Population Genetics: Principles, Forces, and Applications

Population Genetics

Introduction to Population Genetics

Population genetics is the study of genetic variation within populations and how allele frequencies change over time. It connects molecular genetics, Mendelian genetics, and evolutionary biology to explain how populations evolve.

• Key Question: How does genetic information change in populations over time?

• Focus: Understanding the forces that drive genetic diversity and evolution in populations.

Key Questions in Population Genetics

• Why are populations genetically different?

• What forces change allele frequencies?

• How do we measure evolution in populations?

Connecting Genetics to Population Genetics

The flow of genetic information from DNA to population-level evolution involves several steps:

• DNA (molecular level): Mutation creates new alleles.

• Alleles (Mendelian genetics): Segregation and independent assortment distribute alleles.

• Genotypes (chromosomal genetics): Variation enters the population.

• Allele Frequencies (population): Changed by evolutionary forces.

• Evolution (population genetics): Long-term change in allele frequencies.

Key Terms

Term	Definition
Population	Group of interbreeding individuals
Gene pool	All alleles present in a population
Allele frequency	Proportion of an allele in the gene pool
Genotype frequency	Frequency of a specific genotype
Evolution	Change in allele frequency over generations

Allele and Genotype Frequencies

Allele Frequency Calculation

Allele frequencies (commonly denoted as p and q) represent the proportion of each allele in the population.

• If p is the frequency of one allele, q is the frequency of the other allele at the same locus.

• The sum of all allele frequencies at a locus equals 1: $p + q = 1$

Example: If p = 0.8, then q = 1 - 0.8 = 0.2

General formula:

$\text{Allele freq} = \frac{\#\ \text{of given allele}}{\text{Sum of all alleles sampled}}$

Worked Example

Given a population of 100 individuals:

• 36 AA

• 48 Aa

• 16 aa

1. Total alleles: 100 individuals × 2 alleles each = 200 alleles

2. Count A alleles: AA gives 2 A's (36 × 2 = 72), Aa gives 1 A (48 × 1 = 48). Total A alleles = 72 + 48 = 120

3. Frequency of A (p): $p = \frac{120}{200} = 0.6$

Evolutionary Forces

How Populations Evolve

Allele frequencies in populations change due to several evolutionary forces:

1. Mutation: Introduction of new alleles; rare but essential for long-term evolution.

2. Genetic Drift: Random changes in allele frequencies, especially significant in small populations.

3. Gene Flow: Migration introduces or removes alleles, increasing genetic variation within populations and decreasing it between populations.

4. Natural Selection: Differential survival and reproduction based on phenotype, affecting allele frequencies.

5. Nonrandom Mating: Changes genotype frequencies but not necessarily allele frequencies.

Mutation

• Source of new alleles in a population.

• Usually rare but provides the raw material for evolution.

Genetic Drift

• Random fluctuation in allele frequencies, more pronounced in small populations.

• Founder Effect: A few individuals start a new population, leading to reduced genetic variation.

• Bottleneck Effect: Population size is drastically reduced by a catastrophic event, causing loss of genetic diversity.

Example: If a population of 10 loses 5 individuals (all with blue eyes), the allele frequency can shift dramatically (p=1, q=0). In a population of 1000, the same event has a much smaller effect (p=0.503, q=0.497).

Natural Selection

• Acts on phenotypes, indirectly changing allele frequencies.

• Types of selection:

  • Directional: Favors one extreme phenotype.

  • Stabilizing: Favors average/intermediate phenotypes.

  • Disruptive: Favors both extreme phenotypes.

Gene Flow

• Movement of alleles between populations via migration.

• Increases genetic variation within populations, decreases variation between populations.

Hardy-Weinberg Principle

Definition and Conditions

The Hardy-Weinberg Principle states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.

• Conditions for equilibrium:

  1. Large population size

  2. Random mating

  3. No mutation

  4. No migration

  5. No natural selection

Hardy-Weinberg Equations

• Allele frequencies: $p + q = 1$

• Genotype frequencies:

  • Homozygous dominant: $p^2$

  • Heterozygous: $2pq$

  • Homozygous recessive: $q^2$

Worked Example: Hardy-Weinberg Calculations

If 36% of individuals show the recessive trait (aa):

1. Convert percent to proportion: 36% = 0.36

2. Find $q$: $q^2 = 0.36 \implies q = \sqrt{0.36} = 0.6$

3. Find $p$: $p = 1 - q = 0.4$

4. Genotype frequencies:

   • AA: $p^2 = 0.16$ (16%)

   • Aa: $2pq = 0.48$ (48%)

   • aa: $q^2 = 0.36$ (36%)

5. Check: $0.16 + 0.48 + 0.36 = 1.0$

Example: Scarlet Tiger Moth (Panaxia dominula)

Genotype	Count	Frequency
BB (homozygous dominant)	452	0.909
Bb (heterozygous)	43	0.087
bb (homozygous recessive)	2	0.004

Allele frequency calculations:

• $f(B) = \frac{(2 \times 452) + 43}{2 \times 497} = 0.95$

• $f(b) = \frac{(2 \times 2) + 43}{2 \times 497} = 0.05$

Genetic Drift: Population Size Effects

Population #1 (10 people)	Population #2 (1000 people)
5 BB, 5 bb p = q = 0.5 5 killed (all bb): p=1, q=0	500 BB, 500 bb p = q = 0.5 5 killed (all bb): p=0.503, q=0.497

Conclusion: Genetic drift is only significant in small populations.

Bottleneck Effect

A bottleneck occurs when a population is drastically reduced in size due to a catastrophic event (e.g., disease, natural disaster), leading to loss of genetic variation. Example: A population reduced to fewer than 10 individuals may lose much of its genetic diversity.

Gene Flow and Migration

Gene flow occurs when individuals migrate between populations, introducing new alleles and altering allele frequencies. This process can be modeled mathematically to predict changes in allele frequencies after migration.

Applications of Population Genetics

• Conservation Biology: Maintaining genetic diversity in endangered species.

• Human Genetics: Studying the frequency of disease alleles in populations.

• Agriculture: Breeding programs and development of resistance traits.

Additional info: These notes cover core concepts from Chapter 25: Population and Evolutionary Genetics, including the Hardy-Weinberg principle, evolutionary forces, and real-world applications.