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Chapter 25: Phylogeny and Systematics


Chapter 25: Phylogeny and Systematics Macroevolutionary Relatedness Microevolution is Macroevolution is Connections... Fossils Classifying Organisms Taxonomic ... – PowerPoint PPT presentation

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Title: Chapter 25: Phylogeny and Systematics

Chapter 25 Phylogeny and Systematics
Macroevolutionary Relatedness
Shown is a phylogenetic tree
Taxonomy is the ordered division of organisms
into categories based on similarities and
differences. p. 495, Campbell Reece (2005)
Systematics is an analytic approach to
understanding the diversity of relationships of
organisms p. 491, Campbell Reece (2005)
Phylogenies are evolutionary histories
Microevolution is
  • Any time you consider
  • natural selection
  • genetic drift
  • mutation
  • gene flow between populations
  • the randomness of mating within populations
  • you are considering microevolutionary processes
  • Indeed, microevolution is all those evolutionary
    things up to and just about including the act of
    speciation itself

Macroevolution is
  • Any time you consider
  • the likelihood of births of new species
    (speciation events)
  • the likelihood of the death of species
  • the adaptive radiation of lineages (birth of many
  • mass extinction (death of many species)
  • evolutionary relationships between species
  • the evolutionary history of a lineage
  • biogeography, or
  • shared derived characters
  • you are considering macroevolutionary processes

  • Adaptation of a species to its natural
    environment (microevolution) will not necessarily
    have a positive impact on the ability of that
    species to give rise to descendant species
  • That is, microevolution and macroevolution are
    not identical processes
  • However, microevolutionary processes do impact on
    macroevolutionary processes
  • For example, macroevolution is affected by the
    adaptations displayed by organisms (e.g.,
    dispersal or dormancy ability) and adaptations
    are a product of selection (a microevolutionary

Classifying Organisms
  • Fossils allow us to compare extinct organisms
    with modern organisms
  • People study fossils to understand past
    environments plus to discover clues to
    evolutionary relationships among organisms
  • Taxa (sing. taxon) are units of classification of
  • So far we have considered the taxonomic
    categories species domain
  • Ideally, all taxonomic categories group organisms
    according to evolutionary (blood) relationships
  • Also ideally, more closely related species should
    be grouped into more-similar taxonomic categories

Taxonomic Categories
Do (Domain) Keep (Kingdom) Privates
(Phylum) Clean (Class) Or (Order) Forget
(Family) Getting (Genus) Sex (Species)
Other Mnemonics
  • Darn Kids Picking Cacti On Fridays Get Stuck
  • Dumb Kids Playing Catch On Freeways Get Squished
  • Did King Phillip Come Over For Great Sex?
  • Do Kiss Pigs Carefully Or Face Grimy Smiles
  • Do Kings Play Chess On Fine Grained Sand
  • Did King Phillip Came Over From Germany Stoned?
  • Did King Peter Came Over From Geneva Switzerland?
  • Do Kindly Produce Credit Or Furnish Good Security
  • Do Keep Peeling Cold Onions For Good Smells
  • Do Keep Putting Coal On the Fire Grate Slowly
  • Did King Phillip Cry, "Oh, For Goodness Sake!"?
  • Do Keep Putting Cheese On Five Green Spoons
  • Did Karen's Pups Chew On Furry Grey Squirrels?
  • Did King Phillip Come Over For Good Spaghetti?
  • Did King Phillip Court Ophelia For Good Sex?
  • Did Karl Push Cliff Over Football Grand Stand?
  • Did King Peter Come Over From Germany Saturday?

Binomial Nomenclature
  • Examples Escherichia coli, E. coli, Escherichia
    spp., and the genus Escherichia
  • The genus name (Escherichia) is always
  • The specific epithet (coli) is never capitalized
  • The specific epithet is never used without the
    genus name (e.g., coli standing alone, by itself,
    is a mistake!)
  • The genus name may be used without the specific
    epithet (e.g., Escherichia may stand alone,
    though when doing so it no longer actually
    describes a species)
  • When both genus and specific epithet are present,
    the genus always comes first (e.g., Escherichia
    coli, not coli Escherichia)
  • Both genus and specific epithet are always
    underlined or italicized
  • Genera may be abbreviated, e.g., the E. in E.
    coli, but should be spelled out in full the first
    time they are used in a document

Phylogenies Cladograms
  • A typical goal of systematics (and paleontology)
    is the construction of phylogenies
  • A phylogeny is a description of the blood (i.e.,
    evolutionary) relationships between organisms
  • A phylogeny can be a description of the
    macroevolutionary history of a group of species
  • A cladogram is a graphical representation of a
  • Cladograms show patterns of shared
  • Cladograms come in a variety of types
  • Typical among all is an attempt to properly sort
  • Nodes are speciation events
  • Descendant species should be properly connected
    to their ancestral species
  • Species should be grouped more closely to related
    species than they are to less-well related
  • The goal of a cladogram or phylogeny is to
    properly represent correct evolutionary

Classifying Clades
Each deeper branch point represents greater
Classifying Proper Clades
A clade consists of an ancestral species plus all
descendant species
This is missing descendant species
The goal of systematics is to define monophyletic
taxa (a.k.a., clades)
Here is a missed ancestor species
All descendants (of the shown set, i.e., of D, E,
G, H, J, and K)
A correct grouping
A clade
  • Homologies are similarities between organisms
    that are present due to common descent
  • Common descent means that organisms share
    ancestry / are evolutionarily related
  • Clades are defined by shared derived characters
    (a.k.a., Synapomorphies)
  • Synapomorphies are those homologies that are
    unique to individual taxa
  • That is, the common ancestor and all of the
    descendant species share (in one form or another)
    a given homology
  • And, organisms not found in a given clade do not
    share the shared derived character
  • Contrast, shared primitive characters

Convergent evolution?
This is a mistake
Polyphylies happen when species are included in
clades that they dont belong in
Convergent Evolution
  • Polyphyletic taxa occur as a consequence of
    mistaking analogies for homologies
  • Analogies are two structures that superficially
    resemble each other, i.e., which appear (at least
    at first glance) to be homologous but are not
  • Analogies result from convergent evolution the
    two species do similar things in similar
    environments so consequently evolve similar
    structures to perform these similar functions
  • The key difference between an analogy and a
    homology are two-fold
  • The common ancestor between the two species will
    have lacked the common structure
  • The development of the structure will differmore
    generally, homologies predict other homologies
    between two species whereas analogies provide
    much less predictive power

Analogies Homoplasies
These moles only look similar
This is an analogy
This is a polyphyletic grouping
This is the true clade
This is a mistake
But one made for legitimate reasons
Mono, Para, or Polyphyletic?
Birds are modified reptiles
Turtles may be incorrectly included here (if
mammals are excluded)
Retention of this mistake is often done
deliberately, i.e., not grouping birds with
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  • Cladistics is a technique by which organisms are
    assigned to different (monophyletic) taxa
  • Cladistics works by identifying homologies and
    grouping together organisms such that within taxa
    individuals share more homologies than they do
    with individuals found in different taxa
  • Cladistics also rejects the inclusion of
  • that result from convergent evolution (i.e.,
  • that are homologies that are shared with other
    taxa (i.e., shared primitive characters)
  • Note, that this is not to say that it is
    necessarily easy to distinguish analogies from
  • Cladistic techniques are not able to judge
    evolutionary divergence in terms of the time
    between nodes (speciation events)
  • Time information is derived from the fossil record

Shared Derived Characters
Cladogram based on S.D.C.
An outgroup is a species that is related to the
species being studied, but less closely they are
like a negative control
More Familiar Synapomorphies
Shared PrimitiveCharacters
  • Not all shared characters are shared derived
  • For example, a cladist would not use the fact
    that both dogs and bears have hair as a means of
    classifying both dogs and bears as carnivores,
    since the ancestor of the ancestral carnivore, a
    mammal, also had hair
  • However, a cladist would use the fact that dogs
    and bears both have hair to include both among
    the mammals
  • That is, hair cannot be employed to distinguish
    mammals because hair is a shared by all mammals
  • However, though hair is a shared primitive
    character when comparing among mammals, it is a
    shared derived character when grouping mammals as
    distinct from other lineages

Primitive Shared Derived Characters
These are analogies
For mammals these are primitive
For birds this is shared derived
Molecular Systematics
  • Molecular systematics has come to dominate the
    study of evolutionary relations (rather than
    supplanting other approaches, e.g., the fossil
    record, it instead serves as an increasingly
    important tool)
  • Molecular systematics seeks out homologies in
    molecules, such as DNA sequences
  • "One advantage of this molecular tool of
    systematics is that it is objective and
  • A second advantage is that it can be used to
    assess relationships between groups of organisms
    that are so physiologically distant that they
    share very few morphological similarities
  • Third molecular comparisons go right to the
    heart of evolutionary relationships.

Molecular Homologies
Note differences in branch lengths (reflecting
differences in rates of sequence evolution)
In phylograms the length of the branch reflects
the degree of divergence (here in terms of
changes in DNA sequence of a gene)
Ultrametric Tree
Note similarities in branch lengths
An ultrametric tree is a phylogram that provides
time-since-divergence information
Maximum Parsimony
Homologous Genes
The previous phylogenies were based on
orthologous genes
Universal Tree
Fusion bacteria with archaea eukaryote
Last common Ancestor
The End