STUDYFETCH AUDIO RECAP — CHAPTERS 22 + 23 ONLY
CHAPTER 22 — EVOLUTION (Quick Audio Summary)
Evolution means a change in allele frequencies in a population over time.
There are four mechanisms of evolution:
1. Natural selection. This is not random. Individuals with traits that help them survive and
reproduce leave more offspring. Natural selection requires variation.
2. Genetic drift. This is random change, especially in small populations. Two types: the
founder effect, when a small group leaves to start a new population, and the bottleneck
effect, when a disaster suddenly reduces population size.
3. Gene flow. Movement of individuals or gametes between populations. Gene flow makes
populations more genetically similar.
4. Mutation. The only source of brand‑new alleles. Mutation creates the raw material for
evolution.
Types of natural selection:
Directional selection favors one extreme.
Stabilizing selection favors the average.
Disruptive selection favors both extremes and can lead to speciation.
CHAPTER 23 — SPECIATION (Quick Audio Summary)
A species is defined by the ability to interbreed and produce fertile offspring.
Reproductive isolation prevents gene flow.
Prezygotic barriers occur before fertilization:
• Habitat isolation
• Temporal isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation
Postzygotic barriers occur after fertilization:
• Hybrid inviability
• Hybrid sterility
• Hybrid breakdown
Two ways new species form:
Allopatric speciation: populations become geographically separated.
Sympatric speciation: occurs without geographic separation. Often caused by polyploidy in
plants, or niche differences or sexual selection in animals.
Polyploidy causes instant speciation.
Autopolyploidy: chromosome doubling within one species.
Allopolyploidy: hybridization between two species followed by chromosome doubling.
Hybrid zones are places where two species meet and produce hybrids.
Outcomes:
Reinforcement: hybrids are weak, species stay separate.
Fusion: hybrids are strong, species merge.
Stability: hybrids continue to be produced at a steady rate.