Download Genetic drift is the change in allele frequencies of a population due

Genetic drift is the change in allele frequencies of a population due to
random chance events, such as natural disasters.
LEARNING OBJECTIVES [ edit ]
Distinguish between selection and genetic drift
Describe the results of genetic drift due to a population bottleneck
KEY POINTS [ edit ]
Genetic drift is the change in the frequency of an allele in apopulation due to random
sampling and the random events that influence the survival and reproduction of those
individuals.
The bottleneck effect occurs when a natural disaster or similar event randomly kills a large
portion (i.e. random sample) of the population, leaving survivors that have allele frequencies that
were very different from the previous population.
The founder effect occurs when a portion of the population (i.e. "founders") separates from the
old population to start a new population with different allele frequencies.
Small populations are more susceptible genetic drift than large populations, whose larger
numbers can buffer the population against chance events.
TERMS [ edit ]
founder effect
a decrease in genetic variation that occurs when an entire population descends from a small
number of founders
genetic drift
an overall shift of allele distribution in an isolated population, due to random sampling
random sampling
a subset of individuals (a sample) chosen from a larger set (a population) by chance
Give us feedback on this content: FULL TEXT [ edit ]
Genetic Drift vs.Natural
Selection
Genetic drift is the converse of natural
selection. The theoryof natural selection
maintains that some individuals in a
population have traits that enable to
survive and produce more offspring, while
other individuals have traits that are
detrimental and may cause them to die
before reproducing. Over successive
Register for FREE to stop seeing ads
generation, these selection pressures can change the gene pool and the traits within the
population. For example, a big, powerful male gorilla will mate with more females than a
small, weak male and therefore more of his genes will be passed on to the next generation.
His offspring may continue to dominate the troop and pass on their genes as well. Over time,
the selection pressure will cause the allele frequencies in the gorilla population to shift
toward large, strong males.
Unlike natural selection, genetic drift describes the effect of chance on populations in the
absence of positive or negative selection pressure. Through random sampling, or the survival
or and reproduction of a random sample of individuals within a population, allele
frequencies within a population may change. Rather than a male gorilla producing more
offspring because he is stronger, he may be the only male available when a female is ready to
mate. His genes are passed on to future generation because of chance, not because he was the
biggest or the strongest. Genetic drift is the shift of alleles within a population due to chance
events that cause random samples of the population to reproduce or not.
Effect of genetic drift
Genetic drift in a population can lead to the elimination of an allele from that population by chance. In
this example, the brown coat color allele (B) is dominant over the white coat color allele (b). In the first
generation, the two alleles occur with equal frequency in the population, resulting in p and q values of .5.
Only half of the individuals reproduce, resulting in a second generation with p and q values of .7 and .3,
respectively. Only two individuals in the second generation reproduce and, by chance, these individuals
are homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele
is lost.
Small populations are more susceptible to the forces of genetic drift. Large populations, on
the other hand, are buffered against the effects of chance. If one individual of a population of
10 individuals happens to die at a young age before leaving any offspring to the next
generation, all of its genes (1/10 of the population's gene pool) will be suddenly lost. In a
population of 100, that individual represents only 1 percent of the overall gene pool;
therefore, genetic drift has much less impact on the larger population's genetic structure.
The Bottleneck Effect
Genetic drift can also be magnified by natural events, such as a natural disaster that kills a
large portion of the population at random. The bottleneck effect occurs when only a few
individuals survive and reduces variation in the gene pool of a population. The genetic
structure of the survivors becomes the genetic structure of the entire population, which may
be very different from the pre­disaster population.
Effect of a bottleneck on a population
A chance event or catastrophe can reduce the genetic variability within a population.
The Founder Effect
Another scenario in which populations might experience a strong influence of genetic drift is
if some portion of the population leaves to start a new population in a new location or if a
population gets divided by a physical barrier of some kind. In this situation, it is improbable
that those individuals are representative of the entire population, which results in the
founder effect. The founder effect occurs when the genetic structure changes to match that of
the new population's founding fathers and mothers.
The Founder Effect
The founder effect occurs when a portion of the population (i.e. "founders") separates from the old
population to start a new population with different allele frequencies.
The founder effect is believed to have been a key factor in the genetic history of the Afrikaner
population of Dutch settlers in South Africa, as evidenced by mutations that are common in
Afrikaners, but rare in most other populations. This was probably due to the fact that a
higher­than­normal proportion of the founding colonists carried these mutations. As a
result, the population expresses unusually high incidences of Huntington's disease (HD) and
Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital
abnormalities, even cancer.
Drift and fixation
The Hardy–Weinberg principle states that within sufficiently large populations, the allele
frequencies remain constant from one generation to the next unless the equilibrium is
disturbed by migration, genetic mutation, or selection.
Because the random sampling can remove, but not replace, an allele, and because random
declines or increases in allele frequency influence expected allele distributions for the next
generation, genetic drift drives a population towards genetic uniformity over time. When an
allele reaches a frequency of 1 (100%) it is said to be "fixed" in the population and when an
allele reaches a frequency of 0 (0%) it is lost. Once an allele becomes fixed, genetic drift for
that allele comes to a halt, and the allele frequency cannot change unless a new allele is
introduced in the population via mutation or gene flow. Thus even while genetic drift is a
random, directionless process, it acts to eliminate genetic variation over time.
Genetic drift over time
Ten simulations of random genetic drift of a single given allele with an initial frequency distribution 0.5
measured over the course of 50 generations, repeated in three reproductively synchronous populations
of different sizes. In these simulations, alleles drift to loss or fixation (frequency of 0.0 or 1.0) only in the
smallest population.Effect of population size on genetic drift: Ten simulations each of random change in
the frequency distribution of a single hypothetical allele over 50 generations for different sized
populations; first population size n=20, second population n=200, and third population n=2000.