Speciation

Speciation acts as the bridge between microevolution (allele frequency changes) and macroevolution (evolutionary patterns above the species level).

The Biological Species Concept

  • Species: A group of populations whose members interbreed in nature to produce viable, fertile offspring, and are reproductively isolated from other groups.

  • Limitations: Cannot be applied to fossils, asexual organisms (prokaryotes), or distinct species that still exchange genes (e.g., grizzly and polar bears).

Reproductive Isolation

Biological barriers that prevent different species from producing viable, fertile offspring:

1. Prezygotic Barriers (Prevent Fertilization)

  • Habitat Isolation: Different locations (e.g., apple vs. blueberry maggot flies).

  • Temporal Isolation: Different breeding times (e.g., summer vs. winter skunks).

  • Behavioral Isolation: Unique rituals (e.g., blue-footed booby dances).

  • Mechanical Isolation: Physical incompatibility (e.g., shell spiral direction).

  • Gametic Isolation: Sperm/egg protein incompatibility.

2. Postzygotic Barriers (Prevent Hybrid Success)

  • Reduced Hybrid Viability: Offspring are frail or fail to develop.

  • Reduced Hybrid Fertility: Offspring are sterile (e.g., mules).

  • Hybrid Breakdown: Second-generation hybrids are weak or sterile.

Alternative Species Concepts

  • Morphological Species Concept: Based on structural/physical features.

  • Ecological Species Concept: Based on ecological niche/environmental interactions.

Types of Speciation

  • Allopatric Speciation: Occurs through geographic isolation (e.g., separation by the Isthmus of Panama).

  • Sympatric Speciation: Occurs in the same geographic area via:

    • Polyploidy: Extra chromosome sets (2n \to 4n). Includes autopolyploids (from one species) and allopolyploids (from two species).

    • Sexual Selection: Mate choice based on traits.

    • Habitat Differentiation: Switching to new niches.

Hybrid Zones and Outcomes

A hybrid zone is a region where different species interbreed. Three outcomes include:

  1. Reinforcement: Hybrids are less fit; reproductive barriers strengthen.

  2. Fusion: Hybrids are fit; species merge into one.

  3. Stability: Persistent production of hybrids in a local zone.

Rates of Speciation

  • Punctuated Equilibria: Long periods of stasis followed by sudden, rapid change.

  • Gradualism: Slow, steady change over long periods.