The presentation explains that how nature plays an important role in the selection of organisms for survival include the postulates provided by Charles Darwin with examples. The presentation also explains different types of selections with examples.

1. Presented by – Dhananjoy kalita
M.Sc 1𝑠𝑡 Semester ,
Roll No.-204141005
School - Royal school of life science
Royal global university, guwahati

2. •CONTENTS :
 Introduction
 Darwin’s points on natural selection
— Overpopulation
— Competition
― Variation
— Survival of the Fittest
— Inheritance of Useful Variation
— New Species
 Examples of natural selection
 Types of natural selection
— Stabilizing Selection
— Directional Selection
— Disruptive Selection

3. • INTRODUCTION :
• Natural selection is Darwin’s most famous theory.
• The machanism for adaptation and evolution is called “natural
selection”
• This theory states that -
“ Those organisms best adapted to their environment have a better
chance of surviving and reproducing.”
• Darwinism : Darwin's Theory of Natural Selection.
• The theory of the evolution of species by natural selection advanced
by Charles Darwin announced on July 1, 1858.
• “On the origin of species by Means of Natural Selection “,
November,1859.

4. DARWIN’S 6 POINTS OF
NATURAL SELECTION
1.Overpopulation
2.Competition
3.Variation
4.Survival of the Fittest
5.Inheritance of Useful
Variation
6.New Species
Source - https://cdn.nybooks.com/wp-content/uploads/2020/03/darwing_charles-042320.jpg

5. 1. OVERPOPULATION
 Organisms tend to produce many offspring – more than the environment can support
 Organisms will reproduce until something stops them (usually food)
e.g. -
 Paramecium divide 3 times in 48 hours with this rate, a single paramecium will produce in 5 year a
mass of paramecium equal to ten thousand times the size of earth.
 Plant produce thousands of seed each year.
 Cod fish may produce over a million egg in a year. If all the egg developed into fishes, the whole
Atlantic Ocean will be full of cods in Five years.
Source: http://www.mosaictraining.ca/fishing/images/stories/whatson%2013.jpg

6. 2. COMPETITION
•Because of overpopulation,
individuals compete with one another
over limited resources → food, water,
shelter, mates
•The Struggle for existence is 3 fold for
every individual :-
 Intraspecific struggle - Within
species
 Interspecific struggle - Different
species
 Environmental struggle - Change in
environmental factor (heat, cold,
drought, flood etc)
Source - https://biologydictionary.net/wp-content/uploads/2018/10/Intraspecific-competition.jpg
Source - https://static.todamateria.com.br/upload/54/35/5435978db2860-predatismo.jpg
Source - https://theaimn.com/wp-content/uploads/2019/01/roo-at-menindee.jpg

7. 3. VARIATION
• Understanding how individuals of the same
species vary was key to Darwin developing
his theory.
• Because the environment changes, the
more variation within a species, the more
likely it will survive.
• Each individual has a unique combination
of inherited traits.
• An inherited traits that increase an
organism's change of survival is called
“Adaptation”. Source - https://i.pinimg.com/originals/4a/55/c7/4a55c76a43b30ac8c8d5a90f52e399c0.gif

8. 4. SURVIVAL OF THE FITTEST
(NATURAL SELECTION)
• The individuals with the best traits / adaptations
will survive and have the opportunity to pass on it’s
traits to offspring.
• Natural selection acts on the phenotype (physical
appearance), not the genotype (genetic makeup).
Source - https://img.favpng.com/21/12/15/natural-selection-selective-
breeding-evolution-survival-of-the-fittest-adaptation-png-favpng-
g4aA4ww82f3J9VkiYDB4mSnws.jpg
Source - https://images.app.goo.gl/U6V4vg6q7KfEnzGS8

9. 5. INHERITANCE OF THE USEFUL
VARIATION
• The individual, selected
by nature, pass on their
useful variation to the next
generation.
• Thus, offspring of fit
individual also be fit.
Source - https://images.app.goo.gl/486ZVXNkgxMUuFG57

10. 6. NEW SPECIES
• New species results by
inheritance of trait(s), on
genes, that give them another
advantage over others.
• In each generation, new
favorable variations appear
and supplement the favorable.
• After a number of generation
the variations become so many
which generate a new species.
• New species evolve! Source - https://kullabs.com/uploads/253.jpg

11. EXAMPLES OF NATURAL
SELECTION
1. Darwin’s finches
2. Peppered moth
3. Pesticide resistant
mosquitoes
Source - https://encrypted-
tbn0.gstatic.com/images?q=tbn:ANd9GcSFzCHLIx84oM-
qKYyL0rBCw9jApjGWz0uqIw&usqp=CAU
Source - https://i.redd.it/b855rcxdtknz.jpg
Source -
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6
e/Pest_resistance_labelled_light.svg/1200px-
Pest_resistance_labelled_light.svg.png

12. TYPES OF NATURAL SELECTON
1. STABILIZING SELECTION –
 This type of natural selection occurs when
there are selective pressures working against
two extremes of a trait and therefore the
intermediate or “middle” trait is selected for.
 If we look at a distribution of traits in the
population, it is noticeable that a standard
distribution is followed:
 Example: For a plant, the plants that are
very tall are exposed to more wind and are at
risk of being blown over. The plants that are
very short fail to get enough sunlight to
prosper.
 Therefore, the plants that are a middle
height between the two get both enough
sunlight and protection from the wind.
Source -
https://sites.google.com/site/naturalselection12345/_/rsrc/146888619888
3/process-1/url.gif?height=211&width=320

13. 2. DIRECTIONAL SELECTION
This type of natural selection occurs when
selective pressures are working in favour of
one extreme of a trait.
Therefore when looking at a distribution of
traits in a population, a graph tends to lean
more to one side:
Example: Giraffes with e longest necks are
able to reach more leaves to each.
Selective pressures will work in the
advantage of the longer neck giraffes and
therefore the distribution of the trait within
the population will shift towards the longer
neck trait.
Source - https://image.slidesharecdn.com/typesofselection-100612202246-
phpapp02/95/types-of-selection-5-728.jpg?cb=1276374223

14. 3. DISRUPTIVE SELECTION
This type of natural selection occurs when selective
pressures are working in favour of the two extremes
and against the intermediate trait.
This type of selection is not as common.
When looking at a trait distribution, there are two
higher peaks on both ends with a minimum in the
middle as such:
Example: An area that has black, white and grey
bunnies contains both black and white rocks. Both
the traits for white and black will be favored by
natural selection since they both prove useful for
camouflage.
The intermediate trait of grey does not prove as
useful and therefore selective pressures act against
the trait.
Source -
https://sites.google.com/site/naturalselection12345/_/rsrc/1468886200
067/process-1/url-2.gif

15. Source - https://o.quizlet.com/xZHfGcc44lgk44wnnrkfgw_b.jpg
Source -
https://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Directional%2C_Disruptive_a
nd_Stabilizing_Selections.svg/1200px-
Directional%2C_Disruptive_and_Stabilizing_Selections.svg.png

16. Thank you