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Reproductive Isolating MechanismsA biological species is defined as a group of similar organismsable to interbreed (under natural conditions) to produce fertile, viable offspring. Biological species are reproductively isolated from one another.
Evolution of reproductive isolating mechanisms Isolating mechanisms can operate at two basic levels.
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Ecological IsolationDifferent habitat preferences lower their probability of mating. Example 1: Central California populations of Rana spp.
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Ecological Isolation: Turdus spp.
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Ecological Isolation: |
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Temporal Isolation: Rana spp.may have different periods of sexual activity or breeding seasons. Example 1: Closely related Rana species in California Coastal Ecosystems
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Temporal Isolation: Fruit Flies
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Behavioral Isolationusually perform species-specific "call and response" signals between male and female before actual mating takes place. These rituals prevent wasted mating effort that would halt gene transmission due to sterile or inviable hybrids. |
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Mechanical Isolation: Shell Coiling
Example 1: Shell Coiling in Euhadra |
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Mechanical Isolation: PollinationExample 2: The Bucket Orchid and the Orchid Bee
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Gametic Isolationplasma membrane receptors, they cannot form a zygote.
Example: Sympatric Sea Urchin Species
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Hybrid Inviability: Drosophilais insufficient to carry the organism through normal development. The embryo dies after a few cleavages, or some time before birth/hatching.
Example 1: Drosophila spp.
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Hybrid Inviability: |
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Hybrid Inviability: Rana spp.Example 3: Rana draytonii and Rana catesbeiana
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Hybrid Sterilityto produce viable hybrids, but the hybrids are sterile.
Example 1: Tigers (Panthera tigris) and Lions (Panthera leo)
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Hybrid SterilityExample 2: Horse (Equus caballus) and Donkey (Equus asinus)
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Hybrid BreakdownBut successive generations (F2 and beyond) suffer lower viability or fecundity. Thus, they cannot become an established population.
Example: Rice cultivars
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Revisiting the Question:
Reproductive isolating mechanisms |
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2. Species FusionSpecies fusion
This has been seen in some species of Darwin's Finches. |
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3. Hybrid EquilibriumSpecies stability/Hybrid equilibrium
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4. Hybrid SpeciatiounHybrid speciation |
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Hybrid Speciation: Tragopogon
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Tragopogon HybridsT. dubius, T. pratensis, and T. porrifolius had established sympatry, two new species of Tragopogon were found.
Neither hybrid can interbreed with its parent species. |
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Tephritid Fruit Fly Hybridsthat lay eggs in many crops, from apples to blueberries.
The two species of Rhagoletis occasionally hybridize..
Because flies almost always breed on the plant species they ate as larvae, relatively rapid reproductive isolation can occur. |
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