In-depth Notes on Speciation

Origin of Species

• Speciation: The creation of distinct species from an ancestral species; can occur rapidly or gradually.

• Key Processes in Speciation:

  • Genetic Isolation: A barrier to gene flow isolates two populations within the same species, leading to genetic divergence.
  • Genetic Divergence: Occurs through mutation, natural selection, and genetic drift in isolated populations.

Mechanisms of Speciation

• Biological Species Concept: Based on reproductive isolation, where distinct species do not interbreed or do not produce viable, fertile offspring if they do.

  • Limitations: Not applicable to fossils, asexual organisms, or geographically isolated populations.

• Morphological Species Concept: Distinguishing species based on size, shape, or other morphological features.

  • Limitations: Fails to distinguish between polymorphic and cryptic species; subjective criteria.

• Phylogenetic Species Concept: Based on evolutionary history and the smallest monophyletic groups.

  • Limitations: Limited phylogenetic data; could overestimate species numbers.

Premating Isolating Mechanisms

1. Habitat Isolation: Species occupy different habitats and therefore do not mate.

   • Example: Different fig wasp species breed in different fig species.

2. Temporal Isolation: Species breed at different times, even if they occupy the same habitat.

   • Example: Bishop pine and Monterey pine have different pollination times.

3. Behavioral Isolation: Differences in courtship behaviors prevent mating.

   • Example: Unique mating displays in male Raggiana birds of paradise.

Pre-fertilization Isolating Mechanisms

1. Mechanical Incompatibility: Species have incompatible reproductive structures.

   • Example: Snails with left-handed shells cannot mate with those with right-handed shells.

2. Gametic Incompatibility: Sperm from one species cannot fertilize eggs of another.

   • Example: Red and purple sea urchin sperm fail to fuse with eggs from another species.

Post-zygotic Isolating Mechanisms

1. Hybrid Inviability: Cross-species fertilization occurs, but hybrids do not survive.
2. Hybrid Infertility: Hybrids, such as ligers, are sterile due to chromosomal mismatches.
3. Hybrid Breakdown: Hybrid F1 generation is fertile, but subsequent generations are not.

Allopatric vs. Sympatric Speciation

• Allopatric Speciation: Begins with geographic isolation; mechanisms include:

  • Dispersal: Movement leading to genetic divergence, e.g., ground finches on Daphne Major.
  • Vicariance: Physical barriers split populations, e.g., rivers creating isolation between trumpeter birds.

• Sympatric Speciation: Occurs within the same geographic area but can be driven by:

  • External factors: Disruptive selection based on ecological niches.
  • Internal factors: Chromosomal mutations leading to reproductive isolation, e.g., polyploidy in plants.

Hybridization and Secondary Contact Outcomes

• When isolated populations come into contact, outcomes include:
  • Fusion: Populations interbreed freely, potentially leading to extinction of one.
  • Reinforcement: Traits evolve to reduce interbreeding when hybrid fitness is low.
  • Hybrid Zone Formation: Interbreeding produces hybrids in specific regions.
  • Formation of New Species: Hybrids occupy new ecological niches.