Species And Speciation

Species Definition and Concepts

  • Definition Difficulty: It is challenging to define species universally.

Species Concepts

  • Morphological Concept:

    • Distinctions based on shape and appearance.

    • Limitations: Does not account for sexual dimorphism (e.g., anglerfish).

  • Genetic Species Concept:

    • Defines species based on identical DNA sequences.

  • Biological Species Concept:

    • Groups of interbreeding natural populations that cannot produce viable offspring with others.

    • Reproductive Isolation: Defines barriers that prevent species from interbreeding.

    • Limitations: Not applicable to bacteria or extinct species.

Types of Reproductive Isolation

  • Prezygotic Barriers: Prevents zygote formation.

    • Behavioral Isolation: Different mating behaviors or rituals (e.g., mating dances).

    • Mechanical Isolation: Incompatibility of reproductive organs.

    • Gametic Isolation: Inability of gametes to fuse.

  • Postzygotic Barriers: Occur after zygote formation.

    • Hybrid Inviability: Offspring do not survive long.

    • Reduced Hybrid Fertility: Offspring are sterile.

Evolutionary Species Concept

  • Definition: A single lineage of ancestor-descendant organisms until a branching occurs.

  • Applicability: Can be applied to all species, but lacks a specific testing method in its definition.

Speciation

  • Origin of Species: A species can split into two different species.

Two Main Patterns of Speciation

  • Allopatric Speciation:

    • Original species splits due to geographic separation.

  • Sympatric Speciation:

    • Original species splits while still in the same location.

    • Results in groups becoming different over time.

Reinforcement and Speciation Completion

  • Reinforcement: Hybrids have less evolutionary fitness compared to non-hybrids (specifically in allopatric speciation).

  • Natural selection favors prezygotic isolating mechanisms.

Example of Sympatric Speciation

  • Hawthorn Flies:

    • Reproductive timing changes with the introduction of apples, leading to differences in mating.

Sympatric Speciation via Polyploidy

  • An error during meiosis causes changes in chromosome number, leading to instant reproductive isolation.

  • More common in plants but also seen in animals.

Parapatric Speciation

  • Definition: Occurs when there is a partial barrier and possible habitat differences between the two groups.