Speciation and Species Concepts

Origin of Species Overview

• Speciation: The process of forming new and distinct species from an ancestral species; can occur rapidly or gradually.

• Key Concepts:

  • Genetic Isolation: A reproductive barrier isolates populations, leading to genetic divergence.

  • Gene Flow: The transfer of genetic material between populations is disrupted, allowing for independent evolution.

Speciation Mechanisms

• Types of Speciation:

  • Allopatric Speciation: Geographic isolation leads to the formation of new species.

  • Sympatric Speciation: New species arise from populations in the same geographic area, often through different ecological niches or mate preferences.

Species Concepts

1. Biological Species Concept:

   • Defines species based on reproductive isolation, indicating distinct species do not interbreed or produce viable, fertile offspring.

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

2. Morphospecies Concept:

   • Identifies species based on morphological differences (size, shape).

   • Limitations: Subjectivity in distinguishing features, misidentifies polymorphic and cryptic species.

3. Phylogenetic Species Concept:

   • Based on evolutionary history; identifies monophyletic groups (clades) using genetic data.

   • Limitations: Incomplete phylogenetic data for many species may lead to overestimation of species diversity.

Reproductive Isolation Mechanisms

• Premating Mechanisms:

  • Habitat Isolation: Different habitats prevent interbreeding.

  • Temporal Isolation: Different breeding seasons prevent mating.

  • Behavioral Isolation: Different mating behaviors lead to unrecognizable courting signals.

• Pre-fertilization Mechanisms:

  • Mechanical Incompatibility: Incompatible reproductive structures prevent mating.

  • Gametic Incompatibility: Sperm and egg from different species cannot fuse.

• Post-zygotic Mechanisms:

  • Hybrid Inviability: Hybrid offspring fail to survive.

  • Hybrid Infertility: Hybrids are sterile (e.g., ligers).

  • Hybrid Breakdown: Hybrid offspring are viable but subsequent generations are not.

Isolating Populations and Divergence

• Allopatric Speciation: Geographic isolation (e.g., dispersal or vicariance).

  • Dispersal Example: Ground finches on islands evolving larger beaks due to natural selection.

  • Vicariance Example: Formation of rivers leads to population subdivision.

• Sympatric Speciation: Occurs in the same area, often initiated by disruptive selection or chromosomal mutations.

  • Ecological Niche: Resources required by a species influence speciation.

Polyploidy in Plants

• Polyploidy: A common mechanism in plant speciation involving more than two complete sets of chromosomes.

  • Autopolyploidy: Chromosome duplication within the same species.

  • Allopolyploidy: Hybridization between species leading to fertile offspring.

Outcomes of Population Contact

• Possible outcomes upon secondary contact:

  • Fusion: Two populations interbreed freely.

  • Extinction: One population outcompetes the other.

  • Reinforcement: Natural selection enhances reproductive isolation if hybrids have lower fitness.

  • Hybrid Zone Formation: Area where distinct populations produce hybrids; hybrids may have adaptations that allow them to occupy new ecological niches.