Grade 11 Biology - Speciation

Mechanisms of Evolution Part 2: Speciation

Definition of Species

  • A species is defined as a group of interbreeding organisms that produce viable, fertile offspring in nature.

  • Species members typically interbreed within their group but not with other organisms.

Microevolution

  • Definition: Small changes within a population over a short time.

  • Examples: Antibiotic resistance in bacteria, changes in moth coloration.

  • Mechanisms: Natural selection, genetic drift, gene flow, mutation.

  • Scale: Occurs within a single species or population.

Macroevolution

  • Definition: Large-scale evolutionary changes over long time periods that lead to the formation of new species or higher taxonomic groups.

  • Examples: Evolution of mammals from reptile ancestors, emergence of birds from dinosaurs.

  • Mechanisms: Accumulation of microevolutionary changes, speciation events.

  • Scale: Encompasses changes across species, genera, families, etc.

Types of Speciation

Allopatric vs Sympatric Speciation

  • Allopatric Speciation: Occurs when a population is geographically separated from its parent population.

  • Sympatric Speciation: Occurs without geographic separation, often due to ecological or behavioral differences.

Allopatric Speciation Process

  • Population is divided by a physical barrier (e.g., mountain, river).

  • Isolation prevents gene flow, leading to independent evolution and genetic divergence.

  • Examples: Darwin’s finches on the Galápagos Islands, squirrels separated by the Grand Canyon.

Adaptive Radiation

  • Organisms rapidly diversify into new forms due to environmental changes creating new opportunities.

  • This is a form of allopatric speciation involving rapid speciation from a common ancestor.

  • Each species adapts to different ecological niches, minimizing competition and accumulating unique traits.

Sympatric Speciation Process

  • Occurs without physical barriers; instead, reproductive isolation arises from ecological or behavioral differences.

  • Examples: Cichlid fish in African lakes with specialized feeding habits, apple maggot flies adapting to different host plants.

Reproductive Isolation

  • Mechanisms that prevent interbreeding between different species, crucial for speciation.

  • Types of Barriers:

  • Pre-zygotic Barriers: Occur before fertilization.

  • Post-zygotic Barriers: Occur after fertilization.

Pre-zygotic Barriers

  • Habitat Isolation: Species occupy different habitats (e.g., land vs. aquatic garter snakes).

  • Temporal Isolation: Species breed at different times (e.g., different frog species).

  • Behavioral Isolation: Different mating rituals (e.g., unique bird songs or dances).

  • Mechanical Isolation: Physical differences prevent mating (e.g., incompatible flower shapes).

  • Gametic Isolation: Sperm and egg are incompatible (e.g., sea urchin species).

Post-zygotic Barriers

  • Reduced Hybrid Viability: Hybrids may develop but do not survive well (e.g., frail salamander hybrids).

  • Reduced Hybrid Fertility: Hybrids are sterile (e.g., mules, offspring of horses and donkeys).

  • Hybrid Breakdown: First-generation hybrids are fertile, but later generations are weak or sterile.

Types of Evolution

Convergent Evolution

  • Unrelated species evolve similar traits due to adaptations to similar environments.

  • Examples: Wings in bats and birds, similar streamlined bodies in sharks and dolphins.

  • Analogous Structures: Traits that serve similar functions but evolved independently.

Divergent Evolution

  • Species with a common ancestor evolve different traits, leading to the formation of new species.

  • Examples: Darwin’s finches with various beak shapes, mammalian limbs adapted for swimming or grasping.

  • Homologous Structures: Traits inherited from a common ancestor, modified for different functions.

Gradualism

  • Evolutionary change occurs gradually and steadily over long periods.

  • Involves small, incremental changes accumulating over time.

  • Example: Evolution of the horse through gradual changes in body size and structure.

Punctuated Equilibrium

  • Evolution occurs in rapid bursts, followed by long periods of stability.

  • Sudden changes happen in response to significant environmental shifts.

  • Characterized by long periods with minimal evolutionary change.