Which Best Describes the Relationship Between Evolution and Natural Selection

Fitness
(often denoted

w

in population genetics models) is a fundamental concept in evolutionary theory. Information technology describes the capability of an individual of certain genotype to reproduce, and ordinarily is equal to the proportion of the individual’s genes in all the genes of the adjacent generation. If differences in individual genotypes touch on fitness, then the frequencies of the genotypes will modify over generations; the genotypes with college fitness become more common. This process is called natural pick.

An individual’s fitness is manifested through its phenotype. As phenotype is affected past both genes and environment, the fitnesses of dissimilar individuals with the aforementioned genotype are not necessarily equal, but depend on the environment in which the individuals alive. Even so, since the fitness of the genotype is an averaged quantity, it volition 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 side by side generation. That is, for an private information technology is as “beneficial” to reproduce itself, or to help relatives with similar genes to reproduce, as long as similar amount of copies of private’s genes get passed on to the next generation. Option which promotes this kind of helper behaviour is called kin option.

Measures of fitness

There are two commonly used measures of fettle; accented fitness and relative fitness.

Absolute fettle

(
w
_abs
) of a genotype is defined as the ratio between the number of individuals with that genotype afterward selection to those before selection. Information technology is calculated for a single generation and may be calculated from absolute numbers or from frequencies. When the fitness is larger than ane.0, the genotype increases in frequency; a ratio smaller than 1.0 indicates a subtract in frequency.

Accented fitness for a genotype can also be calculated as the product of the proportion survival times the average fecundity.

Relative fitness

is quantified equally the boilerplate number of surviving progeny of a particular genotype compared with average number of surviving progeny of competing genotypes afterwards a single generation, i.east. ane genotype is normalized at

w
= one
and the fitnesses of other genotypes are measured with respect to that genotype. Relative fitness can therefore accept whatever nonnegative value, including 0.

While researchers can usually mensurate absolute fitness, relative fitness is more difficult. It is often difficult to make up one’s mind how many individuals of a genotype there were immediately afterward reproduction.

The ii concepts are related, and both of them are equivalent when they are divided past the mean fitness, which is weighted by genotype frequencies.

This leads to the well known Fisher’s fundamental theorem of natural selection. Fisher’s theorem states that:
“The charge per unit of increase in the mean fitness of any organism at any time ascribable to natural selection acting through changes in cistron frequencies is exactly equal to its genic variance in fitness at that fourth dimension”.

This may be somewhat dubious because selection takes place on the individual level, ruling the enrichment of genes (Mayr 2001). In addition, co-ordinate to Maynard Smith, a population may reach a state of selective equilibrium, in which case the increase of mean fettle is equal to cipher, but not necessarily the variance in fettle.

Considering fettle is a coefficient, and a variable may be multiplied by information technology several times, biologists may work with “log fitness” (especially 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 fashion of visualising fitness in terms of a iii-dimensional surface on which peaks correspond to local fitness maxima; it is ofttimes said that natural selection always progresses uphill but can merely 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 fashion to achieve 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 case
nosotros may mention the definition of fitness given by Maynard Smith in the following way: ”Fitness is a holding, non of an individual, merely of a form of individuals – for instance homozygous for allele A at a particular locus. Thus the phrase ’expected number of offspring’ means the average number, non the number produced by some one individual. If the first human infant with a gene for levitation were struck past lightning in its pram, this would not evidence the new genotype to have low fitness, just only that the particular child was unlucky.” This mensurate is certainly useful in breeding programs, but hardly as a basis of a model of an development selecting individuals, considering evolution would hardly know if the individual may be selected or non.

Yet another possible measure out
has been formulated past Hartl,1981: “The fitness of the individual – having an array x of phenotypes – is the probability, s(10), that the individual volition be included among the grouping selected as parents of the next generation.” Then, the hateful fettle may be determined every bit a hateful over the set of individuals in a big population.

where N is the p. d. f. of phenotypes in the population, and m is its centre of gravity. This measure out 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 instance N is a Gaussian it is fairly easily proved that the average information (information entropy, disorder, diversity) of a big population may exist maximized past Gaussian adaptation – keeping the hateful fitness abiding – in accordance with recapitulation, the central limit theorem, the Hardy-Weinberg constabulary and the 2nd police force of thermodynamics. This is in contrast to Fisher’s cardinal theorem of natural option.

History

The British sociologist Herbert Spencer coined the phrase “survival of the fittest” (though originally, and mayhap more accurately, “survival of the best fitted”) in his 1851 piece of work
Social Statics
and later used information technology to characterise what Charles Darwin had called natural selection. The British biologist J.B.Due south. Haldane was the starting time 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 Bogus 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 Development of Social Behavior.

References

• Haldane, J.B.S. (1924) “A mathematical theory of natural and artificial selection” Role 1
  Transactions of the Cambridge philosophical society: 23: xix-41 link (pdf file)
• Hamilton, W.D. (1964) “The evolution of social beliefs”
  Journal of Theoretical Biology
  i:…
• Hartl, D. L. A Primer of Population Genetics. Sinauer, Sunderland, Massachusetts, 1981.
• Maynard Smith, J. Evolutionary Genetics. Oxford Academy Press, 1998.

Further reading

• Sober, East. (2001). The Ii Faces of Fitness. In R. Singh, D. Paul, C. Krimbas, and J. Beatty (Eds.),
  Thinking nigh Evolution: Historical, Philosophical, and Political Perspectives. Cambridge University Press, pp.309-321. Total text

Come across likewise

• Gene-centered view of evolution
• Inclusive fitness
• Natural pick
• Reproductive success