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Term 'fitness' is pretty obvious, but, often 'fitness effects' is commonly used in many research articles in the field of evolutionary biology.

I suspect that 'fitness effects' has a different meaning than just 'fitness'. I found that it's really difficult to find the distinction between the two terms just by googling.

So, if some expert in the subject is reading, please clarify.

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Fitness

Fitness is some function of the probability of survival and fecundity for a given genotype. Fitness is therefore a single value per genotype.

Fitness effect

A fitness effect is a component of fitness. For example, for a given bi-allelic locus, the fitness effect of allele A is -0.05 in comparison to the allele B, meaning that the allele A is deleterious and the allele B is beneficial. There is a fitness effect for each allele, for each trait and, if you interested in genetic interaction (epistasis) and genetic-environment interaction, then there can be a fitness effect for each gene and each trait in each genotype in each environment.

Example

It is typically common to assume a model of $n$ loci, where the fitness effects at each loci are $1$, $1-hs$ and $1-s$ for the three genotypes AA, AB and BB respectively (diploid population). $s$ is the selection coefficients and $h$ is the dominance coefficient. The fitness of the individual is often obtained by multiplying out the fitness effects at each loci. The worst possible fitness is therefore $\left(1-s\right)^n$.

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  • $\begingroup$ Fitness effect of an allele seems to be an 'effect size' which is calculated relative to fitness of other allele. $\endgroup$
    – user345394
    Commented Nov 29, 2017 at 5:05

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