1. The Theory of Evolution by Natural Selection

2. Charles Darwin challenged traditional views of a
young Earth inhabited my unchanging species.
In 1859, Darwin published convincing evidence that
the many species of organisms
presently inhabiting Earth are
descendants of ancestral species,
that is, species evolve.
Most importantly,
Darwin proposed a reasonable
mechanism explaining how
species evolve by natural selection.

3. Darwin’s
Theory of Evolution
by Natural Selection

4. 1. Overproduction & Struggle for Existence
Individuals produce more offspring
than can survive, that is,
than the environment can support.
Individuals in the population
must compete to obtain food,
living space, etc.
Darwin described this as
the struggle for existence.

5. 2. Variation and Adaptation
Individuals in a population have natural variations among
their heritable traits. Some variations make an organism
better suited to life in its environment. Any heritable
characteristic that increases an organism’s ability to survive
and reproduce in its environment is called an adaptation.
Some variations may include:
Some adaptations may include:
Color of fur
speed
size of feet

6. Adaptions may also involve behaviors such as
staying still to avoid predation,
looking larger and
standing your ground, etc.

7. 3. Survival of the Fittest
Individuals with the best adaptations are better suited to
their environment, as a result they can survive and reproduce.
These individuals are said to have greater fitness.
Fitness describes how well an organism can survive and
reproduce in its environment. The individuals that have
the greatest fitness survive
and reproduce more offspring.
Individuals with low fitness
die before reproducing or
reproduce few offspring.

8. 4. Natural Selection
Over generations, the individuals with the adaptation
tend to accumulate in the population.
Natural selection will result in species with adaptations
that are well suited for survival and reproduction
in an environment.
Natural selection is the process
by which organisms with variations
most suited to their environment
survive and leave more offspring.
The environment (nature) “selects”
which organisms will survive.

9. Variations in Populations
The individuals in a population are the same species,
but they are not identical.
The individuals in a population
vary in their characteristics or traits.
A variation is the difference in a physical trait
of an individual from those of other individuals
in the population.
Variation? ___________________
Color and pattern

10. Adaptation A variation that allows an individual
to better survive in its environment.

11. Example of Natural selection in jaguars.
11,000 years ago, jaguars faced extinction.
There were fewer of their usual prey, mammals.
There was an abundance
of turtles.
But eating turtles
required large jaws.

12. Most jaguars in the population had small jaws.
But there was variation in the population
in the size of the jaw.
Some jaguars in the population had larger jaws.
Variation?
Size of jaw

13. Those jaguars with
larger jaws were able
to eat the turtles.
Favorable variation?
The larger jaw is an
Larger jaw
Adaptation

14. The jaguars with larger jaws
survived, and produced
more offspring.
Their offspring inherited
the larger jaws and they
survived and produced
more offspring.

15. In time, the number of individuals that carry the
favorable adaptation will increase in the population.
More and more jaguars in the population
Will have larger jaws.
Simultaneously, individuals that do not have
the adaptation will die, or produce fewer offspring.
Eventually, the population will consists
only of individuals with large jaws.
The nature of the population has changed.
Evolution has occurred.

16. Larger jaws became a more common trait in the population.
Over many generations,
jaguars evolved larger jaws.

17. Darwin further suggested that organisms differ
from place to place because their habitats present
different challenges to survival and reproduction.
Each species has accumulated adaptations
in response to its particular environment
resulting in evolution of the species.

18. Common Descent Every organism alive today is descended from parents
who survived and reproduced. Those parents descended
from their parents, and so forth back through time.

19. Darwin proposed that living species are
descended with modification, from common ancestors.
Darwin also proposed that, over many generations,
adaptations could cause successful species to evolve
into new species.
All species –
living and extinct –
are descended from
ancient common ancestors.
A single “tree of life”
links all living things.

20. How is evolution defined in genetic terms?
Variation is the raw material for natural selection.
Where does this variation come from?
Individuals in a population may have many different alleles.
These alleles give rise to many different phenotypes.

21. What are the sources of genetic variation?
Genetic Recombination in Sexual Reproduction:
New individuals are a random combination
of the alleles of two parents.

22. What are the sources of genetic variation?
Genetic Recombination in Sexual Reproduction:
Mutations:
A mutation is a change in the DNA of a cell,
a change in a gene.
Mutations may give rise to new phenotypes.
Ex: Blue eyes