IB DP Biology Diversity of organisms Study Notes

A4.1 Evolution and speciation

1. Evolution and Mechanisms of Evolution

• Evolution: The gradual change in the genetic makeup of populations over generations, leading to biodiversity and adaptation to different environments. Evolution acts on populations rather than individuals.
• Mechanisms of Evolution:
  
  • Natural Selection: The process where individuals with advantageous traits (in a specific environment) are more likely to survive, reproduce, and pass these traits to the next generation. Natural selection is a non-random process.
  • Genetic Variation: Crucial for natural selection to act upon. Sources of genetic variation include:
    
    • Mutations: Random changes in DNA that can create new alleles, providing new traits.
    • Sexual Reproduction: Leads to recombination of genes, creating unique genetic combinations in offspring.
    • Gene Flow: Movement of alleles between populations (e.g., migration), introducing new alleles into a population.
  • Adaptation: Traits or characteristics that enhance survival and reproduction within a particular environment.
  • Selective Pressure: Environmental factors (e.g., predators, climate, resources) that influence an individual’s survival and reproduction rates, affecting population traits over time.

2. Types of Natural Selection

1. Stabilizing Selection:
   
   • Favors the average phenotype, reducing variation in a population.
   • Example: Human birth weight, where very low and very high weights have higher mortality rates.
2. Directional Selection:
   
   • Favors one extreme phenotype, shifting the average trait value in one direction.
   • Example: The evolution of antibiotic resistance in bacteria, where resistant strains survive and multiply.
3. Disruptive Selection:
   
   • Favors individuals at both extremes of a trait, potentially leading to the formation of two distinct groups within a population.
   • Example: Beak sizes in certain bird species where small and large beaks are favored for different food sources, while intermediate sizes are less advantageous.

3. Speciation and Types of Speciation

• Speciation: The process by which one species splits into two or more separate species, usually due to genetic and reproductive isolation.
  
  • Reproductive Isolation: A set of mechanisms that prevent different species from interbreeding, leading to speciation. These can be pre-zygotic (before fertilization) or post-zygotic (after fertilization).
  • Pre-Zygotic Barriers:
    
    • Temporal Isolation: Different mating or flowering times.
    • Behavioral Isolation: Different mating rituals or behaviors.
    • Mechanical Isolation: Differences in physical compatibility.
    • Gametic Isolation: Incompatibility of sperm and egg.
  • Post-Zygotic Barriers:
    
    • Hybrid Inviability: Hybrid offspring fail to develop or are weak.
    • Hybrid Sterility: Hybrids are sterile (e.g., mules).

Types of Speciation:

• Allopatric Speciation:
  
  • Occurs when populations are geographically separated, causing isolated populations to evolve independently.
  • Example: Squirrels on opposite sides of the Grand Canyon evolved separately due to geographic isolation.
• Sympatric Speciation:
  
  • Occurs within a shared habitat without physical barriers, often due to genetic differences or behavioral changes.
  • Example: Certain cichlid fish in African lakes show sympatric speciation due to differences in feeding preferences and behaviors.
• Polyploidy in Plants:
  
  • Common mechanism of sympatric speciation in plants, where errors in cell division result in extra chromosome sets. Polyploid plants can reproduce with each other but not with their diploid ancestors, leading to reproductive isolation.

4. Genetic Drift

• Definition: A change in allele frequencies due to random chance, having a greater impact in small populations.
• Types of Genetic Drift:
  
  • Founder Effect: Occurs when a small group establishes a new population, leading to reduced genetic diversity and possible increased prevalence of certain traits.
  • Bottleneck Effect: Drastic reduction in population size due to events like natural disasters, reducing genetic variation. The surviving population may not represent the genetic makeup of the original population.

5. Adaptive Radiation

• Definition: The diversification of a common ancestor into a variety of species adapted to different ecological niches.
• Characteristics:
  
  • Common in environments with many available niches and low competition.
  • Often follows events like mass extinctions or the colonization of new areas.
• Example: Darwin’s finches, which evolved from a common ancestor on the Galápagos Islands into multiple species with different beak shapes suited to specific food sources.

6. Convergent and Divergent Evolution

• Convergent Evolution:
  
  • Occurs when unrelated species evolve similar traits independently due to similar environmental pressures.
  • Example: Wings in birds, bats, and insects evolved independently as adaptations for flight.
• Divergent Evolution:
  
  • Occurs when related species evolve different traits due to adaptation to different environments, leading to increased biodiversity.
  • Example: The evolution of different limb structures in mammals (e.g., human hands, bat wings, whale flippers).

7. Coevolution

• Definition: The process where two or more species influence each other’s evolutionary pathways, often resulting in mutual adaptations.
• Examples:
  
  • Predator-Prey Relationships: Adaptations in predators and prey, such as speed or camouflage.
  • Mutualistic Relationships: Bees and flowers coevolve, with bees adapted to collect nectar and flowers evolved to facilitate pollination.