BIOL 3542 Lecture Notes - Synapomorphy, Nuclear Membrane, Eukaryote

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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
o The most fundamental principle of phylogeny inference is that only certain types of
homologous characters are useful in estimating phylogenetic trees
o A 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
o Any 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)
o Two 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
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
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