex explanations
The preferred tree is the one that minimizes the total
amount of evolutionary change that has occurred
It is valid to assume that convergence and reveal will be
rare relative to similarity that is due to modification from a
common ancestor
oReversals and convergence both require multiple
evolutionary changes
oSo, it makes sense that the tree that minimizes the
total amount of changes implied by the data will also
be the one that minimizes the amount of homoplasy.
4.2 The Phylogeny of Whales
Prof said read this and be familiar with this is about because it could be used
for the midterm