Fitness (often denoted w in population genetics models) is a
 
central concept in evolutionary theory. It describes the
 
capability of an individual of certain genotype to reproduce,
 
and usually is equal to the proportion of the individual's genes
 
in all the genes of the next generation. If differences in

individual genotypes affect fitness, then the frequencies of the
 
genotypes will change over generations; the genotypes with
 
higher fitness become more common. This process is called
 
natural selection.

An individual's fitness is manifested through its phenotype. As
 
phenotype is affected by both genes and environment, the
 
fitnesses of different individuals with the same genotype are

not necessarily equal, but depend on the environment in which
 
the individuals live. However, since the fitness of the genotype
 
is an averaged quantity, it will reflect the reproductive outcomes

of all individuals with that genotype.

As fitness measures the quantity of the copies of the genes of an

individual in the next generation, it doesn't really matter how the
 
genes arrive in the next generation. That is, for an individual it is

equally "beneficial" to reproduce itself, or to help relatives with
 
similar genes to reproduce, as long as similar amount of copies

of individual's genes get passed on to the next generation. Selection

which promotes this kind of helper behavior is called kin selection.

The concept is particularly difficult to understand and frequently

misunderstood; J.B.S. Haldane when discussing it with John Maynard
 
Smith is reported to have described it as "a bugger".

Because fitness is a coefficient, and a variable may be multiplied by
 
it several times, biologists may work with "log fitness" (particularly

so before the advent of computers). By taking the logarithm of
 
fitness each term may be added rather than multiplied. A fitness
 
landscape, first conceptualized by Sewall Wright, is a way of

visualising fitness in terms of a three-dimensional surface on

which peaks correspond to local fitness maxima; it is often said

that natural selection always progresses uphill but can only do

so locally. This can result in suboptimal local maxima becoming

stable, because natural selection cannot return to the less-fit

"valleys" of the landscape on the way to reach higher peaks.

The related concept of genetic load measures the overall fitness

of a population of individuals of many genotypes whose fitnesses

vary, relative to a hypothetical population in which the most fit

 genotype has become fixed.

As another example we may mention the definition of fitness given

 by Maynard Smith in the following way: ”Fitness is a property, not

of an individual, but of a class of individuals – for example homozy

gous 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.” This measure is certainly useful in

breeding programs, but hardly as a basis of a model of an evolution

selecting individuals, because evolution would hardly know if the

individual may be selected or not.