Hardy–Weinberg Equilibrium & Evolutionary Change

I. Key Definitions

Population
A local group of interbreeding individuals that produce fertile offspring.

Gene pool
All alleles for all genes in a population at a given time.

Alleles
Different versions of the same gene.

Allele frequency
Proportion of a specific allele in the gene pool (expressed as a decimal).

Evolution (population level)
A change in allele frequencies over generations.

II. Hardy–Weinberg Principle (H–W)

A model describing a non-evolving population.

Five required conditions (MEMORIZE)

1. Random mating

2. Very large population

3. No migration (gene flow)

4. No mutation

5. No natural selection

If any condition is violated → evolution is occurring.

III. Hardy–Weinberg Equations

Let:

• p = frequency of dominant allele (A)

• q = frequency of recessive allele (a)

Allele equation

p + q = 1

Genotype equation

p² + 2pq + q² = 1

IV. Using Hardy–Weinberg (Problem Strategy)

Step-by-step method

1. Identify what is given (usually % recessive phenotype).

2. Convert % → decimal.

3. If recessive phenotype → q².

4. Take square root → q.

5. Find p = 1 − q.

6. Plug into equations as needed.

Example 1: Allele frequencies

36% albino rabbits (recessive)

• q² = 0.36

• q = 0.6

• p = 0.4

Example 2: Genotype frequencies

49% yellow butterflies (bb)

• q² = 0.49

• q = 0.7

• p = 0.3

BB = p² = 0.09 (9%)
Bb = 2pq = 0.42 (42%)

V. What Breaks Hardy–Weinberg? (Mechanisms of Evolution)

If allele frequencies change → evolution is happening.

VI. Mutation

Definition
A change in DNA that creates a new allele.

Key points

• Only source of new genetic variation

• Usually rare

• Can be harmful, neutral, or beneficial

• Can increase rapidly if beneficial (e.g., antibiotic resistance)

VII. Migration (Gene Flow)

Definition
Movement of individuals (and alleles) between populations.

Effects

• Introduces new alleles

• Reduces differences between populations

• Increases genetic diversity

More common in animals than plants

VIII. Natural Selection

Definition
Individuals with advantageous traits survive and reproduce more.

Key requirement

• Traits must be heritable

Classic example

Peppered moth

• Environment changed → selection favored dark moths

• Allele frequencies shifted

Types of Natural Selection

IX. Genetic Drift

Definition
Random changes in allele frequencies due to chance.

Most significant in

• Small populations

• Isolated populations

Can reduce genetic diversity.

A. Founder Effect

• Small group starts a new population

• Gene pool ≠ original population

• Certain alleles overrepresented

Example

• Amish population → high frequency of rare traits

B. Bottleneck Effect

• Sudden population reduction (disaster)

• Survivors are not genetically representative

• Leads to reduced variation

X. Comparison Table (Exam Favorite)

XI. One-Line Exam Lock-Ins

• Hardy–Weinberg describes no evolution

• Evolution = change in allele frequencies

• q² = recessive phenotype

• Mutation is the only source of new alleles

• Small populations evolve faster by drift