Organismal Biology #09
SPECIATION
Performance Objectives:
Species are evolutionary units within which gene flow occurs. Natural populations belong to the same species only if they can interbreed and leave fertile offspring. Different species are reproductively isolated from each other. Most new species originate geographically.

Importance of species: Species characteristics are passed from parents to offspring. Species were long considered real and important because these characteristics seemed to be constant and unchanging.

Species definitions: morphological, biological

Morphological (typological) species definition: Each species is defined by characteristics thought to be "essential" or "typical".

Biological species definition: Species are groups of interbreeding populations that are reproductively isolated from other species.
  • biological species definition(2)
  • reproductive isolation
  • reproductive isolating mechanisms (premating, postmating)
  • example: mallards and pintails
  • sibling species
Reproductive isolating mechanisms that act prior to mating:
  • Ecological isolation: Potential mates do not meet because they live in different habitats or breed at different times or seasons.
  • Behavioral isolation: Mating calls or mating rituals differ.
  • Mechanical isolation: "Lock and key" mismatch of genitalia.
Reproductive isolating mechanisms that act after mating:
  • Gametic mortality: Gametes die before fertilization.
  • Zygotic mortality: Fertilized eggs fail to divide properly.
  • Embryonic or larval mortality: They die prematurely.
  • Hybrid inviability: Hybrids never reach reproductive age.
  • Hybrid sterility: Hybrids cannot reproduce, as in mules.
  • F2 breakdown: Offspring of hybrids are inviable.

What is speciation?
  • Phyletic transformation (anagenetic; no new RIM)
  • True speciation or splitting (cladogenetic; new RIM must evolve)
Geographic speciation: Most speciation occurs geographically.
    Stages of geographic speciation:
  • A species develops geographic variation over its range.
  • Geographic barriers prevent contact between populations.
  • Reproductive isolation may now evolve.
  • Geographic isolation ends, with two possible outcomes:
    1. No reproductive isolation-- still a single species.
    2. Reproductive isolation is effective-- two species now exist; selection will enhance differences between them.
      Speciation *
      Speciation.ppt
Other models of speciation (sometimes controversial):
  • By doubling or tripling of chromosome number in plants (polyploidy).
  • "Semigeographic" or "parapatric" speciation if adjacent ranges touch.
  • "Sympatric" or "stasipatric": no geographic separation.
Geographic speciation: Most speciation occurs geographically.
    Stages of geographic speciation:
  • A species develops geographic variation over its range.
  • Geographic barriers prevent contact between populations.
  • Reproductive isolation may now evolve.
  • Geographic isolation ends, with two possible outcomes:
    1. No reproductive isolation-- still a single species.
    2. Reproductive isolation is effective-- two species now exist;
      selection will enhance differences between them.
  • Incomplete speciation:
    • clines of reproductive isolation (e.g., Rana pipiens)
    • clines with circular overlap (e.g., Parus major,   Ensatina)
    • species in statu nascendi (e.g., Drosophila paulistorum)
  • "Semigeographic" or "parapatric" speciation if adjacent ranges touch.
  • Character displacement
Alternative possibilities for speciation
  • SUDDEN speciation:
    • Genic (DeVries, Goldschmidt) (no confirmed examples; does not produce new species)
    • Chromosomal, by aneuploidy, etc. (no confirmed examples; does not produce new species)
    • Polyploidy:   doubling or tripling of chromosome number in plants.
  • GRADUAL speciation:
    • Sympatric speciation (sometimes claimed, but controversial)
    • Nonsympatric speciation (spectrum of possibilities):
      • Parapatric
      • Alloparapatric
      • Allopatric (geographical) speciation (includes nearly all confirmed cases):
        • old (pre-existing) barriers are crossed by a species
        • new barriers develop within a geographic range
        • intervening populations within a cline become extinct
      • ?Symparapatric ("stasipatric")
Hybridism and polyploidy
  • hybridism and its effects
  • speciation by polyploidy
  • REVIEW:         Study guide and vocabulary

  • Index             Syllabus
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