CHAPTER 13

Chapter 13: The Origin of Species

Introduction to the Course

  • Course: SI BIOL 214

  • Instructor: Dr. Butler

  • SI Leader: Faith Hall

    • Junior Biology major with a minor in Psychology

    • Second semester as SI leader

    • Earned an A in the course during Fall 2024

  • Important links: Session times and materials available on SI website and GroupMe.

Meeting Times

  • Tuesdays: 6:30pm-7:45pm in HELD 118

  • Thursdays: 5:30pm-6:45pm in HECC 202

  • Invitation to join GroupMe for communication.

Key Learning Objectives

  • Compare and contrast various species concepts:

    • Phylogenetic species concept

    • Biological species concept

    • General lineage species concept

  • Discuss gene flow influenced by:

    • Geographic barriers

    • Reproductive isolation barriers

  • Examine speciation types:

    • Allopatric

    • Parapatric

    • Sympatric

  • Enumerate evidence used to evaluate speciation models.

  • Explore challenges in studying speciation.

  • Identify factors affecting the rate of speciation across organisms.

  • Understand significance of cryptic species discoveries.

  • Discuss difficulties in applying species concepts to bacteria and archaea.

Species Definitions

  • Various definitions exist for 'species':

    • Biological Species Concept: Groups of potentially interbreeding populations that are reproductively isolated from others. Most reliable for sexually reproducing animals.

    • Phylogenetic Species Concept: The smallest group descending from a common ancestor, characterized by unique, derived traits. Valuable for systematics, focusing on phylogenetic history.

    • General Lineage Species Concept: Defines species as metapopulations that exchange alleles frequently enough to form a single gene pool.

    • Metapopulation: Groups of spatially separated populations of the same species that interact to some extent.

  • Ongoing scientific efforts develop more precise species definitions but consensus suggests no single definition fits all taxa.

Speciation

  • Definition: Speciation is the evolutionary process that results in the emergence of new species through the splitting of populations.

  • The diagram of genetic exchanges illustrates this concept.

Isolating Barriers

  • Definition: An isolating barrier is an aspect of environment, genetics, behavior, physiology, or ecology which reduces or impedes gene flow from one population to another.

  • Types of isolating barriers:

    • Geographic Barriers: External landscape features obstruct gene flow (e.g., allopatry).

    • Reproductive Barriers: Features intrinsic to organisms that prevent interbreeding, effective in both sympatry and allopatry.

Prezygotic Barriers

  • Definition: Mechanisms that prevent mating or fertilization before zygote formation.

    • Types of Prezygotic Barriers:

    • **Premating: **

      • Behavioral isolation

      • Ecological isolation

      • Habitat Isolation

      • Temporal Isolation

      • Pollinator Isolation

      • Mechanical isolation

    • Post-mating:

      • Copulatory behavioral isolation

      • Gametic isolation

Postzygotic Barriers

  • Definition: Mechanisms that reduce viability or reproductive capacity of hybrid offspring after fertilization.

    • Types of Postzygotic Barriers:

    • Intrinsic:

      • Hybrid inviability (e.g., sheep and goats hybrid dies early)

      • Hybrid sterility (e.g., mules)

    • Extrinsic:

      • Ecological inviability (e.g., outbreeding depression)

      • Behavioral sterility (e.g., hybrids unable to find mates).

Speciation Examples

  • Example: Elk and Red Deer.

    • Geographically isolated populations that reproduce when brought together after a history of separation (e.g., after the latest ice age).

    • Size difference noted (average bull elk = 720 lbs; red deer stag = 400 lbs).

Specific Cases of Reproductive Isolation

  • Corals: Timing of reproduction prevents interbreeding.

    • Notable spawning events in Coral species.

  • Fireflies: Distinct flashing patterns in males lead to female responses, preventing hybridization.

  • Monkeyflowers: Different pollinators lead to non-hybridization due to isolation.

Speciation Mechanisms

  • Allopatric Speciation:

    • Results from geographic isolation leading to divergence. After isolation, reinforcement increases reproductive barriers through selective pressures.

  • Sympatric Speciation:

    • Occurs without geographic barriers; relies on nonrandom mating potentially driven by phenotypic traits.

  • Parapatric Speciation:

    • Populations are only partially separated geographically with some gene flow. E.g., differences in tolerance to heavy metals lead to divergence.

Additional Speciation Concepts

  • Isolation by Distance: Members tend to mate with nearby individuals, contributing to population differentiation.

  • Ecological Speciation: Adaptive traits in different ecological contexts can lead to reproductive barriers.

  • Cryptic Species: Species that are morphologically similar but genetically distinct; important for biodiversity.

Speciation in Microbial Species

  • Definition of microbial species based on genetic similarity (e.g., 97% similarity in 16S rRNA).

  • Horizontal Gene Transfer complicates classical species definitions, influencing microbial classifications.

  • Genetic interchanges may lead to new adaptations even among different taxa.

Conclusion: Key Concepts

  • Ongoing refinement of species definitions due to new scientific insights into gene flow.

  • Geographic and reproductive barriers play critical roles in controlling gene flow.

  • Speciation is complex with multiple models based on extensive evidence and research findings.

  • Identification and understanding of cryptic species are crucial for biodiversity assessments.