Evolution by Natural Selection In-Depth Notes

The Theory of Evolution by Natural Selection

• Essential Question: How does natural selection cause evolutionary change?

Natural Selection Overview

• Natural selection and evolution are fundamental concepts in biology, broadly accepted today.
• Darwin theorized that populations must compete for limited resources, leading to:
  • Inherited variation among individuals.
  • Differential reproduction where some traits lead to greater survival.

Four Principles of Natural Selection

• 1. Variation: Individuals in a population exhibit variation in traits, e.g., sunflower height.

  • Genetic variation is essential for traits to vary.

• 2. Heritability: Traits must be inherited through genes.

  • For example, taller sunflowers produce seeds that develop into tall sunflowers.

• 3. Overproduction: Populations typically produce more offspring than the environment can support.

  • Example: A sunflower can produce hundreds of seeds but only a few will germinate and survive.

• 4. Reproductive Advantage: Traits that enhance survival and reproduction become more common over generations.

  • Example: Larger beaks in certain birds may improve their ability to access food, increasing their reproductive success.

Natural Selection's Impact on Populations

• Evolution refers to changes in allele frequencies within a population over generations.
• Alleles that provide advantages in a given environment will increase in frequency.
• Natural selection operates on populations, not individuals, and results in changes in allele frequencies driven by:
  • Differential reproductive success influenced by the traits that offer survival benefits.

Mechanisms of Evolution

1. Natural Selection: The primary mechanism discussed.

   • Alters populations through stabilizing, directional, and disruptive selection:
     • Stabilizing Selection: Favors average traits (e.g., average birth weights in humans).
     • Directional Selection: Favors extreme traits (e.g., increasing proportion of silent crickets due to predation).
     • Disruptive Selection: Favors two extreme traits at the expense of average traits (e.g., beak size in seedcrackers).

2. Sexual Selection: A subtype of natural selection based on mating success.

   • Traits evolve that enhance attractiveness or competition (e.g., peacock tails).

3. Genetic Drift: Random changes in allele frequencies, more pronounced in small populations.

   • Founder Effect: Small group establishes a new population, potentially carrying rare alleles.
   • Bottleneck Effect: Population size drastically reduces, leading to decreased genetic diversity.

4. Gene Flow: Movement of alleles between populations through migration, enhancing genetic diversity.

5. Mutation: Random changes in genetic material that can introduce new traits.

   • Beneficial mutations may spread through populations.

6. Nonrandom Mating: Selective mating can alter allele frequencies if certain traits are preferred.

7. Genetic Recombination: During sexual reproduction, it creates new allele combinations, contributing to genetic diversity.

Hardy-Weinberg Equilibrium

• Population genetics examines gene frequencies and how they change.
• Hardy-Weinberg Principle: States allele frequencies in a population remain constant unless acted upon by evolutionary forces:
  • Conditions for equilibrium: large population, no immigration/emigration, random mating, no mutations, and no natural selection.

Conclusion

• The theory of evolution via natural selection provides insights into the mechanisms driving the diversity of life.
• Understanding these principles is crucial for studying biological evolution and the adaptation of species to their environments.