Darwin's three main principles of natural selection state that, in order for the process to occur, most characteristics in the population must be inherited, more offspring must be produced than can survive, and the fittest offspring must be more likely to survive and reproduce.In the mid-19th century, a man came up with a very powerful idea, the idea that species could change. Today, all the time, we hear about animals adapting, endangered species going extinct, viruses mutating. But in the 1800s, people conceptualized a much more static world. One man looked past all that. His name was Charles Darwin. Darwin called his idea the theory of natural selection. Natural selection is defined as a natural process that results in the survival and reproduction of organisms with genetic traits best suited to their environment. A shorter (but no less accurate) definition might be "survival of the fittest." Within any population, the fittest individuals, or the ones who fit the environment best, usually survive and reproduce, passing on their genetic traits to future generations.

1. Evolution, Darwinism and natural
selection
Zoology 7th semester

2. “QUOTES…

3. OUTLINE
1. Introduction
2. Factor of Darwinism
 Rapid multiplications
 Limited environmental resources
 Struggle for existence
 Variation
 Survival of the fittest
 Inheritance of useful variation
3. Formation of new species
4. Darwinism v/s Lamarckism

4. Biological classification
Evolution: The change in the characteristics of a
species over several generations.

5. CHARLES ROBERT DARWIN
 12 February 1809 – 19 April 1882( England- Shrewsbury )
 Field- naturalist, geologist and biologist.
 Europe- beagle- south America( Galapagos )-Africa-Australia (5 years)
 Species of south America were different from Europe (flora and fauna )
 Study fossil record-evolve modern ssp
 13 type of finches( beak)
 Giant turtles ( long and short neck) due to geographical barrier
 1844- Wallace and Darwin- unpublished paper- linnean society in London
 Result :
 Origin of new ssp and adaptation- closely related process
 New ssp arise from ancestral-by adaptation because of different environment
 Evolution ( modification) fork ,tree
 Book publish 1859- on the origin of species(20 years)
 Award- doctor of laws

6. Darwinism : Darwin's Theory of Natural
Selection.
DARWIN’S THEORY OF NATURAL SELECTION
1. The theory of the evolution of species by natural selection
advanced by Charles Darwin announced on July 1, 1858.
2. Darwin travelled round the world in the 19th century on the ship
H.M.S. Beagle with a surveying expedition for 5 years (1831-1836)
3. “On the origin of species by Means of Natural Selection “,
November,1859.

7. Natural Selection
Darwin’s most famous and most important theory of evolution.
Only process that promotes adaptation.
The mechanism 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.”

9. Factor of Darwinism
Rapid multiplication
Limited environmental resources
Struggle for existence
Variation
Survival of the fittest
Inheritance of the useful variation
Formation of new species

10. Rapid multiplication
over production
 Organism reproduce more offspring than can survive. There is high
death rate. 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

11. Limited environment resources
Space and food
• Increase of population in animal & plants requires more space and
food but the universe remains constant.
• Individuals compete for limited resources
• food,
• water,
• spaces,
• mates.

12. Struggle for existence
competition
Competition among members of a species for food, living space,
and other necessities of life.
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..)

13. Variation & Adaptation
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”.
Fig : the original species of the finch had a small beak where it only was able
to eat insects and nectar. As centuries pasted the finch's beak started to curve
downward making the advantage for the bird to tears bark from trees. However,
current finches are nectar feeders who look closely to a iiwi and that their beaks
are long and curved downward suggest that Darwin's theory of natural selection
made the finches advantage being able to get nectar from bark and and flowers.

14. Survival of 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).
Fig : some Giraffes have long neck and other have short neck.
Something caused low-lying shrub to die out, the giraffes with short
neck would not get enough food. After a few generation all the
giraffes would have long necks.

15. 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.

16. Formation of new species
• 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 specie.

17. Conditions of Natural Selection
1. Reproduction
2. Heredity
3. Variation in individual characters among the members of the population
4. Variation in the fitness of organisms according to the state they have for a heritable character.
If these conditions are met for any property of a species, natural selection automatically results.

18. TYPES OF NATURAL SELECTION

19. Types of Natural Selection
Natural selection can occur with or without environmental change.
It is of three types:-
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection

20. Directional Selection
 The first type of natural selection is called directional selection.
 It derives its name from the shape of the approximate bell curve
that is produced when all individuals' traits are plotted.
 Instead of the bell curve falling directly in the middle of the axes on
which they are plotted, it skews either to the left or the right by
varying degrees. Hence, it has moved one direction or the other.

21. Directional selection curves are most often seen when
one coloring is favored over another for a species.
E.g. If thicker-shelled oysters are more resistant to
breakage than thinner- shelled oysters, crabs will be less
able to prey upon them, and thicker-shelled oysters will
be more likely to survive to reproduce.

22. Stabilizing Selection
 The most common of the types of natural selection is stabilizing
selection.
 In stabilizing selection, the median phenotype is the one selected
for during natural selection (favors the intermediate states of
continuous variation).
This does not skew the bell curve in any way. Instead, it makes the
peak of the bell curve even higher than what would be considered
normal.
Over time, the intermediate states become more common and each
extreme variation will become less common or lost.

23. Continuing our oyster example, very light-colored or
very dark-colored oysters might be more frequently
preyed upon by shore birds, simply because they are
more obvious on the oyster bar; as a result, the
intermediate hues become more common.
 A classic example of this is human birth weight.
Babies of low weight lose heat more quickly and get ill
from infectious diseases more easily, whereas babies of
large body weight are more difficult to deliver through
the pelvis. Infants of a more medium weight survive
much more often. For the larger or smaller babies, the
baby mortality rate is much higher

24. Disruptive Selection
 Disruptive selection is also named for the way the bell curve skews
when individuals are plotted on a graph.
 It occurs when natural selection favours both extremes of
continuous variation. Over time, the two extreme variations will
become more common and the intermediate states will be less
common or lost.
To disrupt means to break apart and that is what happens to the
bell curve having one peak in the middle, disruptive selection's graph
has two peaks with a valley in the middle of them.
Disruptive selection can lead to two new species.

25. This might happen in shallow water among rocks. Light-colored
oysters are more cryptic (less easy for a predator to see) because
they match the rock color.
Dark-colored oysters blend into the shadows cast by the rocks. In
this case, intermediate-colored oysters would be most heavily
preyed upon by the crabs, and very light and very dark oysters would
survive to reproduce.

26. Artificial selection
The process of selection conducted under human
direction.
Natural selection
The process occurs naturally without human interference.
Both two methods involved in the breeding of organisms.
Both natural selection and artificial selection allow selected
characters to pass to the next generation.

27. Lamarck’s theory of Evolution
Use and disuse
Organism could change the size or shape of organs by using them or not using
them
Example: Giraffe’s necks longer (from stretching)
Theory “Giraffes inherited long necks from short necked ancestors who
continually stretched their necks to reach food.”

28. DARWINISM V/S LAMARCKISM
COMPETITIVE COMPARISON
1. It doesn’t believe in the internal vital force.
2. These vital force do not form part of Darwin’s natural
selection.
3. An organ can develop further or degeneration only due
to continuous variation
4. Struggle for existence is vary important.
5. Only useful variation are transferred to next generation.
6. Darwin’s natural selection theory is based on survival of
the fittest.
1. 1. There is an internal vital force in all organism.
2. 2. It consider new need or desire or produce new structure
& change habits of the organism.
3. 3. According to this theory if organ is contently used it
would be better developed where as disuse of organ results
in its degeneration.
4. 4. No concept of struggle for existence
5. 5. All the acquired characters are inherited to the next
generation .
6. 6. Lamarckism does not believe in survival of the fittest.

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