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Evolution
Ms.
Edwards
Darwin’s Theory
• Charles Darwin (English) set sail
around the world on the Beagle in
December 1831.
• He was a naturalist and wanted to
learn about the living things in the
world.
• The diverse animals and plants he saw
lead him to create the Theory of
Evolution by Natural Selection.
Darwin’s Observations
• He stopped in South America (S.A),
and then went to the Galapagos
Islands.
• He observed:
1. The diversity of living things
2. Remains of ancient organisms
3. Characteristics of life on the
Galapagos Islands.
Diversity of Living Things
• In South America, Darwin saw diverse
insects, mammals, and plants.
• Species- a group of organisms that can
mate and produce fertile offspring.
• To date, there are more than 1.7 million
species of organisms on earth!
Fossils
• Fossil- the preserved remains of an
organism that lived in the past.
• The fossils Darwin saw made him
wonder about life in the past.
Galapagos Organisms
• Darwin compared the organisms on the
Galapagos to each other and to the
organisms elsewhere in the world.
Comparisons to S.A. Organisms
• Many birds and plants on the islands were
similar to those on the mainland.
• Iguanas on the island had larger claws to
grip rocks. Iguanas on mainland had
smaller claws to climb trees.
• Darwin hypothesized that a small number
of animals were blown or drifted from
the mainland to the islands.
• Then those animals reproduced and slowly
became different from their mainland
relatives.
Comparisons Among the Islands
• Differences in tortoise shell shapes.
• Differences in finches’ beaks.
• Finches that ate worms had slender
beaks.
• Finches that ate seeds had strong, wide
beaks.
• Adaptation- a change that helps an
organism survive and reproduce.
Evolution
• Darwin returned to England and spent 20
years analyzing his findings.
• Darwin reasoned that organisms on the
island faced conditions that were
different from those on the mainland.
• Therefore, the species gradually changed
over time to adapt to their new
environment. This is called evolution.
Theory of Evolution
• Scientific Theory- a well-tested concept
that explains a wide variety of
observations.
• Darwin wasn’t sure how to explain his
theory, so he studied selective
breeding.
• He studied how English sheep were bred
to have fine wool.
Natural Selection
• Darwin published his findings in a book
entitled The Origin of Species (1858).
• Darwin explained that evolution occurred
because of natural selection.
Natural Selection- the process by which
individuals that are better adapted to
their environment are more likely to
survive and reproduce than other
members of the same species.
Factors of Natural Selection
• Overproduction- species produce more
offspring than can survive.
• Variation- differences between members
of the same species.
• Competition- food and resources are
limited, so members of a species must
compete to survive.
- Darwin’s Theory
Overproduction and Variation
• Natural selection is the process by which
individuals who are better adapted to their
environment are more likely to survive and
reproduce than other members of the same
species.
Natural Selection
• Darwin proposed that over a long time,
natural selection can lead to change.
• Helpful variations accumulate in a species
while unfavorable ones disappear.
Genes and Natural Selection
• Darwin did not know how these changes
came about.
• Scientists later discovered that changes
occur because of mutations in genes.
• Genes are then passed from parent to
offspring.
• Only traits that are inherited, or
controlled by genes, can be affected by
natural selection.
Evidence of Evolution
• Evidence tells us that evolution still occurs
today.
• Consider pesticides and insects:
1. Bugs are sprayed with pesticide
2. A few survive, reproduce, and pass
their genes to offspring
3. Offspring now have resistance
4. After many generations, all bugs are
resistant.
Interpreting the Evidence
• Fossils
• Embryonic Development
- Even though many adult organisms
look different, they look similar
while they are developing
- This suggests that they have a
common ancestor.
Interpreting the Evidence
• Some organisms have similar body
structures.
• Bird’s wing, dolphin’s flipper and dog’s leg
all have similar bone structures, which
suggest they share a common ancestor.
• homologous structures- similar structures
that organisms have inherited from a
common ancestor.
• Fossil evidence gives us clues about
homologous structures.
- Evidence of Evolution
Homologous Structures
The structure of the bones in a dolphin’s flipper,
a bird’s wing, and a dog’s leg is similar.
Homologous bones are shown in the same color.
Inferring Species Relationships
• Scientists have combined the evidence from
DNA, protein structure, fossils, early
development, and body structure to
determine the evolutionary relationships
among species.
• Closely related species have similar DNA,
which they inherited from a common
ancestor.
• Recent DNA evidence shows that dogs are
more closely related to wolves than
coyotes.
Branching Trees
• Branching Tree- a diagram that shows how
scientists think organisms are related.
How Do New Species Form?
• A new species can form when a group of
individuals becomes isolated from the
rest of its species long enough to evolve
different traits.
How Do Fossils Form?
• Most fossils form when organisms die and
become buried in sediment.
• Petrified fossils- Fossils that become
buried and turn into rock.
• Mold- a hollow space in sediment in the
shape of an organism.
• Cast- a copy of the shape of the organism
that made the mold.
- The Fossil Record
How Do Fossils Form?
Most fossils form when organisms that die
become buried in sediments.
Determining a Fossil’s Age
• Two methods
1. Relative Dating- uses the position of
the fossils. Can only be used when
rock layers have been preserved in
their original order.
2. Radioactive Dating- used to
determine the exact age of fossils.
Uses radioactive elements.
• Half-life- the time it takes for half of
the atoms in a sample to decay.
- The Fossil Record
Radioactive Decay
The half life of potassium-40, a radioactive element,
is 1.3 billion years. This means that half of the
potassium-40 in a sample will break down into
argon-40 every 1.3 billion years. The graph shows
the breakdown of a 1-gram sample of potassium-40
into argon-40 over billions of years.
What Do Fossils Reveal?
• Fossil Record- the millions of fossils that
scientists have collected.
• Extinct- a species is extinct if no
members of that species are alive.
• Geologic Time Scale- the calendar of
Earth’s history.
- The Fossil Record
Earth’s History as a Clock
Fossils found in rock layers tell the history of life
on Earth. The history of life can be compared to
12 hours on a clock.
- The Fossil Record
Precambrian Time and the Paleozoic Era
- The Fossil Record
Mesozoic and
Cenozoic Eras
Unanswered Questions
• 2 questions about evolution:
1. What caused the mass extinctions?
2. How fast does evolution occur?
Rate of Evolution
• Gradualism- theory that says evolution
occurs slowly but steadily.
• Punctuated Equilibria- theory that
accounts for the gaps in the fossil
record. Says species evolve quickly
during short periods of time and then
have long periods of no change.