Download Genetic Drift - Ms. Stevens` Class

Genetic Drift
• Random change in allele frequencies from 1
generation to the next.
• Individuals who survive & pass on their genes
are not necessarily the “fittest”
• Leads to a loss of genetic variation
• 2 types: Bottleneck & Founder Effect
Genetic Drift:
Bottleneck Effect
• A bottleneck effect is a sudden reduction in the number of alleles in a
population
Genetic Drift:
Founder Effect
Occurs when a small part of a population
moves to a new locale (or when the
population is reduced to a small size
because of some environmental change)
The genes of the "founders" of the new
society are disproportionately frequent in
the resulting population.
The smaller the new population, the more
likely the allelic frequencies will differ
from the original population.
The Founder Effect
The Amish community in eastern
Pennsylvania was formed by ~200
German immigrants.
It is a closed community, and
members marry other Amish
people.
This creates a small gene pool (lack
of genetic variability) for offspring,
and a founder effect occurred
because of this
The Founder Effect
Ellis–van Creveld syndrome is a
relatively rare autosomal recessive
disorder for the US population
This disorder occurs more frequently in
the Amish community because of the
limited gene pool, and is an example of
the founder effect.
The mutated genes have been traced
back to one couple who came to the area
in 1744, and it has now been passed on to
their kids, grandkids, great
grandchildren, and so on.
Founder Effect
Original
mainland
population
Some people from
the mainland
colonize an island
Island gene pool
is not as variable
as the mainland’s
Genetic Drift Examples
Cheetahs are so closely
related to each other that
skin grafts from one cheetah
to another do not provoke
immune responses. This
suggests an extreme
population bottleneck in the
past.
Elephant Seals were reduced to
20 individuals in 1896. Now
there are 30,000 individuals
with no detectable genetic
diversity.
Overhunting almost
caused extinction,
now has begun to
recover.
Gene Flow
Movement of alleles from one population to
another.
Occurs when individuals leave one population, join
another, and breed.
Immigration - alleles entering the gene pool
Emigration - alleles leaving the gene pool
Gene Flow
Offspring of
immigrated bird
has a genotype
of Hh
Population A
Selection pressure against recessive
phenotype has created a homozygous
homozygous dominant population
Offspring of
immigrated
bird has a
genotype of
Hh
Population B
Selection pressure against dominant
phenotype has created a homozygous
recessive population (hh)