THE ORIGIN OF SPECIES

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THE ORIGIN OF SPECIES
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WHAT IS A SPECIES?

• A population of group of populations whose
  members have the potential to interbreed with one
  another in nature to produce viable, fertile
  offspring
• This biological species concept emphasizes
• Ability to productively interbreed within the
  species
• Reproductive isolation from closely related
  species

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REPRODUCTIVE ISOLATION

• Any factor impeding two species from producing
  viable, fertile offspring contributes to
  reproductive isolation
• Species are often separated by multiple barriers

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REPRODUCTIVE ISOLATION

• Prezygotic barriers
• Habitat isolation
• Behavioral isolation
• Temporal isolation
• Mechanical isolation
• Gametic isolation

• Postzygotic barriers
• Reduced hybrid viability
• Reduced hybrid fertility
• Hybrid breakdown

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REPRODUCTIVE ISOLATION

• Habitat isolation
• Two species living in separate habitats within
  the same area may encounter each other rarely
• e.g., Two species of garter snakes (genus
  Thamnophis) occur in the same areas, but one is
  primarily aquatic and the other is primarily
  terrestrial

These may not be the correct species within this
genus
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REPRODUCTIVE ISOLATION

• Behavioral isolation
• Elaborate courtship behavior and other signals
  unique to a species are likely the most important
  reproductive behaviors between closely related
  species
• e.g., Firefly flashing patterns, bird songs,
  elaborate mating dances, etc.

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REPRODUCTIVE ISOLATION

• Temporal isolation
• Two species may breed during different times of
  the day, different seasons, or different years
• e.g., Ranges of western spotted skunk (Spilogale
  gracilis) and eastern spotted skunk (Spilogale
  putorius) overlap, but S. gracilis mates in late
  summer and S. putorius mates in late winter

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REPRODUCTIVE ISOLATION

• Mechanical isolation
• Anatomical incompatibility may prevent the
  consummation of an attempted mating between
  closely related species
• e.g., ? and ? copulatory organs may not fit
  together
• e.g., Floral anatomy may be adapted to require
  different pollinators

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REPRODUCTIVE ISOLATION

• Gametic isolation
• Gametes may be unable to fuse to form a zygote
• e.g., Sperm of one species may not be able to
  survive in the female reproductive tract of a
  closely related species
• e.g., Molecules on the surfaces of gametes that
  are required for fertilization may differ
  between related species

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REPRODUCTIVE ISOLATION

• Reduced Hybrid Viability
• Zygotes may fail to complete embryonic
  development
• e.g., Many species of frogs (genus Rana)

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REPRODUCTIVE ISOLATION

• Reduced Hybrid Fertility
• Some hybrids between closely related species are
  vigorous, but are sterile
• e.g., Horse donkey ? mule
• Genomes mix temporarily, not permanently
• Often related to inability to produce viable
  gametes via meiosis
• Parent species differ in chromosome structure
  and/or number

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TIGONS
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REPRODUCTIVE ISOLATION

• Hybrid Breakdown
• Though first generation hybrids may be viable and
  fertile, further generations are often feeble or
  sterile
• e.g., Different species of cotton

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REPRODUCTIVE ISOLATION
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BIOLOGICAL SPECIES CONCEPT

• The biological species concept has limitations
• Difficult to check for interbreeding
• Impossible to determine for fossils
• Not useful for asexually reproducing species

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SPECIATION

• Two general modes
• Allopatric speciation
• Geographically separated populations
• Sympatric speciation
• Geographically overlapping populations

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ALLOPATRIC SPECIATION

• Geographical processes can separate a population
  into multiple isolated populations
• e.g., Mountain formation, land bridge formation,
  canyon formation, etc.

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ALLOPATRIC SPECIATION

• Separated subpopulations may diverge enough to
  become separate species
• Small populations more likely to change through
  genetic drift and natural selection
• Unable to productively reproduce when they regain
  contact new species

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SYMPATRIC SPECIATION

• New species can arise within the range of parent
  populations
• Geographic isolation unnecessary
• e.g., Autopolyploidy in plants

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SYMPATRIC SPECIATION

• New species can arise within the range of parent
  populations
• Geographic isolation unnecessary
• e.g., Allopolyploidy in plants

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SYMPATRIC SPECIATION

• The origin of new polyploid plant species is
  common enough and rapid enough that many such
  speciations have been documented

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PUNCTUATED EQUILIBRIUM

• Large evolutionary changes occur by the
  accumulation of many small changes
• Macroevolution is an extension of microevolution
• However, paleontologists rarely find gradual
  transitions in the fossil record
• Species often appear rather suddenly
• Species often persist unchanged for long periods
  of time, then disappear suddenly
• How can we explain this?

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PUNCTUATED EQUILIBRIUM

• Although each species must have passed through
  numerous transitional stages, it is probable that
  the periods during which each underwent
  modification, though many and long as measured by
  years, have been short in comparison with the
  periods during which each remained in an
  unchanged condition.
• -- Charles Darwin

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PUNCTUATED EQUILIBRIUM

• Stephen Jay Gould and Niles Eldridge (1972)
• Species remain relatively constant over long
  periods of time
• This constancy is punctuated by relatively brief
  periods of rapid evolutionary change
• Typically in response to the population
  encountering a new set of environmental conditions

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PUNCTUATED EQUILIBRIUM