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, American Richard Lewontin and Italian Luigi Luca Cavalli-Sforza, and the Britons John Maynard Smith and W.D. Hamilton
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 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 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.
- Ecological genetics
- Fitness landscape
- Founder effect
- Genotype-phenotype distinction
- Hardy-Weinberg principle
- Molecular evolution
- Muller's ratchet
- Mutational meltdown
- Population bottleneck
- Small population size
- J. Beatty. 1986. "The synthesis and the synthetic theory" in Integrating Scientific Disciplines, edited by W. Bechtel and Nijhoff. Dordrecht.
- John Gillespie Population Genetics: A Concise Guide, Johns Hopkins Press, 1998 ISBN 0-8018-5755-4
- Daniel Hartl Primer of Population Genetics, 3rd edition, Sinauer, 2000 ISBN 0878933042
- Daniel Hartl and Andrew Clark Principles of Population Genetics, 3rd edition, Sinauer 1997 ISBN 0-87893-306-9
- Richard C. Lewontin. 1974. The Genetic Basis of Evolutionary Change. Columbia University Press. New York.
- History of population genetics
|Topics in population genetics
|Key concepts: Hardy-Weinberg law | Fisher's fundamental theorem | neutral theory
|Selection: natural | Censored page | artificial | ecological
|Genetic drift: small population size | population bottleneck | founder effect
|Founders: Ronald Fisher | J.B.S. Haldane | Sewall Wright
|Related topics: evolution | microevolution | evolutionary game theory | fitness landscape
|List of evolutionary biology topics
|Subfields of genetics
|Classical genetics | Ecological genetics | Molecular genetics | Population genetics | Quantitative genetics
|Related topics: Genomics | Reverse genetics
|Basic topics in evolutionary biology
|Processes of evolution: macroevolution - microevolution - speciation
|Mechanisms: selection - genetic drift - gene flow - mutation
|History: Charles Darwin - The Origin of Species - modern evolutionary synthesis
|Subfields: population genetics - ecological genetics - molecular evolution - phylogenetics - systematics - evo-devo
|List of evolutionary biology topics | Timeline of evolution