1. Darwin’s
Theory of Natural Selection:
A Mechanism For Evolution

2. Be careful not to suggest…
 Organisms can acquire/change
characteristics during their
lifetime based on NEED
 These characteristics can only be
passed on genetically.
 Example: giraffes needed
longer necks so they
stretched them!

3. Charles Darwin
1809 -1882
“I have called this
principle, by which
each slight
variation, if useful, is
preserved,
by the term
Natural Selection.”

4. Important Features of Darwin’s
Theory of
Natural Selection:
A Mechanism for
Evolution

5. 1. Organisms
produce more
OFFSPRING
than can
possibly
SURVIVE.

6. Thomas Malthus
1766-1834
What "struck" Darwin in Essay
on the Principle of Population
(1798) was Malthus's
observation that in nature
plants and animals produce far
more offspring than can
survive, and that Man too is
capable of overproducing if left
unchecked. Malthus concluded
that unless family size was
regulated, man's misery of
famine would become globally
epidemic and eventually
consume Man.

7. REMEMBER…
Overproduction
(of Offspring)

8. 2. As populations
grow in size,
NATURAL
RESOURCES
become limited.
This forces
organisms to
COMPETE…

9. What do lions & hyenas
compete for?
 Food
 Water
 Shelter – bear & raise young
 Space – hunt, travel

10. What else to lions compete
for with other lions?
 Mates
 Do all lions mate?

11. REMEMBER…
Competition

12. 3. Because
organisms must
COMPETE for limited
resources, they
STRUGGLE to EXIST.
If they do not obtain
needed
resources, they
____.

13.  VARIATIONS exist in all populations of
organisms.
The 2 causes of these differences can be
attributed to:
– Mutation
– Meiosis (crossing-over, unique
combination of chromosomes in
egg/sperm)

14. Because
organisms are
built somewhat
differently from
others in the
same population,
some have an
ADVANTAGE

16. What determines which variety of
finch has a beak that will help it
survive more successfully???

17. Adaptations make an
organism FIT for the
environment.
 Fitness can be described as
the ability to survive (out
compete) and reproduce.
 Why not just survive?

18. REMEMBER…
Survival
Of The
Fittest

19. Environment creates
challenges & opportunities
 Selective Pressures are applied to each
population of organisms making it
difficult for them to survive.

20. REMEMBER…
The environment
applies a
Selective pressure

21. 5. Organisms that survive can pass on
their traits to their offspring. They are
able to REPRODUCE.

22. WHY?

23. Remember:
Reproductive Advantage

24. How much change?
 Populations of organisms may change
but not evolve into a new species.
 Evolution does not mean that new
species must arise, just that they can
if enough changes take place.

25. REMEMBER…
Darwin referred to this as:
Descent
with
Modification

26. 6. Over time,
hundreds or thousands of
generations, the
characteristics (VARIATIONS)
of a population change.

27. If members of the same species
can no longer reproduce
successfully (their offspring can reproduce),
they are considered to be two
distinct species.

28. REMEMBER…
Speciation -
Over time, members of a
species may become SO
different that they are now 2
different species
How does evolution really work?

29. Overview
1. Overproduction of offspring
2. Competition for limited resources
3. Variations exist which are inherited
from their parents.
4. Those more fit for the environment
have a selective advantage
5. Reproductive advantage
6. Population characteristics may change