Microevolution

3. Objectives

1. Define the Modern Synthesis (Combination of Natural Selection & Genetics).

2. Differentiate between Microevolution & Macroevolution.

3. Understand the Five Mechanisms of Microevolution.

4. Explore real-life examples of these mechanisms.

4. Modern Synthesis

• Evolution debate pre-1930s:

  • Was evolution driven by mutation or natural selection?

• Modern Synthesis (1920s-30s):

  • Recognized that BOTH mutation & natural selection drive evolution.

  • Key Concepts:

    • Variation is inherited.

    • Natural selection acts on variation.

    • Differential reproduction changes allele frequencies.

5. Microevolution vs. Macroevolution

6. The Five Mechanisms of Microevolution

1. Mutation

2. Non-Random Mating

3. Natural Selection

4. Gene Flow

5. Genetic Drift

7. Evolution = Change in Allele Frequency

• Definition: Evolution occurs when the frequency of alleles changes in a population over time.

• Example (Blood Type in 100 people):

  • 50 have Type A → A allele = 50% (0.5)

  • 40 have Type B → B allele = 40% (0.4)

  • 10 have Type O → O allele = 10% (0.1)

📌 Key Idea: Evolution is driven by changes in allele frequencies over generations.

8. Mechanism 1: Mutation

• What is a Mutation?

  • A spontaneous change in DNA sequence.

  • Mutations introduce genetic variation.

  • Some mutations are caused by external mutagens:

    • Chemicals (e.g., smoking)

    • Radiation (e.g., X-rays, UV light)

Effects of Mutations

Example: Sickle Cell Mutation

• Cause: A point mutation in the hemoglobin gene.

• Effects:

  • HbS/HbS (Two copies) → Sickle Cell Anemia (severe disease).

  • HbA/HbS (One copy) → No disease + Malaria resistance.

  • HbA/HbA → Normal blood, but vulnerable to malaria.

📌 Key Idea: In malaria-prone areas, the sickle cell allele persists because it offers a survival advantage.

9. Mechanism 2: Non-Random Mating

• Mating is NOT always random!

• Affects genetic diversity by changing genotype frequencies.

• Two Types of Non-Random Mating:

  1. Assortative Mating (Similar Individuals Mate)

  • Decreases genetic diversity (more homozygosity).

  • Example:

    • Achondroplasia (Dwarfism): High rates of in-group pairing.

    • Habsburg Jaw (Royal Inbreeding): Due to royal family inbreeding.

  2. Disassortative Mating (Different Individuals Mate)

  • Increases genetic diversity (more heterozygosity).

  • Example:

    • Redheads: Tend to mate outside their hair color group.

📌 Key Idea: Mating choice affects genetic variation.

10. Mechanism 3: Natural Selection

• Definition: Favorable traits increase survival & reproduction, while unfavorable traits decrease over generations.

Modern Examples of Natural Selection

1. Lactose Tolerance:

   • A mutation in Europeans & some African populations allowed adults to digest milk.

   • 75% of the world is still lactose intolerant!

2. Wisdom Teeth Evolution:

   • Humans evolved smaller jaws due to softer diets.

   • Some people never develop wisdom teeth (mutation).

📌 Key Idea: Natural selection favors traits that improve survival and reproduction.

11. Mechanism 4: Gene Flow

• Definition: Exchange of genes between populations.

• Occurs due to: Migration & interbreeding.

• Can introduce new genetic variation to a population.

Examples of Gene Flow

1. Europeans & Native Americans Interbreeding.

2. Neanderthal DNA in Modern Humans (~1-4%).

📌 Key Idea: Gene flow increases genetic diversity & connects populations.

12. Mechanism 5: Genetic Drift

• Definition: Random changes in allele frequencies.

• More significant in small populations.

• Types of Genetic Drift:

  1. Founder Effect

  • A small group breaks off & forms a new population.

  • Example: Amish people have higher rates of genetic diseases due to intermarrying within a small founder population.

  2. Bottleneck Effect

  • Disaster or event reduces population size.

  • Example: Cheetahs have low genetic diversity due to past bottleneck events.

📌 Key Idea: Genetic drift is random & can lead to reduced variation in populations.

13. Are Humans Still Evolving?

✅ Yes!

• Unless we start cloning, evolution will continue.

• Modern technology (e.g., genetic modification, medical advancements) influences natural selection.

📌 Key Idea: Evolution is an ongoing process!

14. Summary: Five Mechanisms of Microevolution