Charles Darwin first proposed the theory of evolution by natural selection. Natural selection is the process by which organisms with traits better suited to their environment tend to survive and reproduce more, passing on those favorable traits. For natural selection to occur there must be genetic variation within a population, the variations must be heritable, individuals must compete for limited resources, and favorable traits must increase reproductive success over time. These conditions lead to adaptations becoming more common and populations changing.

2. Theory of Evolution by Natural
Selection
Presented by – Lekhan Lodhi
Research scholar
Department of Zoology
Dr. H. S. Gour university Sagar ( M.P)

3. Darwin and His Theory of Evolution
by Natural Selection
• Evolution - changes in the
characteristics of groups of
organisms over time
• Charles Darwin was the first to
propose a feasible mechanism
for evolution. It is called natural
selection.

4. What is a scientific theory?
Scientific Theory- an idea that is strongly supported by
evidence. It is generally accepted and used to explain many
observations.
*Remember: The word “theory” in everyday language and
in scientific language mean very different things.
In everyday language, people use the word theory to mean
a hypothesis or an educated guess.

5. Natural Selection-
differential success in
survival and
reproduction of
individuals with
different phenotypes
(physical
characteristics)
resulting from
interactions with their
environments

6. Conditions Needed for Natural Selection
1) Variation
There are a variety of forms of traits or phenotypes.

7. What causes genetic variation?
1) Mutations- changes in the
DNA sequence can result in
new traits
2) Sexual Reproduction- New
combinations of traits can be
created in gametes during
meiosis.
The wide variety of gametes
created will join together to
make diverse offspring.

8. 2. Inheritance
● Only traits that are determined by an organism’s genes
will be passed to the next generation.
● Some variation is caused by the environment, such as the
loss of a limb or the building of muscle. These traits are
not heritable, so they can’t be selected for in the next
generation.

9. 3. Survival and Reproduction
● Individuals compete for limited resources, such as food,
shelter, and mates.
● Individuals with traits best adapted for the environment are
able to out compete other individuals for resources, allowing
them to survive and reproduce more.

10. “Survival of the Fittest”
● Evolutionary fitness isn’t a measure of physical
fitness but of reproductive fitness.
● Fitness- Relative ability to survive and produce
offspring in an environment

11. 4) Time
Evolution occurs over generations.

12. 5) Adaptations
● an inherited characteristic that increases an
organism’s chance of surviving and reproducing
● Over time, natural selections causes adaptations to
become more common in a population.

13. Drug Resistant Bacteria Practice
● Variation existed in a population of bacteria. Some were resistant to
antibiotics, while some weren't.
● The resistance was an inherited trait caused by a gene.
● When treated with antibiotic, the resistant bacteria survived and
reproduced more than the nonresistant ones, which were killed by the
antibiotic.
● Over time, the adaptation of drug resistance became more common in
each generation of bacteria.

14. Natural selection can lead to a variety of changes within a population.
These changes can be roughly grouped into three categories:
1) Stabilizing Selection: natural
selection that favors an
intermediate phenotype
Example: Birth weight, Human
height

15. 2) Directional Selection: natural selection that favors an extreme
phenotype and causes a shift in the distribution curve
Example: Birth weight, Human height

16. 3) Disruptive Selection: natural selection that favors two or more extreme
phenotypes are favored over any intermediate phenotype

17. Since Darwin’s time, several other forces of selection have been
explained:
1. Sexual Selection: adaptive changes in males and females that lead to
an increased ability to secure a mate.

18. 2. Artificial Selection: adaptive changes in organisms that occur
because of human forces.