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Speciation
I. What is a species?
A. Biological species: groups of
populations whose membershave the potential to
interbreed in their natural
environmentand produce viable offspring;maintained by reproductive
isolation
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I. A. 1. This doesnt work for extinctpopulations, or
2. Asexually reproducingorganisms
because reproductive
isolation cannot be
evaluated
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Speciation
I. B. Morphospecies: similarities inmorphology with the
underlying assumptionthat distinctive morphologieswould arise in isolated,independent
populations, i.e.,no gene flow
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Speciation
I. C. Phylogenetic species: a clusterof organisms that represents a
genealogy or a lineage ofdescent from a singleancestor
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Speciation
II. How does it happen? A portion ofan ancestral species becomes
reproductively isolated; there aretwo major mechanisms
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II. A. Allopatric speciation:geographic isolation
leads to genetic drift and/ornatural selection to such anextent that the derivedspecies can no
longer mate successfully withmembers of its ancestral
species; this is the major
mechanism of speciation
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Speciation
II. A. 1. Dispersal
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II. A. 2. Vicariance
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II. B. Sympatric speciation: a newspecies arises within the midst
of the ancestral population1. Common in plants
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II. B. 2. Polyploidy
a. Autoploidy
b. Alloploidy
3. Also observed in variousanimal species
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Speciation
III. Maintenance of species: twotypes of reproductive isolation
(prezygotic barriers andpostzygotic barriers)
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III. A. Prezygotic barriers: preventfertilization
1. Temporal isolation: lifecycle timing differences
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III. A. 2. Habitat isolation: physicalseparation
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III. A. 3. Behavioral (or, sexual)isolation: no attraction
between male and female
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III. A. 4. Mechanical isolation:incompatible
reproductivestructures
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III. A. 5. Gametic isolation: chemicalincompatibility of gametes
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III. B. Postzygotic barriers: reductionof the viability or fertility of a
hybrid
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III. B. 1. Hybrid inviability:embryonic death of a
hybrid
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III. B. 2. Hybrid breakdown:offspring of hybrid
unable to reproduce
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III. B. 3. Hybrid sterility: hybridcannot reproduce as an
adulta. Mule: female horse +
male donkey
b. Hinny: female donkey +male horse
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Speciation
IV. Possible outcomes of post-speciation contact
A. Reinforcement
B. Hybrid Zones
C. Hybridization
1. New species2. Species extinction