Fitness Calculators

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In evolutionary biology, fitness is the reproductive success of an individual (or genotype) relative to others in a population — specifically, the contribution an individual makes to future generations through its offspring. Absolute fitness (W) = average number of offspring produced by an individual (or genotype). Relative fitness (w) = W of a genotype / W of the most fit genotype (so maximum w = 1). Selection coefficient (s) = 1 − w (where 0 = neutral; 1 = lethal). Natural selection acts by changing allele frequencies in proportion to differences in relative fitness. Inclusive fitness extends fitness to include the reproductive success of genetic relatives.

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Fitness Concepts

Absolute fitness (W): average number of offspring an individual leaves in the next generation. Relative fitness (w): W / W_max (normalized to the most fit genotype = 1.0). Selection coefficient (s): s = 1 − w; 0 = neutral; 1 = lethal. Example: genotype AA produces 10 offspring; Aa produces 9; aa produces 6. W_max = 10 (AA). Relative fitness: w_AA = 1.00; w_Aa = 0.90; w_aa = 0.60. s_AA = 0; s_Aa = 0.10; s_aa = 0.40.

Components of Fitness

  • Viability: survival to reproductive age
  • Fecundity: number of offspring produced
  • Mating success: probability of obtaining mates (important in sexual selection)
  • Offspring quality: how well offspring survive and reproduce (parental investment)

Inclusive Fitness (Hamilton)

W_inclusive = direct fitness + indirect fitness. Indirect fitness = fitness gained through success of relatives weighted by relatedness (r). Hamilton's rule: altruistic behavior evolves when r × B > C. r = coefficient of relatedness; B = benefit to recipient; C = cost to actor. Explains altruism in eusocial insects (workers, full sisters share 75% of genes → indirect fitness from queen's reproduction).

Fitness Landscape

Sewall Wright's fitness landscape: peaks (high fitness) and valleys (low fitness); evolution moves toward fitness peaks; genetic drift can allow crossing valleys to higher peaks.

Glossary

Relative Fitness (w)
W / W_max; normalized reproductive success where the most fit genotype = 1.0; selection coefficient s = 1 − w; determines the rate at which allele frequencies change under natural selection.
Inclusive Fitness
Direct fitness + indirect fitness (via relatives); Hamilton's rule: altruism evolves when r×B > C; r = relatedness; explains kin selection and eusocial insect behavior.
Fitness Landscape
A conceptual model of genotype × fitness relationships (Sewall Wright); selection drives populations toward peaks; drift can allow crossing valleys; explains local adaptation and epistasis.

Frequently Asked Questions

Evolutionary fitness = the reproductive success of an individual or genotype relative to others, measured by the number of viable offspring that survive to reproduce. Absolute fitness (W): average number of offspring an individual leaves = (probability of surviving to reproduce) × (average fecundity). Relative fitness (w): W of a genotype / W of the most fit genotype → normalized so maximum = 1.0. Selection coefficient (s) = 1 − w: measures how much less fit a genotype is compared to the most fit. Fitness determines which alleles increase in frequency over generations — genotypes with higher fitness leave more offspring → their alleles increase. Fitness is context-dependent: a genotype favored in one environment may be less fit in another (frequency-dependent selection, G×E interactions).

Natural selection occurs when genotypes differ in fitness. Directional selection: one allele consistently increases fitness → that allele increases in frequency over generations. Rate of change: Δp ≈ s × p × q (for a completely dominant favored allele). A mutation with s = 0.01 takes approximately 4,600 generations to go from 1% to 99% frequency. Dominance matters: fully dominant beneficial allele (only AA and Aa selected): increases in frequency rapidly when rare; mostly homozygous by selection. Recessive beneficial allele (only aa selected): very slow increase when rare (because most copies are in Aa heterozygotes, which aren't selected).

Inclusive fitness (W. D. Hamilton, 1964) extends the concept of fitness to include indirect fitness — reproductive success gained through the success of genetic relatives: W_inclusive = direct fitness (own reproduction) + indirect fitness (success of relatives × degree of relatedness). Why: you share genes with relatives; an allele that benefits a relative who carries that allele is also 'successful' from the gene's perspective. Hamilton's rule: altruistic behavior evolves when r × B > C. r = coefficient of relatedness (parent-offspring = 0.5; full siblings = 0.5; half-siblings = 0.25). B = benefit to recipient (increase in recipient's fitness). C = cost to actor (decrease in actor's fitness). Example: a worker bee (r = 0.75 with sisters in haplodiploidy) sacrifices own reproduction to help queen produce sisters → r×B >> C → kin selection drives eusociality.

The adaptive fitness landscape (Sewall Wright, 1932) visualizes fitness as a topographic surface: Axes: allele frequencies or genotype space (the 'genetic landscape'). Height: average fitness of that genotype or population. Peaks: high-fitness combinations; Valleys: low-fitness combinations. Evolutionary dynamics: natural selection 'pushes' populations up the fitness landscape toward local peaks. Problem: populations can get trapped at local peaks that are not the global maximum. Genetic drift (especially in small populations) can allow populations to wander across valleys to reach higher peaks. The fitness landscape is not static — it changes with the environment, which can shift peaks. Modern extensions: NK model, empirical fitness landscapes from experimental evolution; used to understand epistasis and evolvability.