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Darwin & Natural Selection
Evolution Unit
Learning Goals
 1. Define "Evolution" & "Natural Selection".
 2. Describe the 4 steps of Natural
Selection, giving an example of each.
 3. Explain the importance of "Variation".
 4. Explain how environmental changes can
trigger evolution
Theory of Evolution
 Evolution: The process of change over time
 Specifically, a change in the frequency of a
gene or allele in a population over time (it’s all
about the DNA!)
Evolution vs. Natural Selection
 They are not the same
thing!
 Evolution – is the
“what” (organisms
changing over time)
 Natural Selection is the
“how” (the mechanism
that causes the
changes)
Charles Darwin
 Father of Evolution
 Darwin went on a 5-year trip
around the world on the
ship, the HMS Beagle
 As the ship’s naturalist, he
made observations of
organisms in South America
and the Galapagos Islands
What did Darwin’s
travels reveal?
 The diversity of living
species was far greater
than anyone had
previously known!!
 Species separated on
different islands, while
similar, had different traits
How did tortoises and birds differ
among the islands of the Galapagos?
 Each island
had its own
type of
tortoises and
birds that
were clearly
different from
other islands
Galapagos
Tortoises
Darwin’s Travels
 These observations led him to develop
the theory of evolution!!
 He proposed that natural selection was
the mechanism for evolution
 He wrote a book called “The Origin of
Species”
Natural Selection
 Organisms that inherit advantageous
traits will reproduce more successfully
4 Steps of Natural Selection
 Darwin’s Theory of Natural Selection
occurs in four steps:




Overproduction
Genetic Variation
Competition
Selection
1. Overproduction
 Each species produces more offspring
than can survive
 Only some will survive to grow up and
reproduce (passing on their genes)
2. Genetic Variation
 Each individual has a
unique combination of
inherited traits (in the
DNA/genes).
 If an organism has a trait
that helps it survive and
reproduce, we call it an
adaptation
What adaptations do you see?
What adaptations do you see?
Why is Variation Important?
 Because the environment changes.
 The more variation within a species, the more
likely the species will survive
 If everyone is the same, they are ALL vulnerable
to the same environmental changes or diseases
 Organisms that cannot evolve to adapt to a
changing environment, will become extinct.
3. Competition
 Individuals COMPETE for limited resources:
 Food, water, space, mates
 Natural selection occurs through “Survival of
the Fittest”
 Fitness: the ability to survive and reproduce
 Not all individuals survive to adulthood
 Starvation, sickness, eaten by predators
“Survival of the Fittest”
 It’s not always the biggest,
or the strongest, or the
fastest, or the smartest!
 It’s simply the one that is
best able to survive in
the environment
4. Selection
 The individuals with the best traits /
adaptations will survive and have the
opportunity to pass on their traits to offspring.
 Those traits will increase in the population
4. Selection
 The individuals that do not survive will not
pass on their traits.
 Those traits will decline (go down) or disappear in
the population
Selection
 Individuals with traits that are not well suited to their
environment either die or have fewer offspring.
 Evolution occurs when successful traits build up in
a population over many generations and
unsuccessful traits are eliminated by the death of
the individuals.
 The successful traits have been “selected”
✗
✔
Gene Pool
 The gene pool is all of
the different traits in
the population
Gene Pool
 The gene pool will change as some
traits are added (through mutation),
successful traits increase, and
unsuccessful traits decrease
What natural selection is NOT
 It is NOT a change during an
organism’s lifetime
 It is caused by the traits that an organism
is born with…found in the genes/DNA
 It is NOT a conscious act
 Organisms do not “try” to adapt. They only
try to survive and reproduce!
 It is NOT random chance
 There are traits that are more likely to get
passed on, because they are the best fit for
the environment
Peppered Moths
 A famous example of an environmental
change causing evolution
 Click here to watch a Youtube clip:
http://www.youtube.com/watch?v=LyRA807djLc
Peppered Moth
A
 Which moth will the bird catch?
B
Learning Goals
 1. Define "Evolution" & "Natural Selection".
 2. Describe the 4 steps of Natural
Selection, giving an example of each.
 3. Explain the importance of "Variation".
 4. Explain how environmental changes can
cause evolution