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Genetic drift
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Genetic drift or allelic drift is the change in the relative frequency with which a gene variant (allele) occurs in a population that results from the fact that alleles in offspring are a random sample of those in the parents, and because of the role of chance in determining whether a given individual survives and reproduces. Genetic drift may cause gene variants to disappear completely, and thereby reduce genetic variability.
In evolution, genetic drift is one of several processes that change allele frequencies over time.
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Encyclopedia
Genetic drift or allelic drift is the change in the relative frequency with which a gene variant (allele) occurs in a population that results from the fact that alleles in offspring are a random sample of those in the parents, and because of the role of chance in determining whether a given individual survives and reproduces. Genetic drift may cause gene variants to disappear completely, and thereby reduce genetic variability.
In evolution, genetic drift is one of several processes that change allele frequencies over time. In contrast to natural selection, which makes gene variants more or less common due to their causal effects on reproductive success, the changes due to genetic drift have no specific environmental cause, and may be beneficial, neutral, or detrimental to reproductive success.
The effect of genetic drift is larger in small populations, and smaller in large populations. The extent of the effect relative to the other three processes is a subject of debate, for example in the context of the neutral theory of molecular evolution. Scientific views on the effect in large populations vary from insignificant to dominant.
Basic concept
If more than one variant (allele) of a gene exists in a population, allele frequency is the fraction of the population having a given variant. This frequency can change from generation to generation as a result of chance. For example, in a parent generation of 50 individuals, there may be 25 with a certain allele. If the next generation happens to consist of 51 individuals (or any odd number), the population can no longer have the same allele frequency as the parent generation. Drift is the summation of these statistical phenomenon that affect allele frequency; variance results from sampling.
As an analogy, imagine a population of organisms represented as 20 marbles in a jar, half of them red and half blue. These two colors correspond to two different gene alleles in the population. Each generation, the organisms will reproduce at random and the old generation will die. To see the effect of this, randomly pick a marble from the jar. Return the selected marble to its jar after putting a new marble of the same color into a second jar. This procedure represents the "reproduction" of the selected marble, and the second jar holds the next generation of organisms.
Mix the original 20 marbles and pick another to reproduce. After 20 steps, the second jar will contain 20 "offspring" of various colors. Now throw away the marbles from the first jar – since the older generation of organisms eventually die – and repeat this process over several generations. The numbers of red and blue marbles picked will fluctuate by chance, so the more common color in the population of marbles will change over time: sometimes more red, sometimes more blue. It is even possible, purely by chance, that all marbles of one color (say blue) will be lost, leaving the jar containing only red offspring. One color (allele) has been "lost", while the remaining allele (red) has become fixed: all future generations will be entirely red.
Given enough time, especially in a small population, this outcome is nearly inevitable. That is genetic drift – random variations in which organisms manage to reproduce, leading to changes over time in the allele frequencies of a population.
Simple example
Consider the following idealised world. A colony of four bacteria live in a very small drop that contains all kinds of food the bacteria need. The bacteria are genetically identical except for one gene for which there are two alleles. They differ sufficiently to make them distinguishable in a microscope when stained with a particular stain. This difference causes no difference whatsoever in ability to survive and reproduce. We call the alleles A and B. Two bacteria have one of the alleles and the other two have the other allele.
The bacteria divide in synchrony for several generations, until the food is depleted. Then they die off by starvation until only four have survived. We have postulated that there are no differences in ability to survive, so the individuals that remain are a completely random sample of the maximum population. To find the number of A-bacteria and B-bacteria in the surviving population they are studied one after the other, so that four observations are obtained. Since the number of A and B bacteria were the same originally, the reproduction was identical and the ability to survive was the same, each observation has the same probability of finding an A as a B. The probability is 1/2. There are sixteen possible combined outcomes of the four observations,
(A, A, A, A),
(B, A, A, A),
(A, B, A, A),
(B, B, A, A),
(A, A, B, A),
(B, A, B, A),
(A, B, B, A),
(B, B, B, A),
(A, A, A, B),
(B, A, A, B),
(A, B, A, B),
(B, B, A, B),
(A, A, B, B),
(B, A, B, B),
(A, B, B, B) and
(B, B, B, B).
Since each individual observation has the same probability, all the possible combinations have the same probability, 1/2 * 1/2 * 1/2 *1/2 = 1/16. If the combinations with the same number of A and B respectively are counted, we get the following table.
| A | B | Combinations | Probability | | 4 | 0 | 1 | 1/16 | | 3 | 1 | 4 | 4/16 | | 2 | 2 | 6 | 6/16 | | 1 | 3 | 4 | 4/16 | | 0 | 4 | 1 | 1/16 |
The number of combinations with equal number of A and B bacteria is six, and the probability of equal (conserved) number is 6/16. The number of other combinations is ten and the probability of different number is 10/16. The outcomes where the number of A alleles (and B alleles) has changed are instances of genetic drift. In this example the probability of genetic drift is 10/16. This means it is more probable that the population will drift than that it will not drift.
These combinations of numbers are called binomial coefficients and they can be derived from Pascal's triangle. The probability distribution is called binomial distribution.
The formula for the probabilities is
where N is the number of bacteria and k is the number of A (or B).
Probability and allele frequency
Chance events can change the allele frequencies in a population because any individual's reproductive success can be determined by factors other than adaptive pressures. Genetic drift occurs when these allele frequencies change as a consequence of sampling error. In probability theory, the law of large numbers predicts little or no change would take place over time from random sampling when a population is large. When the reproductive population is small, however, the effects of sampling error can alter the allele frequencies significantly. Genetic drift is therefore generally considered a consequential mechanism of evolutionary change only within small, isolated breeding populations.
By definition, genetic drift has no preferred direction, but due to the volatility stochastic processes create in small reproducing populations, there is a tendency within small populations towards homozygosity of a particular allele, such that over time the allele will either disappear or become universal throughout the population. This trend plays a role in the founder effect, a proposed mechanism of speciation. With reproductively isolated homozygous populations, the allele frequency can only change by the introduction of a new allele through mutation.
When the alleles of a gene do not differ with regard to fitness, probability law predicts the number of carriers in one generation will be relatively unchanged from the number of carriers in the parent generation, a tendency described in the Hardy-Weinberg principle. However, there is no residual influence on this probability from the frequency distribution of alleles in the grandparent, or any earlier, population--only that of the parent population. The predicted distribution of alleles of the offspring is a memory-less probability described in the Markov property.
Drift and fixation
The genetic drift halts when an allele eventually becomes fixed, either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.
The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations. The precise measure of population that is important is called the effective population size. The effective population is always smaller than the total population since it takes into account factors such as the level of inbreeding, the number of animals that are too old or young to breed, and the lower probability of animals that live far apart managing to mate with each other.
Genetic drift versus natural selection
Although both processes drive evolution, genetic drift operates randomly while natural selection functions non-randomly. This is because natural selection emblematizes the ecological interaction of a population whereas drift is regarded as a sampling procedure across successive generations without regard to fitness pressures as dictated by the environment. Drift affects genotypic frequencies within a population whereas natural selection concerns itself with both the phenotypes and genotypes present in a population. Moreover, natural selection impels the creation of adaptations (influencing both the phenotypic and genotypic components of a population) while genetic drift does not.
Selection and drift as a function of population size
Genetic drift and natural selection do not act in isolation; both forces are always at play in a population. However, the degree to which alleles are affected by drift and selection varies according to population size.
Especially in small populations, the statistical effect of sampling error (during reproduction) on certain alleles from the overall population may result in an allele (and the biological traits that it confers) becoming more common or rare over successive generations. Often a particular gene either becomes fixed in the population or goes extinct. Given enough time, speciation follows as genetic drift builds up.
In a large population, where probability predicts little change in allele frequencies over many generations will result from sampling error, even weak selection forces acting upon an allele will push its frequency upwards or downwards (depending on whether the allele's influence is beneficial or harmful). However, if the population is very small, drift will predominate. In small populations, weak selective effects may not be seen at all as the small changes in frequency they would produce are overshadowed by drift.
Evolution of maladaptive traits
Drift can have profound effects on the evolutionary history of a population. In very small populations, the effects of sampling error are so significant that even deleterious alleles can become fixed in the population, and may even threaten its survival.
In a population bottleneck, where a larger population suddenly contracts to a small size, genetic drift can result in sudden and radical changes in allele frequency that occur independently of selection. In such instances, the population's genetic variation is reduced, and many beneficial adaptations may be permanently eliminated.
Similarly, migrating populations may see a founder effect, where a few individuals with a rare allele in the originating generation can produce a population that has allele frequencies that seem at odds with natural selection. Founder's effects are sometimes held to be responsible for high frequencies of some genetic diseases.
Examples
- If two competing alleles in a population have exactly a 50 % / 50 % share in one generation, this will change by a small amount because of minor, chance events as each individual comes into existence. In a mid-sized group, this level of randomness will account for a fraction of a percent difference per generation; 50 % to 49.8 %, etc. In large populations, in absence of selective pressure, the share will hover near 50 %; in smaller groups, one or the other allele is likely to become progressively more common until it has taken hold.
Often, the process is driven by more than statistical buzzing.
- Plants broadcast seeds into the wind, or recruit animals and insects to carry them. Occasionally new land is colonized, perhaps by a bird carrying a seed to a new island.
- Population movements can lead to a founder effect where a small number of individuals from a larger group splinters off to form a new population. Genetic diversity is lost as a result, and the smaller new population allows genetic drift to ripple through it. One of the most well-known examples is the peopling of the Americas, when perhaps thousands crossed the Bering land bridge into Alaska, and only 72 individuals left descendants whose lineage lived on through modern times. Other cases are too numerous to count; the Austronesian expansion brought small numbers of pigs to large numbers of islands, where isolated founder populations of both species drifted slowly apart from each other.
- A catastrophe kills large numbers of a species. This often happens as much to unlucky individuals as to unfit ones; a fire burns trees wherever the winds take it, and a mudslide is a very local event. This changes the frequency of competing alleles in the "gene pool." In extreme cases, this is known as a population bottleneck. A well known example in human pre-history is the Toba supervolcano. There have certainly been others, as suggested by Mitochondrial Eve and Y-Chromosomal Adam, or by the lack of genetic diversity in cheetahs. Elephant seals were driven almost to extinction in the 1880s and 1890s, to a minimum of about 25 individuals. While the numbers have rebounded, genetic diversity takes much longer to accumulate.
History of the concept
The concept was first introduced by Sewall Wright in the 1920s. There is debate over the relative significance of genetic drift. Many scientists consider it to be one of the primary mechanisms of biological evolution. Others, such as Richard Dawkins (borrowing from Ronald Fisher), consider genetic drift important (especially in small or isolated populations), but much less so than natural selection.
See also
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