Ch.24-25

Speciation is the formation of new and distinct species in the course of evolution.
Charles Darwin was fascinated by unique species in the Galápagos Islands, suggesting they originated from ancestors arriving from South America.

Definition: A species is a group of populations whose members can interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups.
- Examples:
  - Humans belong to the same species despite geographical differences.
  - Chimpanzees and humans are distinct biological species due to reproductive barriers.
Gene Flow: The transfer of alleles between populations maintains species similarity.

Speciation involves the development of reproductive barriers.
Prezygotic Barriers: Operate before fertilization and include:
- Habitat Isolation: Different habitats restrict encounters.
- Temporal Isolation: Different breeding times (e.g., seasonal) prevent mating.
- Behavioral Isolation: Courtship rituals differ, preventing mating.
- Mechanical Isolation: Morphological differences prevent successful mating.
- Gametic Isolation: Sperm cannot fertilize eggs of another species.
Postzygotic Barriers: Operate after fertilization and include:
- Reduced Hybrid Viability: Hybrids do not develop or reach maturity.
- Reduced Hybrid Fertility: Hybrids are sterile (e.g., mules).
- Hybrid Breakdown: Future generations of hybrids may be inviable or infertile.

Allopatric Speciation: Occurs when populations are geographically isolated, leading to reproductive isolation.
- Example: Flightless cormorants evolved on isolated islands.
Sympatric Speciation: Occurs without geographical isolation, often through mechanisms like polyploidy in plants, sexual selection, or habitat differentiation.

Hybrid zones are regions where different species meet and interbreed.
These zones can reflect the extent of reproductive isolation:
- Reinforcement: Strongening of reproductive barriers occurs if hybrids have lower fitness.
- Fusion: Weakening of reproductive barriers can lead to merging of species into one.
- Stability: Hybrid populations persist over time despite reduced fitness.

Speciation rates can vary greatly. Factors influencing these rates include:
- Geological changes (e.g., continental drift) can create new barriers.
- Climate shifts can influence habitat availability and reproductive strategies.

Fossil records indicate patterns of speciation and extinction across geological timelines.
Molecular studies can reveal genetic changes tied to shared ancestry and speciation events.
- Example: The evolutionary history of cichlid fish in the African Great Lakes showcases rapid diversification in isolated habitats.

Natural Selection: Drives adaptive change which can influence speciation rates.
Gene Duplication: Can lead to new functional genes that facilitate novel adaptations.
Heterochrony: Evolutionary changes in timing/duration of developmental processes may result in significant morphological changes.

Adaptive radiations occur when species rapidly diversify to fill various ecological niches after significant extinctions or environmental changes.
Example: The diversification of mammals after the extinction of dinosaurs.

Understanding speciation provides insight into biodiversity and evolution, highlighting the interconnectedness through shared ancestry and the impact of environmental changes on evolutionary trajectories.