Selection Coefficient and Relative Fitness Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The selection coefficient is calculated with s = 1 − W, where W (relative fitness) is the ratio between the number of viable offspring of the studied genotype and that of the best-performing reference genotype.
Explanation
In population genetics, relative fitness (denoted W) measures a genotype's reproductive success compared to the best-performing genotype in the population, which is conventionally assigned a fitness of 1. The selection coefficient s, complementary to W (s = 1 − W), quantifies how strongly natural selection disfavors a given genotype: an s close to 0 signals a negligible difference in reproductive success, while an s close to 1 signals a nearly counter-selected genotype (very few viable offspring), up to a lethal genotype where s=1. This coefficient complements the inbreeding coefficient calculator, which measures the probability that an individual inherits two identical copies of an allele through common ancestry — a distinct genetic phenomenon from selection, though it can amplify selection's effects by reducing available genetic diversity. The selection coefficient is a central parameter in evolutionary models that predict how fast an allele's frequency changes in a population over generations: the higher s is for a disadvantageous allele, the faster its frequency drops under natural selection, unless offset by other evolutionary forces like mutation or genetic drift. This is exactly the assumption relaxed by the Hardy-Weinberg equilibrium calculator, which models allele frequencies in the theoretical absence of selection, mutation, migration, and drift.
Example: 65 viable offspring out of a maximum of 100
Inputs
Viable offspring of the studied genotype: 65. Viable offspring of the reference genotype: 100.
Calculation
Relative fitness W = 65 ÷ 100 = 0.65. Selection coefficient s = 1 − 0.65 = 0.35.
Result
This genotype has a relative fitness of 0.65 and a selection coefficient of 0.35: it is moderately disadvantaged compared to the reference genotype.
Frequently asked questions
How is the reference genotype chosen?
The reference genotype is conventionally the one producing the most viable offspring in the studied population: its relative fitness is set to 1 by definition, and all other genotypes are compared to it. This choice is relative to the studied population and environmental conditions, not a universal absolute value.
Does a high selection coefficient mean the genotype will disappear quickly?
A high s means natural selection strongly disfavors this genotype each generation, but the actual rate of disappearance also depends on other factors like the allele's mode of inheritance (dominant, recessive, sex-linked), population size, and possible reintroduction of the allele through mutation. An s=1 (lethal genotype) eliminates the allele in one generation if it's dominant, but a deleterious recessive allele can persist for a long time in heterozygous form, protected from selection.
What is the difference between the selection coefficient and the inbreeding coefficient?
These are two distinct genetic measures: the selection coefficient quantifies a reproductive disadvantage tied to the genotype itself, while the inbreeding coefficient measures the probability that an individual inherits two identical alleles through common descent from its parents, with no direct link to that genotype's reproductive performance.