Unit 2: Population Genetics

1. Hardy-Weinberg Law: Concept and Application

The Hardy-Weinberg Principle is a mathematical model that describes a population that is not evolving. It provides a baseline to study evolutionary changes.

The Law

"Allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences."

Mathematical Representation

For a gene with two alleles (dominant p and recessive q):

• Allele Frequency: p + q = 1
• Genotype Frequency: p² + 2pq + q² = 1

Where:

• p² = frequency of homozygous dominant individuals.
• 2pq = frequency of heterozygous individuals.
• q² = frequency of homozygous recessive individuals.

Assumptions of Hardy-Weinberg Equilibrium

1. No mutations.
2. Random mating.
3. No natural selection.
4. Extremely large population size.
5. No gene flow (immigration or emigration).

2. Evolutionary Forces: Natural Selection and Genetic Drift

When the assumptions of Hardy-Weinberg are violated, evolution occurs. The primary drivers are Natural Selection and Genetic Drift.

Natural Selection

Natural selection is the process where individuals with favorable traits are more likely to survive and reproduce. There are three modes of selection:

• Directional Selection: Shifts the population toward one extreme phenotype (e.g., antibiotic resistance in bacteria).
• Stabilizing Selection: Favors intermediate variants and acts against extreme phenotypes (e.g., human birth weight).
• Disruptive Selection: Favors individuals at both extremes of the phenotypic range (e.g., beak sizes in finches).

Genetic Drift

Genetic Drift refers to random fluctuations in allele frequencies from one generation to the next due to chance events. It has a much more significant impact on small populations.

• Unlike natural selection, genetic drift is not adaptive; it can cause neutral or even harmful alleles to become fixed or lost.

3. Founder Effect and Bottleneck Effect

These are two specific scenarios where genetic drift significantly alters a population's genetic structure.

Founder Effect

Occurs when a few individuals become isolated from a larger population and establish a new population. The new gene pool is unlikely to represent the genetic diversity of the original population (e.g., high frequency of certain genetic disorders in isolated human communities).

Bottleneck Effect

Occurs when a sudden change in the environment (e.g., earthquake, flood, or overhunting) drastically reduces the size of a population. By chance alone, certain alleles may be overrepresented, underrepresented, or eliminated among the survivors.

4. Exam Focus Enhancements