Lecture Notes: Speciation and Hybridization

Key Concepts

• Speciation: Occurs when two populations become reproductively isolated and undergo differential selection, resulting in the splitting of one species into two or more.
• Reproductive Isolation: Essential for speciation, it can occur through behavioral, temporal, or ecological differences.

Types of Speciation

Allopatric Speciation

• Definition: Speciation occurring in different geographical locations.
• Mechanism: Populations in different places diverge due to lack of gene flow.

Sympatric Speciation

• Definition: Speciation occurring in the same geographical area.
• Mechanisms:
  • Behavioral Isolation: Differences in mating behavior prevent interbreeding.
  • Temporal Isolation: Differences in timing of reproductive events (e.g., flowering or mating seasons).

Examples of Isolation

• Orchids: Temporal isolation due to different flowering days.
• Frogs: Breeding at different times of the year causing reproductive isolation.

Adaptive Radiation

• Definition: Rapid diversification of a single ancestor into multiple new species, often filling different ecological niches.
• Examples:
  • Galapagos Finches: Different beak shapes to utilize different food sources.
  • Mammals: Diversification after dinosaur extinction.

Hybrids and Hybridization

• Hybrid: Offspring of two different species, can occur naturally or through human intervention.
• Barriers to Hybridization:
  • Prezygotic Barriers: Prevent mating or fertilization (e.g., temporal, habitat, behavioral isolation).
  • Postzygotic Barriers: Affect hybrid viability or fertility after fertilization (e.g., mules).

Polyploidy in Plants

• Definition: Condition of having more than two sets of chromosomes.
• Types:
  • Autopolyploidy: Chromosome duplication within the same species (self-fertilization).
  • Allopolyploidy: Hybridization between two species resulting in chromosome sets from both.

Examples of Polyploidy

• Knotweeds: Different species with varying chromosome numbers leading to new species.
• Strawberries: Cultivated varieties are octoploid with larger fruits and better adaptability.

Advantages and Disadvantages of Polyploidy

• Advantages:
  • Enhanced biological traits and competitive advantages in certain environments.
  • Ability to exploit diverse ecological niches.
• Disadvantages:
  • Problems with gene expression and fertility.
  • Larger cell size affecting function and metabolism.

Conclusion

Understanding speciation, hybridization, and polyploidy helps explain biodiversity and organism adaptation to environmental changes.