Speciation and Extinction

Species Definition: A species is defined as a group of organisms capable of interbreeding and producing viable, fertile offspring.
Speciation: The formation of new species, which drives the diversity of life forms.

Speciation can be influenced by geographical factors, leading to two primary modes:
- Allopatric Speciation: Occurs when a physical barrier (e.g., mountains, rivers) divides a population.
  - This separation prevents gene flow and may be caused by natural events (e.g., earthquakes, habitat fragmentation).
- Sympatric Speciation: Occurs when a new species evolves while inhabiting the same geographic region as its ancestral species, often through exploitation of new niches.

Reproductive Isolation: Fundamental to speciation, preventing gene flow between populations.
- Prezygotic Barriers: Prevent mating or fertilization from occurring.
- Types of Prezygotic Barriers:
  1. Habitat Isolation: Species occupy different habitats; e.g., western (high elevation) vs. eastern bluebirds (low elevation).
  2. Temporal Isolation: Species breed at different times; e.g., western spotted skunk (late summer) vs. eastern spotted skunk (late winter).
  3. Behavioral Isolation: Unique courtship behaviors; e.g., blue-footed boobies have specific mating rituals.
  4. Mechanical Isolation: Incompatibility in reproductive structures; e.g., snails with different shell spirals.
  5. Gametic Isolation: Inability of sperm and egg to fuse; e.g., red and purple sea urchins cannot fertilize each other despite being in the same environment.
Postzygotic Barriers: Impact the viability or fertility of hybrid offspring.
- Types of Postzygotic Barriers:
  1. Reduced Hybrid Viability: Hybrid zygote doesn't develop properly; e.g., domestic sheep and goats produce non-viable hybrid embryos.
  2. Reduced Hybrid Fertility: Fertile hybrids that are sterile; e.g., mules (cross between horses and donkeys).
  3. Hybrid Breakdown: First-generation hybrids are fertile, but their offspring are sterile; e.g., certain cotton plant hybrids.

Microevolution: Involves changes in allele frequencies within a population, influenced by factors like natural and sexual selection, genetic drift, and gene flow.
Macroevolution: Encompasses large evolutionary changes over time, including phenomena such as adaptive radiation and mass extinction events.
Stasis: Periods of no significant evolutionary change.

Speciation can occur at varying rates:
- Punctuated Equilibrium: Rapid evolution following long periods of stability.
- Gradualism: Slow evolutionary changes over extensive periods.

Divergent Evolution: Ancestrally related groups evolve different traits, potentially forming new species; e.g., adaptive radiation in new niches.
Convergent Evolution: Unrelated species develop similar traits due to similar environmental pressures; results in analogous traits.

Definition: The termination of a species. Extinctions have historically occurred through five mass extinction events.
Human Impact: Human activities have accelerated extinction rates, caused by ecological stress, which opens up available niches for other species to exploit after a species goes extinct.