Fitness Biology...
Fitness (often denoted in population genetics models) is a
central idea in evolutionary theory. It can be defined either with
respect to a genotype or to a phenotype in a given environment. In
either case, it describes the ability to both survive and reproduce,
and is equal to the average contribution to the gene pool of the
next generation that is made by an average individual of the
specified genotype or phenotype. If differences between alleles of a
given gene affect fitness, then the frequencies of the alleles will
change over generations; the alleles with higher fitness become more
common. This process is called natural selection.
An
individual’s fitness is manifested through its phenotype. The
phenotype is affected by the developmental environment as well as by
genes, and the fitness of a given phenotype can be different in
different environments. The fitnesses of different individuals with
the same genotype are therefore not necessarily equal. However, since
the fitness of the genotype is an averaged quantity, it will reflect
the reproductive outcomes of all individuals with that genotype in a
given environment or set of environments.
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. 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.” [1] Equivalently, “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.”[2]
There are two commonly used measures of fitness; absolute fitness and relative fitness.
Absolute fitness () of a genotype is defined as the ratio between
the number of individuals with that genotype after selection to
those before selection. It is calculated for a single generation and
must be calculated from absolute numbers. When the absolute fitness
is larger than 1, the number of individuals bearing that genotype
increases; an absolute fitness smaller than 1 indicates an absolute
fall in the number of individuals bearing the genotype. If the number
of individuals in a population stays constant, then the average
absolute fitness must be equal to 1.
Absolute fitness for a
genotype can also be calculated as the product of the probability of
survival multiplied by the average fecundity. Absolute fitness is
used in Fisher’s fundamental theorem.
Relative fitness is
quantified as the average number of surviving progeny of a
particular genotype compared with average number of surviving progeny
of competing genotypes after a single generation, i.e. one genotype
is normalized at and the fitnesses of other genotypes are
measured with respect to that genotype. Relative fitness can
therefore take any non negative value, including 0. Relative fitness
is used in the standard Wright-Fisher and Moran models of
population genetics.
The two concepts are related, as can be
seen by dividing each by the mean fitness, which is weighted by
genotype frequencies.
The British sociologist Herbert
Spencer coined the phrase “survival of the fittest” (though
originally, and perhaps more accurately, “survival of the best
fitted”) in his 1864 work Principles of Biology to characterise what
Charles Darwin had called natural selection.
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