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Chapter 4

BIO342H5 Chapter Notes - Chapter 4: Nuclear Membrane, Monophyly, Synapomorphy

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Don Mc Kenzie

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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 isolated
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
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