Origin of Species patterns of speciation

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					      Origin of Species

Galapagos Tortoise
         Origin of Species
                patterns of speciation

• Anagenesis
  – A single population is transformed enough to be
    designated a new species

• Cladogenesis
  – Branching evolution. A new species arises from a
    small population that “Buds” from a parent species.
  – Most species probably evolved this way. The
    branching evolution is the basis of biological diversity
Two patterns of speciation
         Origin of Species
                 Species definitions

• Biological species = a population whose
  members have the potential to interbreed with
  one another and produce viable, fertile
  offspring, but who cannot produce viable, fertile
  offspring with members of another species.
  (Reproductive Isolation)
  – The biological species concept cannot be applied to
    asexually reproducing organisms
• Morphospecies = species defined by their
  anatomical features
The biological species concept is based on interfertility rather than physical similarity

         Eastern Meadowlark                             Western Meadowlark
                 Reproductive barriers
• Any factor that impedes two species from producing
    fertile hybrids
•   Prevents interbreeding between closely related species
•   Prezygotic barriers (impedes mating or hinders ova
•   Postzygotic barriers (after fertilization)
      Origin of Species
        Prezygotic barriers

1. Habitat isolation
2. Temporal isolation
3. Behavioral isolation
4. Mechanical isolation
5. Gametic isolation
Habitat Isolation
Temporal Isolation

Late-winter mating season   Summer mating season
Blue-footed boobies …    Courtship ritual as a

behavioral barrier   between species.
Mechanical Isolation
  “You can’t fit a square peg in a
  round hole”

Gametic Isolation
  “gamete recognition”
      Origin of Species
        Postzygotic barriers

1. Reduced hybrid viability (vigor)

2. Reduced hybrid fertility

3. Hybrid breakdown
       Origin of Species
The transplantation of alleles between
Alleles occasionally “seep” through all
    reproductive barriers and pass
    between gene pools of closely related
    species. May occur when a fertile
    hybrid mates successfully with one of
    the parent species.
              Hercules the


• Female Ligers are fertile and can
 reproduce. If a Female liger mated with a
 male tiger the offspring would have some
 lion genes. If that offspring could then
 mate with a tiger the lion genes would
 continue on within the Tiger Gene Pool, an
 example of introgression.
A summary of reproductive barriers between closely related
        Origin of Species
          Modes of speciation
• Allopatric speciation
  – A population forms a new species while
    geographically isolated from its parent
• Sympatric speciation
  – Species formation occurs in geographically
    overlapping populations
Two modes of speciation
      Origin of Species
         Allopatric speciation

– occurs when the initial block to gene flow is a
  geographical barrier that physically isolates the

– The genetically isolated group can then follow
  its own evolutionary course as changes in allele
  frequencies occur and are undiluted by gene
  flow from other populations
Has speciation occurred during geographic isolation?
Allopatric speciation of squirrels in the Grand Canyon

      Harris’s antelope squirrel         White-tailed antelope
      (south rim)                        squirrel (north rim)
Speciation in Galapagos Finches
Speciation in Cichlids
      Origin of Species
        Allopatric speciation
Adaptive Radiation = emergence of
   numerous species from a common
   ancestor introduced into an
Example: Darwin’s finches on the
   Galapagos island chain
Adaptive Radiation of Galapagos
A model for adaptive radiation on island chains
Adaptive Radiation in Cichlids
(Specialization in feeding)
       Origin of Species
         Sympatric speciation
Formation of a new species within the
   geographical range of the parent
   population (no geographical isolation)
Can occur quickly (in one generation) if a
   genetic change results in a
   reproductive barrier between mutants
   and the parent population
           Origin of Species
                 Sympatric speciation

Polyploid speciation
•    Many plant species have originated from improper cell
     division that resulted in polypoidy
    – Autopolyploidy – an organism that has more than two
        chromosome sets, all derived from the same species
    – Allopolyploidy – a polyploid hybrid resulting from
        contributions by two different species. More common
        than autopolyploidy. Hybrids are usually sterile but may
        be very vigorous and propagate asexually.

Sympatric speciation by autopolyploidy in plants
One mechanism for allopolyploid speciation in plants
Punctuated Equilibrium vs.
• Punctuated equilibrium model suggests
 evolution occurring in spurts of relatively
 rapid change instead of gradual
Two models for the tempo of speciation

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