Last day discussing speciation covered prezygotic

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Last day discussing speciation - covered
prezygotic isolating mechanisms
Also may have postzygotic isolating mechanisms,
e.g. 1) Reduced hybrid viability (Rana frogs)
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2) Reduced hybrid fertility (mules)
Horse, Donkey Mule
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3) Hybrid breakdown (rice cotton)
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How does speciation work? Most popular model of
speciation allopatric speciation - allopatry
inhabiting different ranges
- suggests original pop. becomes divided by new
geographic barrier, or colonization of new
isolated area
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- isolated pops. gradually diverge genetic
diffs. develop due to selection in diff.
environments, drift, or founder effect

• when if pops. make contact isolating
  mechanisms
• may be reinforced (hybrids selected against)

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An alternative model sympatric speciation
- sympatric living in same area two new
species form while their geographic ranges
still overlap
- controversial some systematists doubt it
occurs in animals, most think probably rare (
hard to prove)
allopatric
sympatric
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• might originate through host specialization
• (e.g. Apple Maggot Fly) or strong assortative
  mating

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Sympatric speciation does occur commonly in
plants via polyploidy
Autopolyploidy chromosomes fail to separate
(nondisjunction), diploid gametes, may
self-fertilize form tetraploid offspring
- new species, cannot interbreed with diploids
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Allopolyploidy more frequent - hybridization
leads to unusual chromosome , either
non-disjunction or second hybridization event
increases chromosome

• new polyploid species, can
• breed with other polyploids

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Vegetative reproduction self-fertilization
keep hybrids alive until plant can
reproduce sexually

• 25-50 of plant species
• may be formed this way
• (e.g. wheat other crops)

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Systematics Repeated speciation leads to
tremendous diversity of life needs to be
organized Systematics study of relationships
between organisms their classification naming
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Modern system begins with Carl Linnaeus 1758 -
first to consistently use binomials (e. g. Homo
sapiens)
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e.g. Bald Eagle Haliaeetus leucocephalus
First part genus name, second part specific
epithet
Bald Eagle, Haliaeetus leucocephalus
Stellers Sea Eagle, Haliaeetus pelagicus
Specific epithet is label that (almost) always
stays with sp. once named (Law of Priority)
Genus name indicates closest relatives (plural
genera)
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Species genus only lowest levels in
classification Kingdom Animalia Phylum Chordata
Class Aves Order Falconiformes Family Accipitr
idae Genus Haliaeetus Species leucocephalus
Any unit in the classification is a taxon (pl.
taxa)
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Linneaus trying to decipher Gods plan Darwin
recognized that classification reflected lifes
family tree
Systematists try to make classification reflect
the phylogeny of the group Phylogeny the
pattern of evolutionary descent of a taxon
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What sort of groups should be recognized in a
classification?
Three types of groups can be distinguished Monop
hyletic - 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?
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Phylogeny often has branches at top
What does x axis represent?
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Branches can be rotated without changing
meaning, only branching pattern is important
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Phylogenies are 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
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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)
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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