Genetic drift is the random change in allele frequencies between generations in a small population due to chance. It can cause loss of genetic variation and lead to homozygosity. The bottleneck effect occurs when a population is drastically reduced in size, such as by a natural disaster, leaving a small surviving population with less genetic diversity than the original. The founder effect is similar, occurring when a new population is established by a small number of colonists from a larger population, resulting in allele frequencies that may not represent the original population. Both effects can increase the prevalence of genetic disorders in the new populations.

1. -SARASWATI PAWAR
BSC. BIOTECHNOLOGY
Genetic Drift

2. Genetic Drift
 The term given by Sewall Wright
 The random and directionless change in the Allele
frequency simply as a result of chance from one
generation to next in a small population is called genetic
drift.
 Genetic drift can cause big losses of genetic variation for
small populations.
 Properties of genetic drift :
1. Involves in evolutionary process
2. Random changes
3. It act to eliminate genetic variation class of genetic
variation
4. Can lead to homozygous condition

3. 1. Act upon the genetic frequency within a
population without regard to their phenotypic
effect
2. It can cause harmful allele to become fixed
3. Bottleneck effect and founder effect does not
represent the correct frequency of actual
population

4.  Bottleneck effect
 Earthquakes, floods, or fires may kill large numbers of
individuals, leaving a small surviving population that is
unlikely to have the same genetic makeup as the original
population. Such a drastic reduction in population size is
called a bottleneck effect
 The bottleneck effect, also known as a population
bottleneck, is when a species goes through an event that
suddenly and significantly reduced its population

5.  Analogous to shaking just a few marbles through a
bottleneck at higher frequency in the surviving
population than in the original population, others
(green marbles) may be present at lower frequency,
and some (orange marbles) may not be present at all.
After a bottlenecking event, genetic drift may continue
for many generations until the population is again
large enough for fluctuations due to chance to have
less of an impact. Even if a population that has
passed through a bottleneck ultimately recovers its
size, it may have low levels.

7.  Example
 Northern elephant seals have reduced genetic
variation probably because of a population bottleneck
humans inflicted on them in 1880s
 Hunting reduced their population size to as few 20
individuals at the end of the 19th century
 Their population has since rebounded to over 30000 ,
but their genes still carry the marks of this bottelneck ;
they have much less genetic variation than population
of southern elephant seal that was not so intensely
hunted.

8.  Founder Effect
 Genetic drift is also likely when a few individuals
colonize an island or other new habitat, producing
what is called the founder effect.
 The smaller the group, the less likely the genetic
makeup of the colonists will represent the gene pool
of the larger population they left.
 The founder effect explains the relatively high
frequency of certain inherited disorders among some
human populations established by small numbers of
colonists.

9.  In 1814, 15 people founded a colony on Tristan
da Cunha, a group of small islands in the middle
of the Atlantic Ocean. Apparently, one of the
colonists carried a recessive allele for retinitis
pigmentosa, a progressive form of blindness. Of
the 240 descendants who still lived on the islands
in the 1960s, four had retinitis pigmentosa, and at
least nine others were known to be heterozygous
carriers of the allele. The frequency of this allele
is 10 times higher on Tristan da Cunha than in the
British population from which the founders came.
 Many genetic diseases are increased in
prevalence in human population due in part to the

11. Bottleneck Effect Founder Effect
 Subpopulation caused
when a natural
disaster reduces the
size of an original
population.
 Results of habitat
fragmentation or over
exploitation of
species.
 Establishes a
subpopulation as a
new colony drawn
from a larger
population.
 Result of migration.