Lecture 3: Hybrid Zones and Speciation Speed
Biological Species Concept Recap
- Definition: Species can only cross with each other and produce viable hybrids.
- Modes of Speciation:
- Allopatric Speciation: Species are geographically isolated.
- Sympatric Speciation: Species evolve in the same geographic area.
Hybrid Zones
- Definition: Regions where two species come into contact and potentially form hybrids.
- Purpose: To study interactions between species and assess if they will merge into one species or stay separate.
- Example: Yellow-bellied frogs in Europe and red-bellied frogs in Asia.
- They interact and form hybrids in mountainous regions separating Europe and Asia.
- Observations:
- Hybrids often have reduced fitness.
Outcomes in Hybrid Zones
- Reinforcement:
- Reduced hybrid fitness strengthens reproductive barriers.
- Two species continue to diverge.
- Fusion:
- Weakened barriers; species merge into one.
- Stability:
- Continuous formation of hybrids; species remain distinct.
- Example: Donkeys, horses, and mules.
Graphical Representation
- Yellow Circles: Represent populations.
- Blue Circles: Represent individuals within populations.
- Gene Flow: Black arrows indicate crossing within populations.
- Isolation: Environmental barriers like rivers or fragmented forests lead to microevolution and potential speciation.
Speciation Speed
- Common Question: Why don't we see new species frequently?
- Speciation takes longer than a human lifespan.
- Fossil record shows rapid appearance of new species which then remain unchanged for long periods.
Models of Speciation Speed
- Punctuated Equilibrium:
- Proposed by Niles Eldridge and Stephen J. Gould.
- Rapid speciation with long periods of stasis.
- Gradualism:
- Slow, continuous change over time.
- Less supported by fossil record which favors punctuated equilibrium.
Macroevolutionary Changes
- Definition: Large changes over time through accumulation of many microevolutionary steps.
- Process: Evolution of new forms occurs in steps; combination of many small speciation events.
End of Chapter 24. Thank you for following along.