The Theory of Evolution
by Natural Selection
Evolution of Evolutionary thinking
(Pre-Darwinian)
Jean-Baptiste Lamarck (1744-1829) –
French naturalist, proposed a theory that
organisms were driven by some inner force
toward greater complexity. But thought that
org. could pass on traits to their offspring that
they acquired during their lives.
(“Lamarckism”, proposed in 1809)
Lamarckism
Lamarck based his theory on two observations
thought to be true in his day:
1. “Use it or lose it” - Individuals lose
characteristics they do not require and
develop those which are useful.
2. Inheritance of acquired traits - Individuals
inherit the acquired traits of their ancestors.
Lamarckism
 Examples include: the stretching
by giraffes to reach leaves leads to
offspring with longer necks;
 Strengthening of muscles in a blacksmith's
arm leads to sons with like muscular
development.
This theory was later disproved!
Darwin’s Voyage
 Charles Darwin
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Set sail on the HMS Beagle in 1831
Became the ship’s naturalist
Arrived in the Galapagos Islands in 1835
Observed that the animals on the islands were
similar to those on the mainland
Darwin’s Voyage
 Galapagos Animals
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The Galapagos animals,
while similar, were also
different from island to
island as well as to the
mainland
Most obvious difference
were the sizes and
shapes of the finches’
(small birds) beaks
Sizes and shapes of the
beaks were adapted to
what the birds ate
Galápagos Finches

Berry
eater
Insect
eaters
Seed Cactus
eaters eaters
Beak
shape
varies
depending
on diet
Darwin’s Voyage
 On the Origin of Species (Darwin’s book)
 For the 20 years that followed his return to England
Darwin studied plants, animals and adaptations
 Darwin wrote about how species can change gradually
over many, many generations and become better
adapted to new environmental conditions.
Evolution
 The gradual change in a
species over time.
Natural Selection
Organisms that are better adapted to an
environment are more likely to survive and
reproduce than organisms that are less well
adapted.
Adaptations
Katydids have camouflage to look like leaves.
Non-poisonous king snakes mimic poisonous coral snakes.
Factors that affect Natural Selection:
Overproduction most species produce far more
offspring than will/can survive
Overproducers
Producers
mature rapidly
mature slowly
short-lived: most die
before they reproduce
live long lives: low juvenile
mortality rate
have many offspring - tend have few offspring at a
to overproduce
time
invest little in individual
youngsters
care for their young
population not regulated
by density: boom and bust
population figures
population stabilizes near
carrying capacity
Factors that affect Natural Selection:
Competition: since food and resources are
limited, the offspring have to compete to
survive
Darwin called it: “Struggle for existence”
Factors that affect Natural Selection:
Variation: Members within a species exhibit
individual differences – these differences must be
inheritable
Natural selection won’t work in a population of
clones! Remember that a key to variation is
sexual reproduction.
Factors that affect Natural Selection:
Survival to reproduce: Only those individuals
that are better suited to the environment will
survive and reproduce (“Survival of the
fittest”).
Fit individuals pass on to a portion of their
offspring the advantageous characteristics.
How do new species form?
 Natural Selection
 Continental Drift
 Changes in environment
 Mutations
 Man
Continental Drift
 Fossil records show that when the continents
were connected animals walked across.
When the continents separated, the animals
were separated.
Changes in the Environment
Example, the pepper moth.
Originally, the pepper moth was white, which
was good because it could blend in.
Then, trains were invented and the soot they
produced covered the trees. Making the
trees black. The moths that were black could
now survive better.
Mutations
Some species are more susceptible to
mutations. Some mutations allow the animal to
survive; other mutations do not allow the animal
to survive. The mutations that are not decrease
the chance of survival remain.
Man: Artificial Selection
 Selective breeding as practiced by humans on
domesticated plants and animals….
 For example: Dogs
Evidence of Evolution
 Fossil Record
 Similarities in Body Structure
 Similarities in Early Development
 Vestigial Structures
 Similarities in DNA
Fossil Record
 How fossils form
 An organism dies and becomes buried in sediment
 Minerals gradually replace the bones and more
sediments cover the fossil
Pterodactyl
Trilobite
Similarities in Body Structure
If the two organisms have body
structures that are similar, they must have
had a common ancestor.
Similarities in Early Development
 Scientists look at
embryos of different
organisms and find
that many embryos
resemble one
another.
Vestigial Structures
 Vestigial Structures A structure found in an organism that is no longer in
use but may have been useful at some point in the organism's life.
Tail Present in human and all vertebrate embryos.
In humans, the tail is reduced; most adults only
have three to five tiny tail bones and,
occasionally, a trace of a tail-extending muscle.
Whales possess a femur
and pelvis, but these bones
are no longer useful to the
mammals.
Vestigial Structures
Why do dogs have
tiny, functionless
toes on their feet
(dewclaws)?
Ancestral dogs had
five toes on
each foot
Similarities in DNA
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The more similar the sequences of DNA are,
the more closely related the organisms are.
Humans and chimpanzees DNA is more
similar than human DNA is to dog DNA.
Patterns of Evolution
 Divergence
 Adaptive radiation
 Convergence
Divergent Evolution
 New species develop traits that differentiate them from
their ancestors
 Divergence accounts for descendants that differ from their
ancestors and from one another
Convergent Evolution
 Unrelated
animals
develop similar
body forms to
fill same niche
Adaptive radiation
 Mammals filled ecological niches
vacated by dinosaurs
 Greatest mass extinction occurred
millions of years before us- the
dinosaurs
 More than 90% of all species died
out – the animals that remained filled
the gap
How we know what happened when
 Radiometric Dating
 Stratigraphy
 Molecular clocks
Radiometric Dating

Radioactive Dating
(Absolute Dating)
 Rocks that fossils are
found near contain
radioactive elements.
 The half-life of a
radioactive element is the
time it takes for half the
atoms in an element to
change into a stable
element (carbon 14 into
carbon 12)
Stratigraphy
Fossils from organisms
that died longer ago are
buried deeper in the
sediment/rock than
fossils from organisms
that died more recently.
Molecular clocks
 When a stretch of DNA does indeed behave
like a molecular clock, it becomes a powerful
tool for estimating the dates of lineagesplitting events.
The End (?)