Last day how to classify organisms

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Last day how to classify organisms
Discussed classification, how that relates to
phylogeny
Three types of group could possibly be
used Monophyletic - all spp. share a common
ancestor, all descendants of that
ancestor are included
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Polyphyletic - does not include the most recent
common ancestor of the species
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Paraphyletic - includes the most recent common
ancestor, but does not include all descendants
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Most systematists today try to use only
monophyletic groups in classification, but
traditional classifications often include
paraphyletic groups
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Paraphyletic groups are most controversial -
usually considered bad, but still frequently
used

• e.g. reptiles or Reptilia

Share many characters, but mostly ancestral
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Reptiles excludes some descendants (birds,
mammals?)
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What does a tree represent?
Relationships between taxa
Relative time since taxa split (sometimes
absolute time)
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Phylogeny often has branches at top
What does x axis represent?
Nothing
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Branches can be rotated without changing
meaning, only branching pattern is important
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Phylogenies constructed using similarities
between species, because related species
are expected to share most traits
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If two taxa share many traits, we expect them to
be closely related

• alternative same characters occur in unrelated
  spp.
• character may have evolved multiple times

Not impossible! Convergent evolution
Actually, Alluaudia
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Try to use only homologous characters characters
whose similarity is due to inheritance from a
common ancestor

• recognized by detailed
• similarity in structure,
• organization, development,
• etc. (but not always easy!)

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However, we assume it is more likely that
shared characters indicate related
species Principle of Parsimony suggests simplest
explanation preferred
Most parsimonious phylogeny has characters
evolving fewest times possible (Mutations and
evolutionary change assumed to be relatively
rare)
William of Ockham, of Occams Razor fame
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e.g. mosses tracheophytes share many characters

• if not closely
• related, those
• characters
• would each have
• to evolve twice,
• many more
• evolutionary
• changes

Instead, we assume they are closely related
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Not all characters are equally useful for
indicating relationships - e.g number of legs
in mammals
Try grouping cats, whales, deer, bats
rodents based on of legs
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Cant group mammals that have 4 legs
together because ancestor had 4 legs all
mammal groups inherited this trait
Shared ancestral traits are not useful
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Instead, try to use shared derived traits
traits that have been modified from
ancestral trait since the group started
evolving (AKA synapomorphies)
Modification of front legs into flippers,
hind legs reduced to vestigial bones can
be used as shared derived trait for whales
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To determine ancestral trait, examine
outgroups taxa that are not part of the
focal group but are closely related
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Look at an example of constructing
a phylogeny
Phylogenies built by finding which taxa
share the largest number of derived
traits
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An example bird-hipped dinosaurs
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What sort of characters can be used in
phylogenetic analysis?

• almost anything that can be measured and has a
• genetic basis

• some characters more useful than others because
• more or less stable, or easier to compare
• may depend on breadth of study (for phylogeny of
  
• a phylum use slow-evolving characters,
• for phylogeny of a genus use fast-evolving
• characters)

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• Morphological characters
• easy to measure, many traits available
• usually only characters available for fossils,
  if used
• can combine modern fossil taxa in analysis

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Developmental characters Essentially
morphological, but traits seen in embryo may
disappear in adult
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Behavioral characters Variable, may be hard to
compare but can add much potential
information e.g. duck courtship displays
King Eiders
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Different wasp families differ in their nest
structures
Crabronidae
Vespidae
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• Molecular characters
• Probably most current studies use molecular
  characters
• potential characters include
• enzyme frequencies
• use electrophoresis to determine variation

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Amino acid sequence of proteins

• Each position
• in sequence is
• a character
• having same
• amino acid
• is a shared
• trait

Must align sequences first
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DNA hybridization

• compares similarity of whole strands of DNA from
  
• 2 spp. by how tightly they bond together
• heat DNA until 2 strands separate, when cooled
  they
• join together again in double helix

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DNA from 2 different species will form
hybrid DNA, but will separate again at
lower temperatures
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Phylogeny must be based on overall DNA similarity
(measured by dissociation temperature), not
by number of shared characters
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DNA sequence data from mitochondrial or nuclear
genes

• Similar process as
• amino acid
• sequence
• align sequence,
• determine shared
• nucleotides

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Development of techniques for amplification
sequencing of DNA leading to explosion of
new data and better understanding of
evolution
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Whatever technique used, phylogenies give insight
into how organisms evolved, help answer
many theoretical and practical questions