Population genetics is the study of the allele frequency distribution and change under the influence of the four evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes account of population subdivision and population structure in space. As such, it 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 R. A. Fisher, who also laid the foundations for the related discipline of quantitative genetics.

Scope and theoretical considerations

The framework of mathematical population genetics is an important achievement of the modern evolutionary synthesis. According to Beatty (1986), for example, it defines the core of the modern synthesis.

According to Lewontin (1974) the theoretical task for population genetics is a process in 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 genotypes (G₁) to a phenotype 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 of molecular genetics, this is clearly a gargantuan task. Visualizing this transformation schematically:

$G\_1\; \backslash ;\; \backslash stackrel\{T\_1\}\{\backslash rightarrow\}\; \backslash ;\; P\_1\; \backslash ;\; \backslash stackrel\{T\_2\}\{\backslash rightarrow\}\; \backslash ;\; P\_2\; \backslash ;\; \backslash stackrel\{T\_3\}\{\backslash rightarrow\}\; \backslash ;\; G\_2\; \backslash ;$

\stackrel{T_4}{\rightarrow} \; G_1' \; \rightarrow \cdots

(adapted from Lewontin 1974, p. 12). XD

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. Proceeding as if we do understand it is enough to analyze many cases of interest. For example, if the phenotype is almost one-to-one with genotype (sickle-cell disease) or the time-scale is sufficiently short, the "constants" can be treated as such; however, there are many situations where it is inaccurate.

Genetic structure

Because of physical barriers to migration, along with limited vagility, and natal philopatry, natural populations are rarely panmictic (Buston et al., 2007). There is usually a geographic range within which individuals are more closely related to one another than those randomly selected from the general population. This is described as the extent to which a population is genetically structured (Repaci et al., 2007).

Population geneticists

The three founders of population genetics were the Britons R.A. Fisher and J.B.S. Haldane and the American Sewall Wright. Fisher and Wright had some fundamental disagreements and a controversy about the relative roles of selection and drift continued for much of the century between the Americans and the British. The Frenchman Gustave Malécot was also important early in the development of the discipline. John Maynard Smith was Haldane's pupil, whilst W.D. Hamilton was heavily influenced by the writings of Fisher. The American George R. Price worked with both Hamilton and Maynard Smith. American Richard Lewontin and Japanese Motoo Kimura were heavily influenced by Wright.

See also

References

• J. Beatty. "The synthesis and the synthetic theory" in Integrating Scientific Disciplines, edited by W. Bechtel and Nijhoff. Dordrecht, 1986.
• Buston, PM; et al. (2007). "Are clownfish groups composed of close relatives? An analysis of microsatellite DNA vraiation in Amphiprion percula". Molecular Ecology 12: 733-742. 
• Luigi Luca Cavalli-Sforza. Genes, Peoples, and Languages. North Point Press, 2000.
• Luigi Luca Cavalli-Sforza et al. The History and Geography of Human Genes. Princeton University Press, 1994.
• James F. Crow and Motoo Kimura. Introduction to Population Genetics Theory. Harper & Row, 1972.
• Warren J Ewens. Mathematical Population Genetics. Springer-Verlag New York, Inc., 2004. ISBN 0-387-20191-2
• John H. Gillespie Population Genetics: A Concise Guide, Johns Hopkins Press, 1998. ISBN 0-8018-5755-4.
• Richard Halliburton. Introduction to Population Genetics. Prentice Hall, 2004
• Daniel Hartl. Primer of Population Genetics, 3rd edition. Sinauer, 2000. ISBN 0-87893-304-2
• Daniel Hartl and Andrew Clark. Principles of Population Genetics, 3rd edition. Sinauer, 1997. ISBN 0-87893-306-9.
• Richard C. Lewontin. The Genetic Basis of Evolutionary Change. Columbia University Press, 1974.
• William B. Provine. The Origins of Theoretical Population Genetics. University of Chicago Press. 1971. ISBN 0-226-68464-4.
• Repaci, V; Stow AJ, Briscoe DA (2007). "Fine-scale genetic structure, co-founding and multiple mating in the Australian allodapine bee (Ramphocinclus brachyurus". Journal of Zoology 270: 687-691. doi:10.1111/j.1469-7998.2006.00191.x. 
• Spencer Wells. The Journey of Man. Random House, 2002.
• Spencer Wells. Deep Ancestry: Inside the Genographic Project. National Geographic Society, 2006.
• Cheung, KH; Osier MV, Kidd JR, Pakstis AJ, Miller PL, Kidd KK (2000). "ALFRED: an allele frequency database for diverse populations and DNA polymorphisms". Nucleic Acids Research 28 (1): 361–3. doi:10.1093/nar/28.1.361. PMID 10592274. 

External links

• Yale University
• EHSTRAFD.org - Earth Human STR Allele Frequencies Database
• History of population genetics
• National Geographic: Atlas of the Human Journey (Haplogroup-based human migration maps)
• Monash Virtual Laboratory - Simulations of habitat fragmentation and population genetics online at Monash University's Virtual Laboratory.
• Nordic and Celtic DNA Project (Saami & Iberian).

Population genetics is the branch of genetics which studies the genetic composition of populations.^[1] It brings together genetics, evolution, natural selection, breeding, statistics and mathematics.^[2] Mathematical and computer models are produced, and field research is done to test the models.

"Population geneticists spend most of their time doing one of two things: describing the genetic structure of populations, or theorizing on the evolutionary forces acting on populations.^[3]

A brief history

Starting, perhaps, with G. Udny Yule's paper in 1902,^[4] population theorists tackled key issues in genetics and evolution. G.H. Hardy and Wilhelm Weinberg showed that if a population had random mating, no selection, migration or mutation, then the proportion of alleles would remain the same generation after generation. This was the Hardy–Weinberg law.^[5] the first great result of this new field of research.

Population genetics made great progress from 1918 to 1937. During this period, Ronald Fisher, J.B.S. Haldane and Sewall Wright worked on the connection between evolution and genetics, using new mathematical techniques, such as statistical probability. E.B. Ford and Theodosius Dobzhansky did field research on the genetics of natural populations of lepidoptera and Drosophila, respectively. Broadly speaking, this work proved that the newly rediscovered Mendelian genetics could be reconciled with Darwinian evolution. This laid the groundwork for the modern evolutionary synthesis, which took place in the following years, from about 1937 to 1953.

In the second half of the 20th century, population geneticists tackled a range of complex evolutionary problems, such as the evolution of sex, sexual selection, kin selection (altruism), mimicry and molecular evolution. The key figures included John Maynard Smith, Motoo Kimura and William Hamilton.

References