Evolution by Natural Selection In-Depth Notes

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.

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.

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.

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).
Sexual Selection: A subtype of natural selection based on mating success.
- Traits evolve that enhance attractiveness or competition (e.g., peacock tails).
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.
Gene Flow: Movement of alleles between populations through migration, enhancing genetic diversity.
Mutation: Random changes in genetic material that can introduce new traits.
- Beneficial mutations may spread through populations.
Nonrandom Mating: Selective mating can alter allele frequencies if certain traits are preferred.
Genetic Recombination: During sexual reproduction, it creates new allele combinations, contributing to genetic diversity.

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.

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.