Selection Coefficient Calculator

Calculate selection coefficients, relative fitness, and allele frequency changes. Predict evolutionary outcomes and measure the strength of natural selection in populations

Calculation Mode

Fitness of AA (Homozygous Dominant)

Usually set to 1.0 as the reference

Fitness of Aa (Heterozygous)

Can be higher than 1.0 for overdominance

Fitness of aa (Homozygous Recessive)

Dominance Coefficient (h)

Determines heterozygote effect

Understanding Selection Coefficients

The selection coefficient (s) measures the strength of natural selection acting on a trait. It quantifies the relative disadvantage (or advantage) of one genotype compared to another. A selection coefficient of 0 means no selection (neutral evolution), while larger absolute values indicate stronger selection.

Key Concepts

Absolute Fitness (W): The average number of offspring produced by a genotype
Relative Fitness (w): Fitness scaled so the fittest genotype has w = 1.0
Selection Coefficient (s): s = 1 - w, where w is relative fitness
Dominance Coefficient (h): Determines the fitness of heterozygotes
- • h = 0: Complete dominance (Aa has same fitness as AA)
- • h = 0.5: Additive/no dominance (Aa intermediate)
- • h = 1: Complete recessiveness (Aa has same fitness as aa)

Fitness Relationships

Genotype	Absolute Fitness	Relative Fitness
AA	W_AA	1
Aa	W_Aa	1 - hs
aa	W_aa	1 - s

Types of Natural Selection

Directional Selection

• Favors one extreme phenotype
• Increases frequency of beneficial allele
• Example: Antibiotic resistance
• s > 0 for favored allele

Balancing Selection

• Maintains genetic variation
• Heterozygote advantage (overdominance)
• Example: Sickle cell anemia in malaria regions
• W_Aa > W_AA and W_aa

Purifying Selection

• Removes deleterious mutations
• Most common type of selection
• Maintains functional genes
• s < 0 for deleterious allele

Disruptive Selection

• Favors both extremes
• Selects against intermediate phenotypes
• Can lead to speciation
• Rare in nature

Interpreting Selection Strength

Weak Selection (|s| < 0.01)

Fitness difference less than 1%. Nearly neutral evolution. Changes occur slowly over many generations. Genetic drift may be more important than selection. Example: Synonymous mutations in genes.

Moderate Selection (0.01 < |s| < 0.1)

Fitness difference 1-10%. Selection is effective in large populations. Allele frequencies change over dozens to hundreds of generations. Example: Many polygenic traits, lactose tolerance.

Strong Selection (0.1 < |s| < 0.5)

Fitness difference 10-50%. Rapid evolutionary change. Beneficial alleles spread quickly; deleterious alleles removed efficiently. Example: Insecticide resistance, major disease resistance alleles.

Very Strong Selection (|s| > 0.5)

Fitness difference greater than 50%. Extremely rapid change. Lethal or semi-lethal mutations. Example: Homozygous lethal alleles (s = 1), severe genetic diseases.

Real-World Examples

Peppered Moths (Biston betularia)

During the Industrial Revolution, dark-colored moths had a selective advantage in polluted areas due to better camouflage on soot-darkened trees.

Selection coefficient: s ≈ 0.3-0.5 for light moths in polluted areas

Sickle Cell Anemia

Heterozygotes (HbA/HbS) have resistance to malaria while suffering minimal effects from sickle cell trait, demonstrating heterozygote advantage.

Selection coefficients: s_AA ≈ 0.1 (malaria deaths), s_aa ≈ 0.8 (severe anemia), w_Aa ≈ 1.0 (protected)

Antibiotic Resistance

Bacteria with resistance mutations have dramatically higher fitness in the presence of antibiotics, leading to rapid evolution of resistance.

Selection coefficient: s ≈ 0.9-1.0 for susceptible bacteria (effectively lethal in presence of antibiotic)

CCR5-Δ32 and HIV Resistance

A 32-base-pair deletion in the CCR5 gene provides resistance to HIV infection. Homozygotes are nearly immune; heterozygotes show delayed progression.

Selection coefficient: In HIV-endemic populations, s ≈ -0.2 for wild-type allele (beneficial for deletion)

References

Selection coefficient calculations are based on established population genetics theory:

Related Calculators

Allele Frequency Calculator Hardy-Weinberg Equilibrium Calculator Mutation Rate Calculator Population Growth Rate Calculator Genotype Ratio Calculator

Note: This calculator uses standard population genetics models. Real populations may experience multiple selective pressures, frequency-dependent selection, environmental variation, and gene interactions that affect actual fitness values. Selection coefficients can vary across environments, life stages, and genetic backgrounds.

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