Understand the relationship
The reasoning behind the result
A selection coefficient is relative to a reference
w = Wfocal/Wref
Absolute survival, fecundity or another life-cycle component becomes relative fitness only after division by a declared comparable reference. Changing the reference can change w and s without changing the observations.
The calculator never supplies a preferred genotype, environmental baseline or fitness component.
The sign follows the declared convention
s = 1 − w
With the reference normalized to one, w below one gives positive s and w above one gives negative s. The result is retained as signed arithmetic rather than relabelled as beneficial or harmful.
Some literature parameterizes an advantaged type as 1+s instead. The displayed equation prevents those conventions from being mixed silently.
Fitness components are stage and environment specific
Viability, mating success, offspring number and lifetime reproductive contribution are not interchangeable measurements. A coefficient derived from one interval does not automatically describe total fitness.
Frequency dependence, sex, age, density and environment can change the relevant comparison.
An entered contrast is not causal inference
Sampling error, linkage, population structure and experimental design remain outside this arithmetic. A difference in an entered component does not by itself estimate a selection coefficient acting in a natural population.
Time-series inference and genotype-frequency likelihood models require their own uncertainty and population assumptions.
Follow the numbers
Normalize two reproductive components
- The focal component is 3.6 viable offspring per parent.
- The reference component is 4.5 under the same observation rule.
- Relative fitness is w = 3.6/4.5 = 0.8.
- The signed coefficient is s = 1 − 0.8 = 0.2.
- The result describes this declared component and reference only.
A value of 0.2 is a normalized contrast under the displayed convention, not a universal property of the focal type.
Quick guide
How to use this calculator
- Declare one comparable fitness component and reference before entering values.
- Use relative-fitness mode only when normalization has already been justified; otherwise enter both components.
- Read the sign with the stated convention: positive s means the focal component is below the reference, while negative s means it is above.
Calculation method
Calculation and interpretation
Keep the reference type, fitness component and sign convention visible instead of treating a selection coefficient as a directly observed universal property.
Relative fitness w = Wfocal/Wref; signed selection coefficient s = 1 − w
Worked example
Normalize two reproductive components
A value of 0.2 is a normalized contrast under the displayed convention, not a universal property of the focal type.
Relative fitness w = Wfocal/Wref; signed selection coefficient s = 1 − w
Supported inputs
Precision and limits
Declared component only
The result inherits the entered life stage, environment, denominator and observation protocol and does not combine omitted fitness components.
Reference dependent
Changing the reference changes relative fitness and the signed coefficient.
No causal estimate
No sampling model, uncertainty interval, genotype-frequency time series, linkage model or treatment-effect inference is supplied.
No biological ranking
The sign does not establish adaptation, health, worth, persistence or an intervention decision.
Numerical support
Entered components support zero through 10¹², the reference component must be positive, and entered relative fitness supports zero through 10⁶. A positive derived ratio must remain representable; otherwise the calculation stops with an explicit range message rather than displaying biological zero.
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