Genetic

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SUBMITTED BY:
DHRUV MANGAL
195 B(LA-2)
SUPERVISOR:
ANNA ZHUKOVA

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* Genetic drift is a mechanism of evolution in which
allele frequencies of a population change over
generations due to chance (sampling error).
* Genetic drift occurs in all populations of non-infinite
size, but its effects are strongest in small populations.
* Genetic drift may result in the loss of some alleles
(including beneficial ones) and the fixation, or rise
to 100\%100%100, percent frequency, of other alleles.
* Genetic drift can have major effects when a
population is sharply reduced in size by a natural
disaster (bottleneck effect) or when a small group
splits off from the main population to found a colony
(founder effect).
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Genetic drift is change in allele frequencies in a population from
generation to generation that occurs due to chance events. To be
more exact, genetic drift is change due to "sampling error" in
selecting the alleles for the next generation from the gene pool of
the current generation. Although genetic drift happens in populations
of all sizes, its effects tend to be stronger in small populations.

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The role of random chance in evolution was first outlined by
Hagedoorn and Hagedoorn in 1921 [47]. They highlighted that random
survival plays a key role in the loss of variation from populations.
Fisher (1922) responded to this with the first, albeit marginally
incorrect, mathematical treatment of the 'Hagedoorn
effect'.[48] Notably, he expected that many natural populations were
too large (an N ~10,000) for the effects of drift to be substantial and
thought drift would have an insignificant effect on the evolutionary
process. The corrected mathematical treatment and term "genetic
drift" was later coined by a founder of population genetics, Sewall
Wright. His first use of the term "drift" was in 1929,[49] though at the
time he was using it in the sense of a directed process of change, or
natural selection. Random drift by means of sampling error came to
be known as the "Sewall–Wright effect," though he was never entirely
comfortable to see his name given to it. Wright referred to all
changes in allele frequency as either "steady drift" (e.g., selection)
or "random drift" (e.g., sampling error).[5

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Larger populations are unlikely to change this quickly as a result of
genetic drift. For instance, if we followed a population
of 100010001000 rabbits (instead of 101010), it's much less likely that
the b allele would be lost (and that the B allele would
reach 100\%100%100, percent frequency, or fixation) after such a short
period of time. If only half of the 100010001000-rabbit population
survived to reproduce, as in the first generation of the example above,
the surviving rabbits (500500500 of them) would tend to be a much
more accurate representation of the allele frequencies of the original
population – simply because the sample would be so much larger.

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Allele benefit or harm doesn't matter
Genetic drift, unlike natural
selection, does not take into account
an allele’s benefit (or harm) to the
individual that carries it. That is, a
beneficial allele may be lost, or a
slightly harmful allele may become
fixed, purely by chance.
A beneficial or harmful allele would
be subject to selection as well as
drift, but strong drift (for example, in
a very small population) might still
cause fixation of a harmful allele or
loss of a beneficial one.

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Unlike natural selection, genetic drift does not depend on an allele’s
beneficial or harmful effects. Instead, drift changes allele frequencies purely
by chance, as random subsets of individuals (and the gametes of those
individuals) are sampled to produce the next generation.
Every population experiences genetic drift, but small populations feel its
effects more strongly. Genetic drift does not take into account an allele’s
adaptive value to a population, and it may result in loss of a beneficial allele
or fixation (rise to 100\%100%100, percent frequency) of a harmful allele in
a population.
The founder effect and the bottleneck effect are cases in which a small
population is formed from a larger population. These “sampled” populations
often do not represent the genetic diversity of the original population, and
their small size means they may experience strong drift for generations.