Evolutionary equilibrium, also known as Hardy Weinberg equilibrium, occurs when allele and genotype frequencies remain constant between generations in a population with no evolutionary forces. There are five main destabilizing forces that disrupt evolutionary equilibrium: 1) genetic drift, such as bottleneck and founder effects, which cause changes in allele frequencies by chance, 2) mutation, which introduces new alleles, 3) migration or gene flow between populations, which prevents divergence, 4) meiotic drive, where some alleles are overrepresented in gametes, and 5) natural selection, where some alleles provide a reproductive advantage. Together, these evolutionary forces ensure that Hardy Weinberg equilibrium is rarely achieved in natural populations.

1. DESTABILISING
FORCES OF
EVOLUTIONARY
EQUILIBRIUM
BY
ANILA VARGHESE
I MSc ZOOLOGY

2.  What is evolutionary equilibrium???
Evolutionary equilibrium = Hardy Weinberg equilibrium
Hardy Weinberg Law states that allele and genotype
frequencies in a population will remain constant from generation
to generation in the absence of other evolutionary forces.
Evolutionary forces = Destabilising forces

3. What is allele frequency?
Allele is the alternative form of a gene.
Allele frequency = no. of copies of a particular allele in a
population
total no. of all alleles for that gene
in a population
No of times a particular allele is present in that
particular locus in a population.

4. p + q = 1 allelic perspective
(p + q) (p +q) = 12
genotype perspective
(𝑝 + 𝑞)2
= 𝑝2
+ 2pq + 𝑞2
= 1
𝑝2 = pp = homozygous dominant
2pq = heterozygous
𝑞2 = qq =homozygous recessive.

5. Evolutionary forces
1. GENETIC DRIFT
 Change in allele frequency due to chance.
 Acts in a small population.
 Reduces genetic variability.
 Examples : Bottleneck effect and Founder effect

6. Bottle neck effect
• It occurs when there is a sudden sharp decline in a
population’s size typically due to environmental
factors (natural disasters such as earthquakes or
tsunamis, epidemics that can decrease the number
etc.).
• Some genes (there is not any distinction) are
extinguished from the population.
• Reduction of the total genetic diversity of the original
gene pool.
• Surviving population is be farther from the original
one in its genetic makeup.

8. Example :
• Northern elephant seals
• Coastal areas of U.S and
Mexico
• Bottleneck event
• Humans hunting (1890s)
• Population size 20 individuals
(at the end of the 19th century)
• Now Population 30,000
• But their genes still carry the
marks of this bottleneck
• Much less genetic variation

9. Founder effect
• Loss of genetic variation that occurs when a new
population is established by a small number of
individuals that are cleaved from a larger population.
• New population does not have the genetic diversity of the
previous one.
• Some genetic traits are becoming more prevalent in the
population.
• Leads to the presence of certain genetic diseases in the
next generations

11. Example :-
Dutch settlers in south Africa
Afrikaner population
High frequency of Huntington's disease allele
( nerve cell in brain degenerate)

12. 2. MUTATION
• Sudden heritable changes
Beneficial Harmful
lead to reduce fitness
reproductive success increase susceptibility
to disorders & illness
better adaptability
to environment
spread and get fixed in
population

13. Example :-
• Influenza A virus, causes human influenza pandemic
• An RNA virus, made of 8 ssRNA
• Sub-divided into hemagglutinin (HA) and
neruaminidase (NA), based on the antigenic protein
present .
• RNA polymerase High error rate
Mutations Antigenic drift Viral
evolution
• Antibody provides only partial immunity.
• Therefore flu vaccine needs to be modified very
often

15. 3. MIGRATION OR GENE FLOW
 In an evolutionary sense, migration is the movement of
alleles among populations.
 A unifying force, prevent population from diverging
 Homogenisation
Is effect of migration on alleles same or opposite of
the effect which of genetic drift and natural selection
have on alleles???

16. • No selection, Only Migration Homogenisation
• Selection stronger than migration More
diversity
• Migration stronger than selection Less
diversity

17. One Island Model
 Scenario:asmallislandclosetothemainland.
 Ifmigrationoccursfromthesmallislandtothe
mainland,thepopulationonthemainlandwillnotbe
affectedmuch.
 Ifmigrationoccursfromthemainlandtothesmall
island,thepopulationcanchangedrastically

18. Example
• One Island Model, Lake Erie Water Snake, Nerodia
sipedon.
• Two varieties – banded and unbanded, genetically
• These snakes can be found on both the islands and on
the mainland.
• On the islands’ limestone rocks the young, small, banded
snakes are much more vulnerable to predation than the
unbanded snakes.
• Why do the banded snakes exist on the islands if
they are selected against?

20. Migration works against
selection
• Every generation several banded snakes move from
the mainland to the islands.
• The migrants bring with them copies of the allele for
banded coloration.
• Interbreeding contributes these copies to the
population.
• Migration is working against selection preventing
fixation of any one allele.

21. Conclusion
• Migration can cause allele frequencies to change
from one generation to another which violates the
first Hardy-Weinberg conclusion.
• Migration can be a powerful mechanism for
evolution.
• Migration tends to homogenize allele frequencies
across populations.

22. 4. MEIOTIC DRIVE
Any process which causes some alleles to be over
represented in the gametes.
Mendel’s law is disobeyed.
One of the alleles is consistently found in more than
half of the offspring.
Example: T-locus in mice.
T-locus affect the tail length and viability.
TT - Homozygotes : normal tail length
Tt - Heterozygote : Short tail and transmit ~90% of
the alleles to the gamete
tt - Homozygote: sterile
Meiotic drive increases the frequency of t allele

23. 5. NATURAL SELECTION
• A directional process that leads to the decrease or
increase in allele frequency.
• Disrupts Hardy Weinberg’s equilibrium.
• One allele gives individual a reproductive
advantage over other individuals.
• These leave more offspring, produces adaptation;
“relative fitness”
• “Survival of the fittest” - what does “fittest” mean?
• Alleles that allow the greatest reproductive success
increase in frequency, other alleles decrease.

24. Is Hardy Weinberg's law
followed in nature?? NO
APPLICATIONS
 To calculate gene frequency and allele
frequency
 To develop hypothesis about populations
 To predict how a population will behave
with modifications of certain environmental
factors.

25. THANK YOU