17.09 Genetic Isolation and Speciation

1. Introduction to Isolation in Evolution

• Isolation is a fundamental mechanism that prevents gene flow between populations, leading to divergent evolution and ultimately speciation.
• Speciation is the formation of new and distinct species in the course of evolution.

2. Genetic Isolation

• Definition: Genetic isolation occurs when two populations of the same species are separated in ways that prevent gene exchange.
• Consequences:
  • Independent Evolution: Isolated populations undergo different evolutionary changes due to distinct selection pressures, genetic drift, mutation rates, and migration patterns.
  • Divergence: Over time, genetic differences accumulate, potentially leading to the emergence of new species.
• Mechanisms Leading to Genetic Isolation:
  • Geographical Barriers: Physical separations like mountains, rivers, oceans.
  • Ecological Factors: Different habitats within the same geographical area.
  • Behavioral Differences: Variations in mating rituals or behaviors.

3. Reproductive Isolation

• Definition: Reproductive isolation refers to a set of mechanisms that prevent members of different populations from interbreeding successfully.
• Importance in Speciation:
  • Prevents Gene Flow: Ensures that genetic differences are maintained between populations.
  • Maintains Species Boundaries: Ensures the integrity of species by preventing hybridization.

4. Types of Reproductive Isolation

A. Prezygotic Barriers

These barriers prevent mating or fertilization between species.

1. Behavioral Isolation
   • Definition: Differences in mating rituals or behaviors that prevent recognition as potential mates.
   • Examples:
     • Bird Songs: Different species have distinct songs that attract only their own kind.
     • Courtship Displays: Unique dances or rituals that are not recognized by other species.
2. Temporal Isolation
   • Definition: Differences in the timing of mating seasons or breeding cycles.
   • Examples:
     • Seasonal Breeding: One species breeds in spring, another in summer.
     • Diurnal vs. Nocturnal: Species active at different times of the day.
3. Ecological Isolation
   • Definition: Species occupy different habitats or ecological niches within the same geographical area.
   • Examples:
     • Different Feeding Sites: One species nests in trees, another on the ground.
     • Habitat Preference: Aquatic vs. terrestrial environments.
4. Mechanical Isolation
   • Definition: Physical differences in reproductive organs that prevent successful mating.
   • Examples:
     • Insect Genitalia: Mismatched structures that prevent coupling.
     • Size Differences: Larger size prevents smaller individuals from mating.
5. Gametic Isolation
   • Definition: Incompatibility between sperm and egg cells, preventing fertilization.
   • Examples:
     • Chemical Barriers: Sperm cannot penetrate the egg’s outer layers.
     • Species-Specific Proteins: Binding proteins that only recognize gametes of the same species.

B. Postzygotic Barriers

These barriers occur after fertilization, affecting the viability or fertility of the hybrid offspring.

1. Zygotic Isolation (Zygotic Failure)
   • Definition: The zygote fails to develop into a viable embryo.
   • Causes:
     • Genetic Incompatibilities: Embryonic development is disrupted due to mismatched genes.
     • Environmental Requirements: Hybrid embryos require conditions not met in their environment.
2. Hybrid Viability
   • Definition: Hybrids are formed but are too weak to survive to reproductive age.
   • Examples:
     • Doomed Hybrids: Offspring cannot compete for resources or are susceptible to diseases.
3. Hybrid Sterility
   • Definition: Hybrids develop into adults but are infertile.
   • Examples:
     • Mules: Offspring of horses and donkeys are sterile.
     • Ligers: Offspring of lions and tigers often have reduced fertility.
4. Hybrid Breakdown
   • Definition: First-generation hybrids are viable and fertile, but their offspring are inviable or sterile.
   • Example:
     • Multi-Generational Issues: Subsequent generations may exhibit genetic defects or infertility.

5. Pathways to Speciation

A. Allopatric Speciation (Geographical Isolation)

• Definition: Speciation resulting from populations being geographically separated.
• Process:
  1. Geographical Barrier Formation: Mountains, rivers, oceans, or other physical barriers split a population.
  2. Genetic Isolation: Limited or no gene flow between separated populations.
  3. Divergent Evolution: Each population adapts to its unique environment through natural selection, genetic drift, and mutation.
  4. Reproductive Isolation: Accumulated genetic differences prevent interbreeding, even if the barrier is removed.
• Examples:
  • Darwin’s Finches: Different islands harbor finch species with distinct beak shapes adapted to specific food sources.
  • Hawaiian Honeycreepers: Isolated on different Hawaiian islands, leading to diverse species.
• Additional Factors in Small Populations:
  • Genetic Drift:
    • Definition: Random changes in allele frequencies, more pronounced in small populations.
    • Impact: Can lead to the fixation or loss of alleles, contributing to genetic divergence.
  • Founder Effect:
    • Definition: A new population started by a small number of individuals carries only a fraction of the genetic diversity of the original population.
    • Impact: Can result in rapid genetic differentiation from the parent population.
• Allopatric Speciation Summary:
  • Geographical barriers lead to genetic isolation.
  • Divergent selection pressures, genetic drift, and founder effects drive speciation.
  • Results in new species that are reproductively isolated from the original population.

B. Sympatric Speciation (No Geographical Isolation)

• Definition: Speciation occurring within a single geographical area without physical separation.
• Mechanisms:
  1. Ecological Separation:
     • Different Niches: Populations exploit different resources or habitats within the same area.
     • Resource Partitioning: Reduces competition and promotes divergence.
  2. Behavioral Isolation:
     • Mating Preferences: Preferences for certain mates or mating times within the same population.
  3. Polyploidy (especially in plants):
     • Definition: An increase in the number of chromosomes, leading to reproductive isolation.
     • Example: Many plant species, such as wheat and strawberries, have polyploid forms that are reproductively isolated from their diploid ancestors.
• Examples of Sympatric Speciation:
  1. Palm Trees on Lord Howe Island (Howea forsteriana and Howea belmoreana)
     • Endemism: Both species are unique to Lord Howe Island.
     • Soil Preferences:
       • H. forsteriana: Grows on alkaline (calcareous) soil.
       • H. belmoreana: Grows on acidic volcanic soil.
     • Flowering Times:
       • H. forsteriana: Blooms earlier.
       • H. belmoreana: Blooms later.
     • Result: Limited overlap in flowering times reduces cross-pollination, leading to genetic isolation and distinct species.
  2. Cichlid Fish in Lake Apoyo, Nicaragua (Amphilophus citrinellus and Amphilophus zaliosus)
     • Colonization: A. citrinellus colonized Lake Apoyo around 10,000 years ago.
     • Ecological Niches:
       • Bottom-Feeders: Adapted with longer jaws for feeding near the lakebed.
       • Open-Water Feeders: Developed shorter jaws for mid-water feeding.
     • Behavioral Isolation:
       • Courtship Behaviors: Different mating rituals reinforce separation.
     • Disruptive Selection: Favors individuals at both extremes of the trait distribution, promoting divergence.
     • Result: Over time, ecological and behavioral changes lead to reproductive isolation and the formation of two distinct species.
• Sympatric Speciation Summary:
  • No geographical barriers; speciation occurs within the same area.
  • Driven by ecological niches, behavioral differences, or polyploidy.
  • Temporal isolation in activities like flowering or mating reinforces genetic separation.
  • Results in genetically distinct species sharing the same geographical region.

6. Important Considerations in Speciation

A. Initial Factors vs. Secondary Reproductive Barriers

• Initial Factors: Traits or conditions that first separate populations (e.g., ecological preferences, behavioral differences).
• Secondary Reproductive Barriers: Mechanisms that develop after initial separation, further preventing interbreeding (e.g., changes in mating rituals, hybrid sterility).
• Example:
  • Cichlid Fish in Lake Apoyo:
    • Initial Separation: Adaptation to different feeding habitats (ecological separation).
    • Secondary Barriers: Development of distinct courtship behaviors (behavioral isolation).

B. Gradual vs. Rapid Speciation

• Gradual Speciation: Occurs over long periods through accumulation of small genetic changes.
• Rapid Speciation: Occurs relatively quickly, often due to strong selective pressures or polyploidy in plants.

C. Role of Natural Selection and Genetic Drift

• Natural Selection: Drives adaptation to specific environments, promoting divergence.
• Genetic Drift: Causes random changes in allele frequencies, especially in small populations, contributing to genetic differentiation.

7. Key Terms and Definitions

• Behavioral Isolation: Divergence in mating behaviors that prevent interbreeding.
• Genetic Isolation: Lack of gene flow between populations, leading to separate evolutionary trajectories.
• Reproductive Isolation: Inability of two populations to produce fertile offspring, maintaining species boundaries.
• Allopatric Speciation: Speciation caused by geographical separation.
• Sympatric Speciation: Speciation within a single geographical area without physical barriers.
• Ecological Separation: Differentiation based on habitat preferences or environmental conditions.
• Genetic Drift: Random changes in allele frequencies, more significant in small populations.
• Founder Effect: Genetic differences arising from a new population established by a small number of individuals.
• Hybrid Viability: The ability of hybrid offspring to survive.
• Hybrid Sterility: The inability of hybrid offspring to reproduce.
• Polyploidy: Having more than two complete sets of chromosomes, common in plant speciation.

8. Diagrams and Visual Aids (Recommended)

• Flowcharts: Illustrating the processes of allopatric and sympatric speciation.
• Graphs: Showing allele frequency changes due to genetic drift or natural selection.
• Illustrations: Depicting types of reproductive isolation (e.g., behavioral isolation through different courtship displays).
• Maps: Highlighting geographical barriers leading to allopatric speciation.