Everything about Fitness Biology totally explained
Fitness (often denoted
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'll 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's 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 behaviour 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".
Measures of fitness
There are two commonly used measures of fitness; absolute fitness and relative fitness.
Absolute fitness (
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's 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 can't 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 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 wouldn't 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.
Yet another possible measure has been formulated by Hartl,1981: "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." Then, the
mean fitness may be determined as a mean over the set of individuals in a large population.
»
where N is the p. d. f. of phenotypes in the population, and m is its centre of gravity. This measure is a suitable basis of a model of an evolution selecting individuals. It may in principle take even the stroke of the lightning into consideration. In the case N is a Gaussian it's fairly easily proved that the
average information (
information entropy,
disorder, diversity) of a large population may be maximized by
Gaussian adaptation - keeping the mean fitness constant - in accordance with
recapitulation, the
central limit theorem, the
Hardy-Weinberg law and the
second law of thermodynamics. This is in contrast to
Fisher's fundamental theorem of natural selection.
History
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
1851 work
Social Statics and later used it to characterise what
Charles Darwin had called
natural selection. The British 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 Evolution of Social Behavior.
Further Information
Get more info on 'Fitness Biology'.
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