The Five Mechanisms of Evolution (H-W Violations)
- Mutation — new alleles introduced; ultimate raw material
- Gene flow/migration — alleles enter/leave; homogenizes populations
- Natural selection — differential reproduction; adaptive mechanism
- Genetic drift — random allele changes; non-adaptive; most powerful in small populations
- Non-random mating — alters genotype frequencies; sexual selection
Natural Selection — Three Modes
| Mode | Favours | Bell Curve Effect | Example |
|---|---|---|---|
| Stabilizing | Intermediate | Narrower and taller | Human birth weight |
| Directional | One extreme | Shifts mean | Industrial melanism |
| Disruptive | Both extremes | Bimodal (two peaks) | African seedcracker birds |
Genetic Drift Subtypes
- Founder effect: Small group establishes new population; non-representative gene pool. Example: Amish Ellis-van Creveld syndrome.
- Bottleneck effect: Population drastically reduced by catastrophe. Example: Northern elephant seal (hunted near to extinction → low genetic diversity).
Speciation Comparison
| Type | Mechanism | Speed | Example |
|---|---|---|---|
| Allopatric | Geographic isolation | Slow (thousands-millions of years) | Galapagos finches on different islands |
| Sympatric | Polyploidy (plants), behavioural | Instantaneous (polyploidy) | Spartina anglica grass |
| Adaptive radiation | Multiple allopatric splittings into different niches | Variable | Darwin's finches; Australian marsupials |
NEET Key Points
- Adaptive radiation = one ancestor → many species → different niches
- Allopatric requires geographic barrier; sympatric does not
- Disruptive selection can drive sympatric speciation
- Gene flow prevents speciation; stopping gene flow enables speciation