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Genetic drift

Genetic drift is a mechanism of evolution that biologists believe acts in concert with natural selection to change species over time. It is a statistical phenomenon that results from the effect of chance on the birth, survival and reproduction of individuals, and is most prone to occur in small populations. The effect causes a trait, and the allele that confers it, to become more common or more rare over successive generations. Ultimately, a drifting allele either disappears from the gene pool or becomes the sole existing version of a gene.

Chance affects the commonness or rarity of an allele, because no trait guarantees survival or a particular number of offspring. Instead these outcomes depend on factors like the weather and being in the wrong place at the wrong time. In other words, even when individuals face the same odds, they will differ in their success. Through a rare succession of chance events, a trait may become universal, causing a population or a species to evolve.

A key aspect of genetic drift is that its rate is expected to depend strongly on population size. When many individuals carry a particular allele, and when all face equal odds, the number of offspring they collectively produce will rarely differ from the expected value: The average per individual times the number of individuals. But with a small number of individuals, a lucky break for one or two causes a disproportionately greater deviation from the expected result. Small populations therefore drift more rapidly than large ones. A manifestation of the "law of large numbers," this dependency of drift on population size is one explanation for the so-called founder's effect, a proposed mechanism of speciation. It also explains effects of population bottlenecks.

Drift and selection can act in parallel on traits that affect the odds of survival and average rates of reproduction. Even as a disease or other pressure favors the carriers of traits that help to cope, runs of luck can produce unexpected trends. Particularly when a population is small or when a trait confers only a tiny advantage, the carriers of even an advantageous trait may dwindle in number. Under such circumstances, the carriers of a deleterious or neutral trait become more common.

Drift is unlike selection in applying even to neutral characteristics. In fact, theorists sometimes appeal to drift to account for evolutionary changes that might have paved a way for adaptations, and yet offered no obvious immediate advantage. On the other hand, the inevitability of drift creates an ambiguity. How does one know whether a characteristic that all the individuals of a species possess--yellow spots, for example--represents an adaptation to selective conditions, or represents an accident? It can be difficult to say, and indeed to a degree the answer may be "both." Teasing apart the relative influence that drift and selection have had over the course of evolution, both with respect to individual species as well as with regard to the history of life in general, is a primary aim of evolutionary biology.

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Chance and sexual reproduction

In populations of sexual organisms, another major source of drift exists. Imagine that the population consists of two people, and each has an allele for brown eyes and an allele for blue eyes. In this generation, the frequency of the brown allele is 50%, as is the frequency for the blue allele. If they mate, they could have one child with two alleles for brown eyes, one child with two alleles for blue eyes, one child with an allele for brown eyes and an allele for blue eyes, and one child with a gene for blue eyes and a gene for brown eyes. The children are phenotypically different from their parents, but in this generation, the frequencies are still 50% brown eyes, 50% blue eyes. This will be true on average if many such couples mate, and it will also be true of large populations--a manifestation of the Hardy-Weinberg principle. But of course deviations from this expected frequency are inevitable for individual couples-- they may have only one or two children, and it is possible for them to have four blue-eyed children. In a large population, these chance variations are likely to balance out, but in a small population they probably will not. This leads to drift.

Other sources of chance and variability

The principle of independent assortment may also be involved in drift. According to this principle, during gamete formation many traits combine randomly. Thus, an individual may inherit alleles that increase fitness along with alleles that are neutral (that neither increase nor decrease fitness). Natural selection favors the alleles that increase fitness, but the associated neutral alleles will also increase in frequency, as an accidental byproduct.

Population genetics perspective

From the statistical perspective of population genetics, drift is a "sampling effect." A chance over-production or under-production of offspring compared to the average represents what statisticians call a sampling error[?]. According to this perspective, the frequency distribution of alleles among a population of offspring (how many carriers there are of each allele) reflects a sampling of the alleles of the preceding generation. When the alleles of a gene do not differ with regard to fitness, on average the number of carrieres in one generation is proportional to the number of carriers in the last. 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 preceding generation.

Many sources of mortality, such as infectious diseases for which no immunity exists, may be regarded as randomly sampling a population. In other words, they randomly select some proportion of individuals for death. Because the sample is random, on average alleles are picked in proportion to how common they are. But because the sample size, the population size and the number of carriers of an allele are finite, deviations from the average or mean often occur. To the extent that the upward and downward deviations over successive generations do not exactly balance out, an allele drifts.

Drifting alleles are liable to disappear all together from the gene pool. When the number of individuals who carry an allele drifts to zero, so that no individuals are left to reproduce it, it disappears forever. Similarly, if all but one of the alleles for a given gene disappears, the proportion of individuals who carry it will never stray from 100%. That is, until in at least one individual a spontaneous mutation or other genetic change affects that carrier's allele. It is also possible in principle for such a change to reintroduce an allele that has disappeared from the gene pool.

When drift comes into play Population bottlenecks, Founder's effect etc.

Examples Genetic drift bears similarities to tossing coins: while there is always a fifty-fifty chance that the coin will turn up heads, it is still possible to toss heads several times in a row. The more times one tosses a coin, the closer the numbers of heads and tails will approach the ideal of 50-50. Similarly, the larger the population, the closer the actual gene frequencies will remain the same from one generation to the next. Conversely, the smaller the population, the greater the chance of deviation.

Chance acts on allele frequency in a variety of ways. Perhaps the most obvious input is lifespan. For example, imagine a collision between a car and a bus. If the collision was caused by the fact that the driver of the car had poor vision, that driver's death might be an example of natural selection. But for the driver and passengers of the bus, death was random. If these people died before reproducing, their death would alter the frequency of their genes in the subsequent generation. In other words, even when individuals are equally fit, they will differ in their success. Simply by being in the wrong place at the wrong time, the death of some and the survival of others can change the distribution of alleles in a population, and thus be a force in its evolution. "Differential morbidity" is the most important cause of drift in populations of asexual (or "clonal") organisms, and it is an important cause of drift in populations of sexual organisms as well.

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