Overview
This lecture explains how closely related species can interact after speciation in hybrid zones, and outlines the two main models of speciation rates: gradualism and punctuated equilibrium.
Hybrid Zones and Reconnection
- Hybrid zones are areas where two closely related species interact and can reproduce, forming hybrid offspring.
- The outcome in hybrid zones depends on hybrid fitness and reproductive barriers.
- If hybrids are less fit, reinforcement occurs, increasing divergence between species.
- If reproductive barriers weaken, fusion happens and the two species merge into one.
- If hybrids are equally fit and reproductive barriers persist, stability results and limited hybridization continues.
Speciation Rate Models
- Gradual speciation model: species diverge slowly and steadily over time through incremental trait changes.
- Punctuated equilibrium model: rapid speciation occurs followed by long periods of little or no change.
- Both models explain species divergence, with environmental changes often triggering faster speciation events.
Factors Affecting Speciation Rates
- Environmental conditions are the main factor influencing how quickly speciation occurs.
- Sudden environmental changes can isolate small populations, leading to rapid evolutionary changes due to smaller gene pools.
Key Terms & Definitions
- Hybrid zone — Region where two closely related species mate and produce hybrids.
- Reinforcement — Increased divergence between species due to low hybrid fitness.
- Fusion — Weakening of barriers so species merge back into one.
- Stability — Ongoing hybridization without significant change in species boundaries.
- Gradual speciation — Slow, stepwise divergence of species.
- Punctuated equilibrium — Rapid divergence followed by stasis (little change).
Action Items / Next Steps
- Review the snail speciation example for further understanding of rapid vs gradual speciation.
- Study the diagrams illustrating hybrid zone outcomes and speciation models in your textbook.