Speciation

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Speciation

• Speciation NB for L/T conservation
  maintain ability of lineages to evolve
  speciate ? but problematic to observe because of
  time ? can construct conceptual models
• Crucial event for speciation reproductive
  isolation

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Reproductive IsolationInterruption of Gene Flow

• Isolating mechanisms
• Pre-mating / Pre-zygotic? prevent formation of
  hybrid zygotes
• Post-mating / Post-zygotic? reduce viability or
  fertility of hybrid zygotes

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Driving Force of SpeciationSelective Pressures
or Accidental

• Reproductive isolation can be achieved through
  adaptive change under natural selection, or be
  accidental through e.g. chromosomal changes,
  drift, and founder events

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Theories of Speciation

• Major questions Why under Which
  circumstances the genetic variant causing
  reproductive isolation should evolve
• Hypotheses
• New spp evolve in geographic isolation from
  ancestor allopatric speciation
• New spp evolve in geographically linked
  population parapatric speciation
• New spp evolve within geographic range of
  ancestor sympatric speciation

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Allopatric
Parapatric
Sympatric
Peripheral isolate
Dumb-bell
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Reinforcement

• Reinforcement selection against hybrids of 2
  populations that have lower fitness than
  crosses within each population ? selection then
  increases reproductive isolation
• Secondary reinforcement reproductive isolation
  has evolved allopatrically, then reinforced
  when 2 populations come into secondary contact
• In allopatric theory reinforcement ? necessary
  for speciation Parapatric sympatric theories
  strongly depend on reinforcement

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Reinforcement

• Although reinforcement theoretically possible
  it is controversial1. A time-window exists
  after contact between 2 populations where
  reinforcement must happen ? before
  speciation is complete positively frequency
  dependent selection will remove the rarer
  genotype until population becomes uniform
  again
• 2. Recombination between genes controlling mating
  preferences and the genes under selection
  could hamper reinforcement
• 3. Difficult to construct mathematical models for
  reinforcement
• 4. Lack of test cases

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• Dumb-bell model
• Different natural selective pressures act on 2
  populations because of ecological geographic
  variation ? complete / incomplete speciation
• If incomplete secondary reinforcement
• Genetic variations mutations take time to
  spread because barrier geographical
  populations large
• Model can often give clear estimate of time of
  speciation
• Lineage must 1st split into 2 ? can then split
  again

B
C
D
A
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Dumb-bell Model
Ammospermophilus harisi
Ammospermophilus leucurus
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• Peripheral isolate model
• Reproductive isolation achieved through drift
  founder events and/or natural selective
  pressures
• RI further increased by reinforcement
• Genetic variations mutations spread fast
  because peripheral populations small
• Many lineages can split from ancestral
  population at the same time

B
C
D
A
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Peripheral Isolate Model
Acanthiza pusilla (Brown Thornbill)
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Peripatric Speciation
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Parapatric
Sympatric
? Parapatric and Sympatric speciation must
overcome considerable gene flow!
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Parapatric
Sympatric

• ? Parapatric and Sympatric speciation must
  overcome considerable gene flow!
• Strong selective differences between
• populations

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Parapatric Speciation
California garter snake (Thamnophis elegans)
(Mannier et al. 2007 J. Evol. Biol.
Doi10.1111/j.1420-9101.2007.01401.x)
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Parapatric Speciation Ring Species
Ensatina salamanders
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Parapatric
Sympatric
? Parapatric and Sympatric speciation must
overcome considerable gene flow!

• ? if polymorphism favoured by disruptive
  selection selection also favours
  assortitive mating within each type then
  sympatric speciation may occur

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Sympatric Speciation Host Shift
Apple Hawthorn maggot flies (Rhagoletis
pomonella)
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Sympatric Speciation? The Case of the Yellow
House-Bat
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Polyploidy Speciation

• Cells are polyploid if they contain more than 2
  haploid (n) sets of chromosomes e.g. triploid
  (3n) and tetraploid (4n) cells
• Polyploidy is very common in plants, especially
  angiosperms
• Polyploidy can happen by a mutation or when
  chromosomes of 2 different spp are mixed
• The resultant hybrid can usually only breed with
  other hybrids ? however, many plants have male
  female flowers thus can self fertilize
  or reproduce vegetatively
• Speciation can happen near instantaneously

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Gilia minor
Gilia clokey
Gilia transmontana
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Tympanoctomys barrerae (red vizcacha rat) 2N
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AutoPolyploidy

• Autopolyploids are polyploids with chromosomes
• derived from a single species. Autopolyploids
  can arise from a spontaneous,
  naturally-occurring genome doubling (e.g.
  potato).

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AlloPolyploidy

• Allopolyploids are polyploids with chromosomes
• derived from different species. Titricale is
  an example, having six chromosome sets, four
  from wheat (Triticum turgidum) and two from
  rye (Secale cereale).

Wheat Rye Titricale
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Artificial Speciation

• New spp created by domesticated husbandry e.g.
  domestic sheep from hybridization

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Artificial Speciation

• New spp created in laboratory