Fitness (often denoted
w
With asexual reproduction, it is sufficient to assign fitnesses to genotypes. With sexual reproduction, recombination scrambles alleles into different genotypes every generation; in this case, fitness values can be assigned to alleles by averaging over possible genetic backgrounds. Natural selection tends to make alleles with higher fitness more common over time, resulting in Darwinian evolution.
The term "Darwinian fitness" can be used to make clear the distinction with physical fitness.[1] Fitness does not include a measure of survival or life-span; Herbert Spencer's well-known phrase "survival of the fittest" should be interpreted as: "Survival of the form (phenotypic or genotypic) that will leave the most copies of itself in successive generations."
Inclusive fitness differs from individual fitness by including the ability of an allele in one individual to promote the survival and/or reproduction of other individuals that share that allele, in preference to individuals with a different allele. To avoid double counting, inclusive fitness excludes the contribution of other individuals to the survival and reproduction of the focal individual. One mechanism of inclusive fitness is kin selection.
Fitness is often defined as a propensity or probability, rather than the actual number of offspring. For example, according to Maynard Smith, "Fitness is a property, not of an individual, but of a class of individuals—for example homozygous for allele A at a particular locus. Thus the phrase 'expected number of offspring' means the average number, not the number produced by some one individual. If the first human infant with a gene for levitation were struck by lightning in its pram, this would not prove the new genotype to have low fitness, but only that the particular child was unlucky."[2]
Alternatively, "the fitness of the individual—having an array x of phenotypes—is the probability, s(x), that the individual will be included among the group selected as parents of the next generation."[3]
In order to avoid the complications of sex and recombination, the concept of fitness is presented below in the restricted setting of an asexual population without genetic recombination. Thus, fitnesses can be assigned directly to genotypes. There are two commonly used operationalizations of fitness – absolute fitness and relative fitness.
The absolute fitness (
W
n(t)
t
n(t+1)=Wn(t)
An absolute fitness larger than 1 indicates growth in that genotype's abundance; an absolute fitness smaller than 1 indicates decline.
Whereas absolute fitness determines changes in genotype abundance, relative fitness (
w
N(t)
t
p(t)=n(t)/N(t)
| p(t+1)= | w |
| \overline{w |
where
\overline{w}
Absolute fitnesses can be used to calculate relative fitness, since
p(t+1)=n(t+1)/N(t+1)=(W/\overline{W})p(t)
N(t+1)=\overline{W}N(t)
\overline{W}
w/\overline{w}=W/\overline{W}
W/\overline{W}
N(t)
Assigning relative fitness values to genotypes is mathematically appropriate when two conditions are met: first, the population is at demographic equilibrium, and second, individuals vary in their birth rate, contest ability, or death rate, but not a combination of these traits.[5]
The change in genotype frequencies due to selection follows immediately from the definition of relative fitness,
\Deltap=p(t+1)-p(t)=
| w-\overline{w | |
Thus, a genotype's frequency will decline or increase depending on whether its fitness is lower or greater than the mean fitness, respectively.
In the particular case that there are only two genotypes of interest (e.g. representing the invasion of a new mutant allele), the change in genotype frequencies is often written in a different form. Suppose that two genotypes
A
B
wA
wB
p
1-p
\overline{w}=wAp+wB(1-p)
\Deltap=
| w-\overline{w | |
Thus, the change in genotype
A
B
A
B
s
wA=(1+s)wB
\Deltap=
| w-\overline{w | |
where the last approximation holds for
s\ll1
The British sociologist Herbert Spencer coined the phrase "survival of the fittest" in his 1864 work Principles of Biology to characterise what Charles Darwin had called natural selection.[6]
The British-Indian biologist J.B.S. Haldane was the first to quantify fitness, in terms of the modern evolutionary synthesis of Darwinism and Mendelian genetics starting with his 1924 paper A Mathematical Theory of Natural and Artificial Selection. The next further advance was the introduction of the concept of inclusive fitness by the British biologist W.D. Hamilton in 1964 in his paper on The Genetical Evolution of Social Behaviour.
See main article: Genetic load. Genetic load measures the average fitness of a population of individuals, relative either to a theoretical genotype of optimal fitness, or relative to the most fit genotype actually present in the population.[7] Consider n genotypes
A1...An
w1...wn
p1...pn
L
L={{wmax-\barw}\overwmax}
Genetic load may increase when deleterious mutations, migration, inbreeding, or outcrossing lower mean fitness. Genetic load may also increase when beneficial mutations increase the maximum fitness against which other mutations are compared; this is known as the substitutional load or cost of selection.