Darwin and Natural Selection

1. Introduction to Natural Selection

• Natural selection acts on individuals, but evolution occurs in populations.

• Another term for reproductive success: Fitness

  • Fitness ≠ physical strength/agility.

  • Fitness = Survival & reproduction in a given environment.

2. Key Concepts of Natural Selection

Natural Selection Requires:

✅ Inherited traits
✅ Traits affecting reproduction
✅ Population variation
✅ Pre-existing variation (selection acts on existing traits)
✅ Environmental & time-specific adaptation

Natural selection does NOT act on:

• Traits acquired during life (e.g., muscle gain, haircuts).

• Traits that do not affect reproduction (e.g., late-onset diseases like Alzheimer's).

• Traits that do not vary within a population.

3. Can These Traits Be Selected?

Example of Constraint in Evolution:

• All mammals have 7 cervical (neck) vertebrae (except sloths & manatees).

• This trait is highly conserved and does not easily change.

4. Examples of Natural Selection

Case Study: Peppered Moths

• Pre-Industrial England: Light-colored moths were more common because they blended into lichen-covered trees.

• Industrial Revolution: Pollution darkened tree bark. Dark-colored moths became more camouflaged → Survived & reproduced more.

• Post-Pollution Control: Light-colored moths became more common again.

🔑 Lesson: Fitness is relative to the environment & can change over time.

Case Study: Nylon-Eating Bacteria

• Paenarthrobacter ureafaciens KI72 (mutant strain found near a Japanese factory).

• Mutation enabled it to digest nylon (synthetic material).

• Example of new adaptation via mutation.

Case Study: Grand Canyon Squirrels

• Kaibab & Abert’s squirrels evolved on opposite rims of the Grand Canyon.

• Geographic isolation caused them to diverge over time.

• Key concept: Speciation via physical barriers.

5. Selective Breeding vs. Natural Selection

Artificial Selection (Selective Breeding)

• Humans selectively breed animals/plants for desired traits.

• Example: Domestic dogs from wolves.

• Unlike natural selection, human preference determines which traits persist.

🤔 Question:
Is choosing sperm donors or embryo selection a form of artificial selection or natural selection?

6. Unresolved Issues in Darwin’s Theory

• What is the mechanism of inheritance?

  • Darwin believed in blending inheritance (incorrect).

  • Mendel’s work (later discovered) explained genetic inheritance.

7. Evolution & Religion

• Evolution is opposed by some religious groups.

• 2024 Gallup Poll:

  • 37% of Americans believe in biblical creationism.

  • 34% accept evolution with God’s guidance.

  • 24% accept purely scientific evolution.

• Legal rulings: Courts reject "creation science" as non-scientific due to lack of testability.

Stephen Jay Gould’s "Nonoverlapping Magisteria" (1997)

• Science & religion address different domains and do not need to conflict.