Evolution- Natural selection and Genetic Drift and modes of natural selection My video: https://youtu.be/g240gKFHSrY

1. 1
Natural selection Genetic Drift
Evolutionary forces involved in speciation
Forms of organisms in a population that are best
adapted to the environment increases in frequency
over a number of generations
Random change in allele frequency as a result of
chance from one gen to next in a finite
population
Causes fixation of alleles
(0= lost 1=fixed- drift halts)
Positive selection(↑fitness ↑frequency in population),
Negative selection(↓fitness), Balancing
selection(advantages only in heterozygous conditions-
maintained)
Operates non-randomly Operates randomly
Non-directional
Guided only by chance
Directional
Guides evolution in a direction that increases
survival & reproductive fitness
Favours spread of alleles whose phenotypic
effects increases fitness and survival
Acts upon the genotypic features within a
population irrespective of their phenotypic effect

2. Impact of genetic drift
Bottleneck effect
● Population drastically reduces to a
significantly smaller size over a
short period of time
2
✗ Reasons- Disaster, hunting,
habitat destruction etc
✗ Genetic diversity↓
✗ Loss of alleles → Higher genetic
drift
✗ Thus, new population genetically
distinct from original

3. Founder effect
● New population develops by a few
members of original large population
✗ Reduced genetic variation from
original
✗ Variation depends upon the number
of founder members

5. 5
MODES OF NATURAL SELECTION
DIRECTIONAL
Stabilizing
Disruptive
Individuals at one end of the
frequency distribution are selected
Due to change in environment
Favours extreme phenotype
Favours accumulation of mutations
that increases fitness in changing env

6. 6
MODES OF NATURAL SELECTION
Stabilizing
Individuals at both the ends are
eliminated
Constant or unchanging environment
Favors average/ normal phenotypes
Checks on accumulation & introduces
homozygosity

7. 7
MODES OF NATURAL SELECTION
Disruptive
Divergent selection pressures in
different parts of its distribution area
Individuals are both extremes are
favoured
Favours both extremes

8. Directional Selection Stabilizing selection Disruptive selection
Individuals at one end of the
frequency distribution are
selected
Individuals at both the ends
are eliminated
Individuals are both extremes
are favoured
Due to change in environment Constant or unchanging
environment
When population is subjected
to divergent selection
pressures in different parts of
its distribution area
Favours extreme phenotype Favors average/ normal
phenotypes
Favours both extremes
Favours accumulation of
mutations that increases
fitness in changing env
Checks on accumulation &
introduces homozygosity

9. 9
Antibiotic resistance among bacteria represents?
a. Balancing
b. Stabilizing
c. Directional
d. Disruptive

10. 10
Self cross between Aa results in AA, Aa,aa F1 progeny, the forces of
evolution preserving either AA or aa is:
a. Stabilizing selection
b. Cyclic
c. disruptive
d. directional
Selection which operates against any direction & do not allow to
disrupt the present mean is termed?
a. Directional
b. Stabilizing
c. Disruptive
d. Balancing

11. 11
A person heterozygous for sickle cell anemia has
advantage for both malaria & sickle cell anemia it
illustrates?
disruptive
a. Directional selection
b. Heterozygote advantage
c. Disruptive selection
d. Directed mutation selection