Variation in Populations and Natural Selection

Definition: Random change in allele frequency across generations, not related to organism fitness.
Characteristics: Greater impact in small populations.
Mechanisms:
- Bottleneck Effect: Sudden, random population decrease leading to reduced genetic diversity (e.g., elephant seals).
- Founder Effect: A small sample from a larger population colonizes a new area, resulting in genetic drift (e.g., Desert Mountain Sheep).
Both effects are quick, random, and short-term changes.

Genetics: Diversity arises from mutations creating new alleles or rearranging existing ones.
- Alleles: Alternative variations of a gene producing distinct phenotypic effects (dominant, recessive, multiple types).

Definition: The relative frequency of an allele in a population (e.g., 50% or 0.5).
In non-evolving populations, allele frequency remains stable, as described by Hardy-Weinberg equilibrium.

Importance for Evolution:
- Gene pool: Diversity within a population due to various alleles.
- Geographic variation: Diversity between populations (e.g., beak size in birds).
- Gene Flow: Transfer of alleles between populations via movement of individuals or gametes.

Populations evolve, not individuals.
Population definition: A group of individuals of the same species in the same area (Henderson’s Dictionary of Biology).

Variation is crucial for natural selection to act upon.
Two categories of changes:
- Random Changes: Processes like genetic drift.
- Non-Random Changes: Natural selection favoring advantageous traits.

Allele frequencies are stable in non-evolving populations.
Microevolution: Dynamic changes in genetic variation or allele frequencies within a population over time.
Genetic variation is reflected in phenotypic variation.
Sources of increased variation: Random mutations or gene flow.
Sources of decreased variation: Chance events like bottlenecks and founder effects (genetic drift).
Genetic/phenotypic variation is essential for effective natural selection.