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TOPIC:FACTORS THAT ALTER ALLELIC FREQUENCIES
ST.THOMAS COLLEGE KOZHENCHERRY
An allele is an alternative form of a gene (one member
of a pair) that is located at a specific position on a
Organisms have two alleles for each trait.(RR,rr)
The gene for seed shape in pea plants exists in two
forms, one form or allele for round seed shape (R) and
the other for wrinkled seed shape (r).
THE FREQUENCY OF ANY GIVEN ALLELE IN A POPULATION,
RELATIVE TO ALL THE OTHER ALLELES AT THE SAME LOCUS, IS
KNOWN AS ALLELE FREQUENCY.
Allele frequency =
Number of copies of a particular allele in
Total number of all alleles for that gene
in a population.
FACTORS THAT ALTER ALLELIC FREQUENCIES
GENETIC DRIFT – BOTTLE NECK EFFECT AND
Are the source of new alleles in the gene pool.
Mutation are essential for evolution.
Once a mutation occurs, the allele frequency is
Alleles resulting from unfavourable mutation are
selected against and only remain in the gene
pool if they are recessive(remain ‘hidden 'in
Neutral or silent mutations are not acted upon by
Eg: A change in a base code(GGG to GGC) that codes
for the same amino acid. The same protein is made-no
change result from this mutation.
In this way, mutations increase the opportunity for
evolution of adaptations different from characteristics of
the ancestral population.
These mutations will affect the frequency of alleles.
It was explained by Sewall Wright(1931)hence called
Sewall wright effect.
Genetic drift is the change in allele frequencies of a
population due to random chance events, such as natural
Genetic drift can happen when a natural disaster or similar
event randomly kills a large portion of the population.
The remaining survivors may have allele frequencies that
were very different from the previous population, a
phenomenon known as the bottleneck effect.
BOTTLE NECK EFFECT
Population may be suddenly reduced in numbers.
Usually from a catastrophic environmental
Or by sudden, severe selection pressure(often
human activities eg:rapid habitat
destruction,introduction of predators).
After the event the population may recover to grow
again to return to normal levels.
As population numbers drop rapidly, it is likely that
the range of alleles decreases and the frequency of
When small population subject to genetic drift.
Another way genetic drift can occur is if a portion of
the population separates from the old population to
start a new population(Island).
The alleles in the new group will be found at higher
frequencies than in the original population, a
phenomenon known as the founder effect.
In extreme cases, a founder population may be a
single individual(wind blown seed).
Migration is the movement of individuals from one
population to another or the movement of breeding
individuals into or out of isolated populations.
Results in evolutionary change because alleles move
with the individuals. We call this movement gene flow.
Immigration: Individuals migrate into a population.
Emigration: Individuals migrate out of a population.
Both processes allow for gene flow between
Gene flow may change the frequency and or the range
of alleles in the populations
If population are large, migration may have little or
no effect on allele frequency.
However, if population are small, migration may have
a big impact on allele frequency.
The force of natural selection tends to reduce the
genetic variability of populations in a way that
It does so by removing or reducing the frequency of
some phenotypes and increasing the frequency of
Environmental factors(biotic&abiotic)act as a
selecting agents of phenotypes.
When environmental factors change, different
phenotypes will be selected for.
As phenotype is largely determined by genotype,
successful genotype alleles will increase in frequency
in the gene pool.
Favourable alleles increase in frequency in a gene
pool, while unfavourable alleles decrease.
After a certain number of generations, the frequency of
alleles and phenotypes might change so that the
population becomes reproductively isolated from
others of that species.
It is now a new species.
Three kinds of selection cause changes in the normal
distribution of phenotypes in a population.
1. Stabilizing selection- eliminates those phenotypes
most different from the norm, thus reducing the frequency
of phenotypic extremes.
•Stabilizing selection using phenotype
variation within a population of
butterflies as an example.
•Imagine a butterfly population that
shows continous variation in wing
•Suppose there is a shift in the
environment, so that forms at either end
of the curve are selected against.
•Overtime, if the environmental pressure
remains constant the more extreme
forms will be eliminated.
•Eventually, intermediate phenotypes
will become predominant in the
2.Directional selection-This selection is always
associated with environmental change.
Examples: Evolution of horse is a good example of
directional selection in which a small forest dwelling
animal, Hyracotherium, had to undergo successive
changes in its body when the environment changed
from forest to grassland, giving rise to a tall, fast-
running, grazing horse.
3.Disruptive selection- eliminates average phenotypes
and encourages the extremes. This tends to result in
distinct phenotypes in the same population.
For example, animals that can avoid predation by being
a certain color.
There are cases of non-poisonous butterflies evolving
directional color changes that make them look like
poisonous butterflies, because their predators tend to
avoid the color of the poisonous butterflies.
Non-random mating means that individuals of many
species have a choice about which partners to mate
The result of nonrandom mating is that some
individuals have more opportunity to mate than others
and thus produce more offspring (and more copies of
their genes) than others.
The mating of closely related individuals such as
cousins, self-fertilized plants,
which tends to increase the number of individuals that are
homozygous for a trait.
Inbreeding coefficient (F)
F measures the probability that two genes at any locus in
an individual are identical from the common ancestor(s) of
the two parents.
This means the degree to which two alles are more
likely to be homozygous (AA or aa) rather
than heterozygous (Aa) in an individual, because the
parents are related.