IB MYP Integrated Science- Biology - Variation, natural selection, and adaptation-Study Notes - New Syllabus

Variation, Natural Selection & Adaptation

Variation

Variation means differences that exist among individuals of the same species. Without variation, evolution cannot occur because natural selection needs differences to act upon.

🧬 Why Variation Occurs

Variation arises from both genetic factors and environmental influences.

Genetic (Inherited) Variation

Comes from DNA differences and is passed from parents to offspring.

• Sources of Genetic Variation:
  • Mutations: Random changes in DNA; May create new alleles (Example: mutation causing sickle-cell hemoglobin).
  • Meiosis: Crossing over creates new allele combinations; Independent assortment mixes chromosomes; Random orientation of homologous pairs.
  • Fertilization: Random fusion of gametes; Each sperm and egg is genetically unique.
• Examples: Blood group (A, B, AB, O), Eye color, Height (partly genetic), Ability to roll tongue.

Environmental Variation

Caused by surroundings and lifestyle.

• Examples: Body weight and nutrition, Muscle mass from exercise, Skin tone from sunlight, Speaking accent, Leaf size in plants depending on sunlight/water.

Combined Variation

Both genes and environment shape the trait.

• Examples: Height (genes + diet), Intelligence (genes + education), Skin color (genes + tanning).

🌿 Natural Selection

Natural selection is the process by which individuals with advantageous traits survive and reproduce more successfully.

Why It Happens

• Populations produce more offspring than can survive.
• There is constant competition for resources.
• Individuals vary in traits.
• Those with better adaptations live longer and reproduce more; Their alleles become more common.

Steps of Natural Selection (Exam-friendly sequence)

1. Variation exists: Individuals differ in size, strength, color, resistance, etc.
2. Struggle for survival: Limited food, space, mates, predators, disease pressure.
3. Survival of the fittest: Individuals with beneficial traits survive better. (“Fittest” means best suited, not strongest).
4. Successful reproduction: Those that survive are more likely to reproduce.
5. Inheritance of advantageous traits: Beneficial alleles pass to offspring.
6. Evolution over generations: Trait becomes more common; Population becomes better adapted.

Classic Example: Peppered Moth

• Light moths common before Industrial Revolution.
• Pollution darkened tree bark.
• Birds preyed on light moths.
• Dark moths survived and reproduced.
• Dark phenotype increased (This is directional selection).

🧠 Adaptation

Adaptations are inherited characteristics that help organisms survive and reproduce.

Why adaptations evolve: Through many generations of natural selection acting on variation.

Types of Adaptations

• A. Structural Adaptations (physical features): Thick fur in polar bears, Webbed feet in ducks, Cacti spines instead of leaves, Long roots in desert plants.
• B. Behavioral Adaptations (actions): Migration in birds, Hibernation in mammals, Courtship dances in birds, Nocturnal activity in desert animals.
• C. Physiological Adaptations (internal processes): Snake venom production, Ability of kangaroo rats to conserve water, Sweating to regulate temperature, CAM photosynthesis in cacti.

🌍 Relationship Between Variation, Natural Selection & Adaptation

Variation → Selection → Adaptation

• Variation provides different traits.
• Natural selection favors useful traits.
• Adaptation occurs when these traits become common in the population.

Example: Some bacteria have mutation for antibiotic resistance. They survive antibiotic exposure, reproduce, and pass the resistance. Over generations, population becomes antibiotic-resistant. This is evolution in real time.

Types of Selection

• 1. Stabilizing Selection: Favors average phenotype (Example: human birth weight).
• 2. Directional Selection: Pushes toward one extreme phenotype (Example: antibiotic resistance).
• 3. Disruptive Selection: Favors both extremes; Can lead to speciation (Example: bird beak size extremes favored depending on food type).

Importance of Natural Selection

• Explains evolutionary change.
• Helps develop medicines and vaccines.
• Foundation of genetics + evolution.
• Predicts how species respond to environmental change.
• Key for conservation biology.

📋 Summary Table

📦 Quick Recap
• Variation gives the raw material for evolution.
• Natural selection chooses the most useful variations.
• Adaptations are traits that help survival.
• These traits increase in frequency across generations.
• Natural selection leads to evolution.