BIOA01H3 Chapter 23: Chapter 23 notes
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23.1 Reading a Phylogenetic Tree
• The point where a branch on a phylogenetic tree splits is called a node
• Phylogenetic trees provide hypotheses f evolutionary relationships.
• A phylogenetic tree depicts a pattern on evolutionary relatedness among groups of
species or other groups by comparing their anatomical or molecular features.
• The search for sister groups lies at the heart of phylogenetics.
• Groups that are more closely related to each other than to any other group are called
sister groups, such as lungfish and tetrapods.
• Evolutionary relatedness therefore is determined by following nodes from the tips to
the root of the tree, and is not determined by the order of the tips from the top to
bottom of a page.
• A monophyletic group consists of a common ancestor and all its descendants.
• Taxa – all the species in a taxonomic entity such as a family or genus.
• Monophyletic – All members share a single common ancestor not shared with any other species
or group of species.
• Paraphyletic – groupings that include some, but not all the descendants of a common ancestor.
• If in order to separate a
group from the rest of the
phylogenetic tree you need
only to make one cut, the
group is monophyletic. If you
need a second cut to trim
away part of the separated
branch, the group is
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• Polyphyletic – groupings that do not include the last common ancestor of all members.
• Taxonomic classifications are information storage and retrieval systems.
• Genus – a group of closely related species
• Family – a group of closely related genera.
• Order – a group of closely related families.
• Class – A group of closely related orders.
• Phylum – a group of closely related classes, defined by having one of a number of
distinctive body plans.
• Kingdom – a group of closely related phyla
• Domains – one of the three largest limbs of the tree of life: Eukarya, Bacteria, and
23.2 Building a Phylogenetic Tree
• Homology is similarity by common descent.
• Characters – anatomical, physiological, or molecular features of organisms that vary
among closely related species.
• Character state – the observed condition of a character, such as a presence or absence
of lungs or an arrangement of petals.
• Characters that are similar in
different species because of
descent from a common ancestor
• Analogous describes similar
characters that evolved
independently in different
organisms as a result of adaptation
to similar environments. That is a
result of convergent evolution.
• Shared derived characters enable
biologists to reconstruct evolutionary history.
• Because homologies result from shared ancestry, only homologies, and not analogies,
are useful in constructing phylogenetic trees
• Synapomorphy – a shared derived character. A homology shared by some, but not all,
members of a group.
• Cladistics – Phylogenetic reconstruction on the basis of shared evolutionary changes in
characters, often called synapomorphies.
• The simplest tree is often favored among multiple possible trees.
• Parsimony – choosing the simplest hypothesis to account for a given set of observations.
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