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Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation and migration. It also takes account of population subdivision and population structure in space. As such, it is the theory that attempts to explain such phenomena as adaptation and speciation.Population genetics was a vital ingredient in the modern evolutionary synthesis, its primary founders were Sewall Wright, J. B. S. Haldane and Ronald Fisher, who also laid the foundations for the related discipline of quantitative genetics. Notable population geneticists of the mid-to-late 20th century include Japanese Motoo Kimura, AmericanThe United States of America also referred to as the United States U. America ¹ or the States is a federal republic in central North America, stretching from the Atlantic in the east to the Pacific Ocean in the west. It shares land borders with Canada in Richard LewontinRichard Charles Lewontin (born March 29, 1929) is an evolutionary biologist, geneticist and social commentator at Harvard University. A leader in developing the mathematical basis of population genetics and evolutionary theory, he pioneered the notion of and ItalianThe Italian Republic or Italy ( Italian: Italia is a country in the south of Europe, consisting mainly of a boot-shaped peninsula together with two large islands in the Mediterranean Sea: Sicily and Sardinia. To the north, where it borders France, Switzer Luigi Luca Cavalli-SforzaThe classification into races has proved to be a futile exercise. Luigi Luca Cavalli-Sforza (born January 25, 1922) is an Italian population geneticist born in Genoa, and currently teaching since 1970 as emeritus professor at Stanford University. One of t, and the Britons John Maynard SmithProfessor John Maynard Smith 1, F. 6 January 1920 19 April 2004) was a classical geneticist and leading theorist in evolutionary biology, particularly population genetics. His contributions also included work as an aeronautical engineer, and as a game the and W.D. Hamilton
1 Scope and theoretical considerations
Perhaps the most significant "formal" achievement of the modern evolutionary synthesis has been the framework of mathematical population genetics. Indeed some authors (Beatty 1986) would argue that it defines core of the modern synthesis.
Lewontin (1974) outlined the theoretical task for population genetics. He imagined two spaces: a "genotypic space" and a "phenotypic space". The challenge of a complete theory of population genetics is to provide a set of laws that predictably map a population of genotypeThe genotype is the specific genetic makeup (the specific genome) of an individual, usually in the form of DNA. It codes for the phenotype of that individual. Typically, one refers to an individual's genotype with regard to a particular gene of interest as (G1) to a phenotypeThe phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. Phenotype is determined to some extent by genotype, or by space (P1), where selection takes place, and another set of laws that map the resulting population (P2) back to genotype space (G2) where Mendelian genetics can predict the next generation of genotypes, thus completing the cycle. Even Leaving aside for the moment the non-Mendelian aspects revealed by molecular genetics, this is clearly a gargantuan task. Visualizing this transformation:
- G1 →T1 P1 →T2 P2 →T3 G2 →T4 G1'... (adapted from Lewontin 1974, p. 12)
T1 represents the genetic and epigenetic laws, the aspects of functional biology, or development, that transform a genotype into phenotype. We will refer to this as the " genotype-phenotype map". T2 is the transformation due to natural selection, T3 are epigenetic relations that predict genotypes based on the selected phenotypes and finally T4 the rules of Mendelian genetics.
In practice, there are two bodies of evolutionary theory that exist in parallel, traditional population genetics operating in the genotype space and the biometric theory used in plant and animal breeding , operating in phenotype space. The missing part is the mapping between the genotype and phenotype space. This leads to a "sleight of hand" (as Lewontin terms it) whereby variables in the equations of one domain, are considered parameters or constants, where, in a full-treatment they would be transformed themselves by the evolutionary process and are in reality functions of the state variables in the other domain. The "sleight of hand" is assuming that we know this mapping, and it is certainly true that it is sufficient to proceed as if we do understand it, to analyze many cases of interest. For example, if the phenotype is almost one-to-one with genotype ( sickle-cell anemia) or the time-scale is sufficiently short, the "constants" can be treated as such; however, there are many situations where it is inaccurate.
