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© 1997
David H.A. Fitch
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Lecture notes


I.  Definitions of "Species"

A.  "Morphological species" ("morphospecies") are differentiated by their appearance (Linnaeus)

B.  A "biological species" (Mayr 1942) is the largest unit of a population in which gene flow is naturally possible

C.  Exceptions suggest that speciation is a process that occurs by the gradual evolution of barriers to gene flow
1.  Ring species
2.  Subspecies such as those of the deer mouse Peromyscus maniculatus do not interbreed, but the gene pools are not isolated

II.  Speciation = the evolution of barriers to gene flow

A.  Reproductive isolation preserves species boundaries
1.  Reproductive isolation = any (biological) mechanism that prevents two species from producing fertile hybrids
2.  Usually restricted to "biological" mechanisms because geographical barriers may not be sufficient to prevent hybridization

B.  Mechanisms that maintain reproductive isolation
1.  Prezygotic = mechanisms that prevent mating or fertilization

  1. Habitat isolation
  2. Temporal isolation
  3. Ethological (behaviorial)
  4. Mechanical isolation
  5. Gametic isolation

2. Postzygotic = mechanisms that prevent the development of viable and fertile adults

  1. Hybrid inviability
  2. Hybrid sterility
  3. "Hybrid breakdown"

3.  Introgression

III.  Mechanisms for the origin of reproductive isolation

A.  Geographical level
1.  Allopatric speciation—gene pools separated by geographical barriers

  1. The same barrier may not isolate all kinds of organisms (like birds vs. rodents)
  2. Conditions that might favor allopatric speciation—
    (1)  Small ("founder") populations that become separated
    (2)  Peripheral isolates (at fringe / frontier of larger population) that become separated from a larger population (but extinction rate is also higher)
  3. Adaptive radiations = rapid and frequent allopatric speciation events as exemplified on Island chains
    (1)  Darwin's finches on the Galapagos Islands
    (2)  Hawaiian Archipelago

2.  Sympatric speciation—gene pools separated within the same region, but by different habitats or other means (parapatric speciation might be sort of included here)—often difficult to prove, since the present reproductive isolation could have originated allopatrically with subsequent sympatry ("secondary contact")

  1. Instantaneous—If a chromosomal or genetic change results in a reproductive barrier between the mutants and parent (could be a single generation)—2 mechanisms known in plants:
    (1)  Autopolyploidy
    (2)  Allopolyploidy
  2. "Gradual"—usually based on disruptive selection
    (1)  For example, assortative mating may result in separation of two inbreeding populations from each other (especially if small populations)
    (2)  Controversial because of the difficulties of actually preventing gene flow

B.  Genetic level
1.  Speciation by divergence—isolation is a secondary consequence of genetic divergence between already separated populations

  1. Populations that are separated for some time accumulate genetic differences by mutation
  2. These changes can affect:
    (1)  Postzygotic Mechanisms
    (2)  Prezygotic Mechanisms

2.  Speciation by peak shifts—gene pools separated as a consequence of the dynamics between gene frequencies of subpopulations and natural selection, such as might be triggered by splinter groups ("founders")

IV.  Pattern of speciation

A.  Gradual

B.  Rarely find gradual transitions in the fossil record
1.  For Darwin, this meant the geological record was imperfect
2.  But as we've seen, speciations could be sudden—Eldredge & Gould's "Punctuated Equilibria"

  1. Rapid divergence
  2. Followed by long periods of stasis

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