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Topic 8 – Natural Selection
DAY 1
THEORY OF EVOLUTION
Natural Selection: A
Mechanism of Change
WHAT ARE WE LEARNING TODAY?
Benchmarks
 SC.912.L.15.13 – Describe
the conditions required for
natural selection, including:
overproduction of offspring,
inherited variation, and the
struggle to survive, which
result in differential
reproductive success.
Learning Objectives
 I will analyze the reasoning
in Darwin’s theory of
evolution by natural
selection.
 I will relate the concepts of
adaptation and differential
reproductive success to the
theory of natural selection.
WHAT IS THE ESSENTIAL QUESTION?
“It is not the strongest of the species that
survives, nor the most intelligent that survives. It
is the one that is most adaptable to change.”
Charles Darwin
After reading the quote above and learning about Darwin’s
theory of natural selection, what do you think he meant by
it?
WHO IS CHARLES DARWIN?
 Greatest contributor to our
understanding of evolution.
 Evolution is:
 the change of allele
frequencies in a population
over time.
 process by which modern
organisms have descended
from ancient organisms.
DARWIN’S TRAVELS TO THE GALAPAGOS
ISLANDS
• 1831: voyage on the HMS Beagle
• 5 year voyage around the world to Australia, S. America,
Galapagos Islands, and Africa
• Made several observations on the different species he
found
• He noticed 3 patterns of biodiversity
1. Species vary Globally
2. Species vary locally
3. Species Vary over time
OTHER SCIENTISTS THINKING ABOUT
EVOLUTION
Jean-Baptiste Lamarck
• Suggested organisms
change during their lifetime
Where did Lamarck go wrong
in is theory?
• Suggested individuals pass
on acquired traits to
offspring
• We now know individuals
do not evolve in their own
life time
• Recognized a link to an
organism’s environment and
its body structures
• We also know that
acquired traits cannot
be passed on to
offspring.
ACQUIRED TRAIT VS. INHERITED TRAIT
Acquired traits
Inherited Traits
• are obtained by the
individual in their life time
• Are passed on from the
genes of parents to
offspring
If you were to get a tattoo,
that would be an acquired
trait
Will affect the traits of future
generations of offspring
OTHER SCIENTISTS THINKING ABOUT
EVOLUTION
James Hutton and Charles
Lyell
Thomas Malthus
• Concluded that earth was
extremely old
• Reasoned that is populations
grew unchecked, there
would not be enough space,
food, resources for everyone
• Processes that changed
earth in the past are the
same ones changing the
earth now
• Principles of Geology
• English Economist
WHAT IS AN ADAPTATION?
Eyes with light
reflective retina to
see in the dark
Stripes to help them hide
Loose abdomen skin to
reduce injury when
kicked

Long, sturdy tail to help
with balance
Razor sharp teeth
with muscular
jaws to kill prey
Long, retractable claws to
grab prey
An adaptation is any
inherited
characteristic that
increases an
organism’s chance of
survival.
WHAT ADAPTATIONS IMPROVE THIS
ORGANISM’S ABILITY TO SURVIVE?
DARWIN’S 4 TENETS OF NATURAL SELECTION
• Natural Selection: process by which organisms with variations most
suited to their environments survive and leave more offspring
• Natural selection is the driving force, the mechanism, of
evolution
• Darwin proposed that in order for evolution to occur by natural
selection, 4 conditions must be met (4 tenets)
1. Overproduction of Offspring
2. Inherited variation within the population
3. Struggle for existence
4. Differential reproduction
WHAT IS OVERPRODUCTION OF
OFFSPRING?
 What is taking place
in this picture?
o Grasshoppers can
lay over 200 eggs
at a time.
o Only a small
fraction of these
offspring survive to
reproduce.
 Organisms produce
more offspring than
can survive.
WHAT IS INHERITED VARIATION?
 What are some of the
differences you see in
the population of
grasshoppers?
 What do you think
causes these
variations?
 Inherited variations
are the heritable
differences that
exists in every
population.
 Mutations
 Recombination
Where this
variation comes
from
WHAT IS THE STRUGGLE TO SURVIVE?
 How would you
describe the
environment
in which the
grasshopper lives?
 Which grasshopper is
better adapted to
survive in this
environment?
 What would happen to
this population if a
drought caused
the grass to turn brown?
WHAT IS THE STRUGGLE TO SURVIVE?
 Certain heritable
variations, called
adaptations, increase
an individual’s chance
of surviving and
reproducing.
o Heritable variation
includes brown and
green body color.
o Green coloration is
an adaptation that
allows grasshoppers
to blend into their
environment to be
less visible to their
predators.
WHAT IS DIFFERENTIAL REPRODUCTIVE
SUCCESS?
 What do you think it
means that “green
grasshoppers have
a greater
differential
reproductive
success than
brown
grasshoppers?”
 Green grasshoppers
survive and
reproduce more
often than do brown
grasshoppers in this
environment.
DIFFERENTIAL REPRODUCTIVE SUCCESS
 Organisms whose traits are
best suited to the
environment experience
better reproductive
success and become more
common
1) more grasshoppers are
born than can survive,
2) individuals vary in color, a
heritable trait, and
3) green individuals have a
higher differential
reproductive success in
the current environment
SUMMARY OF DARWIN’S THEORY
(NATURAL SELECTION)

Individuals organisms within a species are different.

Some of these differences are passed down from parent to
offspring, or heritable.

Individuals produce more offspring that can survive due to
limited resources creating a struggle for existence.

Individuals best suited, or adapted, to their environment
survive to experience higher differential reproductive
success.