Genetic drift
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Genetic drift is the term used in population genetics to refer to the stastical drift over time of gene frequencies in a population due to random sampling effects in the formation of successive generations. In a narrower sense, genetic drift refers to the expected population dynamics of neutral alleles (those defined as having no positive or negative impact on reproductive fitness), which are predicted to eventually become fixed at zero or 100% frequency in the absence of other mechanisms affecting allele distributions.
Whereas natural selection describes the tendency of beneficial alleles to become more common over time (and detrimental ones less common), genetic drift refers to the fundamental tendency of any allele to vary randomly in frequency over time due to statistical variation alone, so long as it does not comprise all or none of the distribution.
Genetic drift may be modeled as a statistically stochastic process that arises from the role of random sampling in the production of offspring. The genes of each new generation are not a simple copy of the genes of the successful members of the previous one, but rather a sampling, which includes some statistical error. Drift is the cumulative effect over time of this sampling error on the allele frequencies in the population.
By definition, genetic drift has no preferred direction. A neutral allele may be expected to increase or decrease in any given generation with equal probability. Given sufficiently long time, however, the mathematics of genetic drift (cf. random walk) predict the allele will either die out or be present in 100% of the population, after which time there is no random variation in the associated gene. In this regard, genetic drift tends to sweep gene variants out of a population over time, such that all members of a species would eventually be homozygous for this gene. Genetic drift is opposed in this regard by genetic mutation which introduces novel variants into the population according to its own random processes.
Like selection, genetic drift acts on populations, altering the frequency of alleles (gene variations) and the predominance of traits. Drift is observed most strongly in small populations and results in changes that need not be adaptive.
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Allele frequencies
From the perspective of population genetics, drift is a "sampling effect." To illustrate: on average, coins turn up heads or tails with equal probability. Yet just a few tosses in a row are unlikely to produce heads and tails in equal number. The numbers are no more likely to be exactly equal for a large number of tosses in a row, but the discrepancy in number can be very small (in percentage terms). As an example, ten tosses turn up at least 70% heads about once in every six tries, but the chance of a hundred tosses in a row producing at least 70% heads is only about one in 25,000.
Similarly, in a breeding population, if an allele has a frequency of p, probability theory dictates that (if natural selection is not acting) in the following generation, a fraction p of the population will inherit that particular allele. However, as with the coin toss above, allele frequencies in real populations are not probability distributions; rather, they are a random sample, and are thus subject to the same statistical fluctuations (sampling error).
When the alleles of a gene do not differ with regard to fitness, on average the number of carriers in one generation is proportional to the number of carriers in the previous generation. But the average is never tallied, because each generation parents the next one only once. Therefore the frequency of an allele among the offspring often differs from its frequency in the parent generation. In the offspring generation, the allele might therefore have a frequency p', slightly different from p. In this situation, the allele frequencies are said to have drifted. Note that the frequency of the allele in subsequent generations will now be determined by the new frequency p'.
As in the coin toss example above, the size of the breeding population (the effective population size) governs the strength of the drift effect. When the effective population size is small, genetic drift will be stronger.
Drifting alleles usually have a finite lifetime. As the frequency of an allele drifts up and down over successive generations, eventually it drifts until fixation - that is, it either reaches a frequency of zero, and disappears from the population, or it reaches a frequency of 100% and becomes the only allele in the population. Subsequent to the latter event, the allele frequency can only change by the introduction of a new allele by a new mutation.
The lifetime of an allele is governed by the effective population size. In a very small population, only a few generations might be required for genetic drift to result in fixation. In a large population, it would take many more generations. On average, an allele will be fixed in 4Ne generations, where Ne is the effective population size.
Drift versus selection
Genetic drift and natural selection rarely occur in isolation of each other; both forces are always at play in a population. However, the degree to which alleles are affected by drift and selection varies according to circumstance.
In a large population, where genetic drift occurs very slowly, even weak selection on an allele will push its frequency upwards or downwards (depending on whether the allele is beneficial or harmful). However, if the population is very small, drift will predominate. In this case, weak selective effects may not be seen at all as the small changes in frequency they would produce are overshadowed by drift.
Genetic drift in populations
Drift can have profound and often bizarre effects on the evolutionary history of a population. These effects may be at odds with the survival of the population.
In a population bottleneck, where the population suddenly contracts to a small size (believed to have occurred in the history of human evolution), genetic drift can result in sudden and dramatic changes in allele frequency that occur independently of selection. In such instances, many beneficial adaptations may be eliminated even if population later grows large again.
Similarly, migrating populations may see founder's effect, where a few individuals with a rare allele in the originating generation can produce a population that has allele frequencies that seem to be at odds with natural selection. Founder's effects are sometimes held to be responsible for high frequencies of some genetic diseases.
See also
External link
- Gene Expression - 'Population genetics notes' (only 1 migrant per generation between populations of any size can prevent divergence in allelic frequencies)
| Basic topics in evolutionary biology |
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| Processes of evolution: evidence - macroevolution - microevolution - speciation |
| Mechanisms: selection - genetic drift - gene flow - mutation |
| Modes: anagenesis - catagenesis - cladogenesis |
| History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis |
| Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics - evo-devo |
| List of evolutionary biology topics | Timeline of evolution | Timeline of human evolution |
| Topics in population genetics |
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| Key concepts: Hardy-Weinberg law | Fisher's fundamental theorem | neutral theory |
| Selection: natural | sexual | 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 |
