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?

Trait

Can Natural Selection Act?

Reason

Tooth loss (diet/hygiene)

❌ No

Not heritable

Alzheimer’s disease (affects age 65+)

❌ No

Happens post-reproduction

Type I Diabetes (heritable, early onset)

✅ Yes

Affects reproductive potential

New bird feather color (future prediction)

❌ No

Selection works on existing traits

Vertebral patterning (same in all mammals)

❌ No

No population variation

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.