The Origin of Species

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The Origin of Species

• Chapter 24

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Origin of Species

• How do we define species?
• A population of organisms that produces viable
  fertile offspring in nature.
• When does this definition fall apart?
• Asexual, extinct and blurred organisms
• What definition is used in these cases?
• Biological species concept

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• What is the main distinction that must occur for
  the origin and integrity of distinct species?
• Reproductive isolation

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Prezygotic barriers

• Habitat isolation
• Temporal isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation

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Postzygotic barriers

• Reduced hybrid viability
• Reduced hybrid fertility
• Hybrid breakdown

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• Which sort of reproductive isolation mechanism is
  at work in the following examples?
• Firefly signals with specific flashes to attract
  mates
• Male dragonfly has appendages that clasps female
  during mating
• Brown trout breed in the fall and rainbow trout
  in the fall
• 1 type of garter snake lives in the water and the
  other lives on land
• Horse and donkey produce sterile mule
• Frogs that mate and produce offspring that do not
  quite develop

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Biogeography of Speciation

• What is the difference between allopatric and
  sympatric speciation?
• Allopatric speciation involves a geographical
  barrier between 2 groups
• Sympatric speciation is the result of a genetic
  isolation without a geographical barrier

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Conditions for Allopatry

• Peripheral isolate where the fringe organisms are
  already somewhat different from mainstream
  population
• Genetic drift can occur to a small peripheral
  isolate
• The genetic drift continues to change the gene
  pool until the group is large
• Natural selection will select the best fit traits
  among the new group to survive

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Sympatric Speciation

• Genetic alterations result in a reproductive
  barrier
• Can occur in a single generation
• More frequently seen in plants
• Nondisjunction and selfing leads to polyploidy
• autopolyploid
• allopolyploid
• Evolution of wheat

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Genetic Mechanisms of Speciation

• Adaptive Divergence
• Reproductive barriers evolve as secondary result
  of divergence
• The barriers evolve to enhance reproduction
  within the group not to eliminate reproduction
  between groups
• Reproductive barriers occur as a side effect of
  the accumulated adaptive divergences

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Hybrid zones

• Secondary contact between 2 related populations
• Reproductive isolation is broken only in this
  hybrid zone
• Are the 2 populations really distinct from one
  another when there exists a hybrid zone?
• Reinforcement- strengthening reproductive
  barriers
• Fusion-weakening reproductive barriers
• Stability-continued formation of hybrid
  individuals

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Tempo of Speciation

• Gradualism
• Punctuated Equilibrium

13
Origin of Novelties

• How do large scale novelties arise?
• Modifications of older structures
• Evolution, however, is not goal oriented

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Allopatric and Sympatric Speciation