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.
Genetic drift is one of several evolutionary processes which lead to changes in 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,[1] the changes due to genetic drift are not driven by environmental or adaptive pressures, 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. Vigorous debates wage among evolutionists over the relative importance of genetic drift compared with natural selection. One of the founders of the modern evolutionary synthesis, Ronald Fisher, held the view that genetic drift plays at the most a minor role. This view remained dominant for several decades. In 1968 population geneticist Motoo Kimura rekindled the debate with his neutral theory of molecular evolution which claims that most of the changes in the genetic material are caused by genetic drift. [2]
[edit] Basic concept
A population's allele frequency is the fraction of the gene copies that have a particular form.[3] Genetic drift is the effect of chance factors on allele frequency: the variation from generation to generation resulting from random 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. The organisms that are reproduced in a generation are represented in another jar. Each new generation the organisms will reproduce at random. To represent this reproduction, randomly pick a marble from the original jar and deposit a new marble with the same color as its "parent" in the second jar. Repeat the process until there are 20 new marbles in the second jar. The second jar will then contain a second generation of "offspring", 20 marbles of various colors. Unless the second jar contains exactly 10 red and 10 blue marbles there will have been a purely random shift in the allele frequencies.
Repeat this process a number of times, randomly reproducing each generation of marbles to form the next. The numbers of red and blue marbles picked each generation will fluctuate: sometimes more red, sometimes more blue. That is genetic drift – random variations in which organisms manage to reproduce, leading to changes over time in the allele frequencies of a population.
It is even possible that no marbles of one color (say red) will be chosen, and the jar representing the new generation will contain only blue offspring. Once this happens, the allele (red) has been lost permanently in the population, while the remaining allele (blue) has become fixed: all future generations will be entirely blue. Given enough time, especially in a small population, this outcome is nearly inevitable.
[edit] Simplified biological example
Consider the following very simple 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.
In this simplified model world, the bacteria all divide at the same time and with complete success 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, an observer studies one at a time, 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 finding 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 can be calculated with the formula
where N is the number of bacteria and k is the number of A (or B). The '()' signifies the binomial coefficient and can be expressed as N choose k.
[edit] Wright-Fisher model
In a diploid population consisting of N individuals there are 2 N copies of each gene. If there are two alleles of this gene, we can call the frequency of one allele p and the frequency of the other q. If these frequencies are at hand in a particular generation, then the probability of obtaining k copies of the allele with frequency p in the next generation is [4]
Here the '!' sign signifies the factorial function. This expression can also be formulated with the binomial coefficient,
[edit] 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.[5]
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.[1] 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.
[edit] 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.[6]
The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[7] 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.[8]
[edit] 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.
[edit] 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.[9]
[edit] 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.
[edit] 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.
[edit] 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.[1] 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.
[edit] See also
[edit] References
- ^ a b c Avers, Charlotte (1989), Process and Pattern in Evolution, Oxford University Press
- ^ Futuyma, Douglas (1998). Evolutionary Biology. Sinauer Associates. p. 320. ISBN 0-87893-189-9.
- ^ Futuyma, Douglas (1998). Evolutionary Biology. Sinauer Associates. p. Glossary. ISBN 0-87893-189-9.
- ^ Hartl, Daniel (2007). Principles of Population Genetics. Sinauer Associates. p. 102. ISBN 978-0-87893-308-2.
- ^ Zimmer, Carl (2002), Evolution : The Triumph of an Idea, New York, NY: Perennial, pp. 364, ISBN 0-06-095850-2
- ^ Lande R (1989). "Fisherian and Wrightian theories of speciation". Genome 31 (1): 221–27. PMID 2687093.
- ^ Otto S, Whitlock M (01 Jun 1997). "The probability of fixation in populations of changing size". Genetics 146 (2): 723–33. PMID 9178020. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9178020.
- ^ Charlesworth B (March 2009). "Fundamental concepts in genetics: Effective population size and patterns of molecular evolution and variation". Nat. Rev. Genet. 10: 195–205. doi:. PMID 19204717.
- ^ Simpson, George Gaylord (1967), The Meaning of Evolution (Second ed.), Yale University Press
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