Charles Darwin developed the theory of evolution by natural selection based on observations he made during a 5-year voyage as the naturalist on the HMS Beagle. Some of his most important observations included the diversity of life in South America and the Galapagos Islands, as well as adaptations and differences between similar species on different islands. These observations, combined with his knowledge of selective breeding, led him to propose natural selection as the mechanism of evolution. Natural selection results from overproduction of offspring, competition for limited resources, and inherited variations between individuals that influence survival and reproduction.

1. Science Explorer Chapter 5
Changes Over Time

2. Darwin’s Voyage
• In 1831, Charles Darwin boarded a British Naval
Ship called HMS Beagle.
– This was a five-year-long trip around the world.
– Darwin eventually became the ships naturalist.
• Naturalist- studies the natural world.
• His job was to learn as much as he could about the living
things he saw on the voyage.
– Darwin observed plants and animals, many he has
never seen before.
– Darwin’s observations led to him to develop one of
the most important scientific theories of all time.

3. Darwin’s Observations
• South America- the Beagle’s first stop.
– Brazil
• Darwin saw insects that looked like flowers, and ants
that marched across the forest floor like huge armies.
– Argentina
• Darwin saw armadillos, and sloths.
• Darwin found bones of animals that had died long ago.
• From the bones, Darwin inferred that the animal looked
like the sloths he had seen. However, the bones were
much larger than those of the living sloths.

4. • Darwin was amazed by the tremendous diversity,
or variety, of living things he saw.
– Today scientists know that living things are even more
diverse than Darwin could ever have imagined.
• Scientists have identified more than 2.5 million
species of organisms on Earth.
• Species- is a group of similar organisms that can
mate with each other and produce fertile
offspring.

5. • Galapagos Islands- 1835
– Group of small islands in the Pacific Ocean off the
west coast of South America.
– The Galapagos Island is where Darwin observed some
of the greatest diversity of life forms.
– Observed organisms on the Islands.
• Giant tortoises, or land turtles- they were so tall they could
look him in the eye.
• Seals covered in fur.
• Lizards that ate nothing but tough, prickly cactus plants.
• Finches- most famous observation.

6. Similarities and Differences
• Darwin was surprised that many of the plants
and animals on the Galapagos Islands were
similar to organisms on mainland South
America.
– For example, many of the birds on the islands,
including hawks, mockingbirds, and finches,
resembled those on the mainland.
– Many of the plants were also similar to plants
Darwin had collected on the mainland.

7. • There were also important differences between
the organisms on the islands and those on the
mainland.
– Large sea birds called cormorants, lived in both places.
The cormorants on the mainland were able to fly,
while those on the islands were unable to fly.
– Iguanas on the islands had long sharp claws, which
allowed them to climb on slippery rocks ate seaweed,
while the iguanas on the mainland had short claws
which allowed the animal to climb trees, so they could
eat the leaves.

8. • Darwin inferred that a small number of
different plant and animal species had come
to the Galapagos Islands from the mainland.
– They might have been blown out to sea during a
storm or set adrift on a fallen log.
– Once the plants and animals reached the islands,
they reproduced.
– Eventually, their offspring became different from
their mainland relatives.

9. • Darwin also noticed many differences among
similar organisms as he traveled from island to
island.
– For example, the tortoises on one island had
dome-shaped shells. Those on the another island
had saddle-shaped shells.
• The governor of one of the islands told Darwin
that he could tell which island a tortoise came
from just by looking at its shell.

10. Adaptations
• Like the tortoises, the finches on the Galapagos
Islands were noticeably different from one island
to another.
– The most obvious differences were the varied sizes
and shapes of the birds’ beaks.
– Darwin studied the different finches, he noticed that
each species was well suited to the life it led.
• Finches that ate insects had sharp, needle-like beaks.
• Finches that ate seeds had strong, wide beaks.
• Adaptations- a trait that helps an organism
survive and reproduce.

11. Evolution
• Evolution- the gradual change in a species over
time.
• Scientific Theory- is a well-tested concept that
explains a wide range of observations.
• Darwin spent 20 years consulting and gathering
other information from other scientist.
• Darwin knew that evolution had occurred on the
Galapagos Islands, but he didn’t know how the
processed had occurred.

12. • Darwin had to draw on other examples of
changes in livings things to help him understand
how evolution occurs.
• Darwin knew that people used selective breeding
to produce organisms with desired traits.
– For example, English farmers used selective breeding
to produce sheep with fine wool.
– Darwin also did this type of selective breeding with
pigeons with large, fan-shaped tails.

13. Natural Selection
• In 1858, Darwin wrote a book called The Origin of
Species, which explained that evolution occurs by
means of natural selection.
• Natural Selection- is 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.
• Darwin identified a number of factors that affect
the process of natural selection: overproduction,
competition, and variations.

14. • Overproduction
– Most species produce far more offspring than can
possibly survive.
– In many species, so many offspring are produced
that there are not enough resources- food, water,
and living space- for all of them.
• For example, each year a female sea turtle may lay
more than 100 eggs. If all the young turtles survived,
the sea would soon be full of turtles.

15. • Competition
– Since food and other resources are limited, the
offspring must compete with each other to
survive.
– Competition does not usually involve direct
physical fights between members of a species.
Instead, competition is usually indirect.
• For example, some turtles may fail to find enough to
eat. Others may not be able to escape from predators.
Only a few turtles will survive long enough to
reproduce.

16. • Variations
– Variations- any difference between individuals of
the same species.
– Members of a species differ from one another in
many of their traits.
• For example, some newly hatched turtles are able to
swim faster than other turtles.

17. • Selections
– Some variations make certain individuals better
adapted to their environments. Those individuals are
more likely to survive and reproduce.
– When those individuals reproduce, their offspring may
inherit the allele for the helpful trait.
– After many generations, more members of the species
will have the helpful trait.
– In effect, the environment has “selected” organisms
with helpful traits to be the parents of the next
generation.

18. • Over a long period of time, natural selection can
lead to evolution. Helpful variations gradually
accumulate in a species, while unfavorable ones
disappear.
– For example, suppose a new fast-swimming predator
moves into the turtles’ habitat. Turtles that are able to
swim faster would be more likely to escape from the
new predator. The faster turtles would thus be more
likely to survive and reproduce. Over time, more
turtles in the species would have the “fast-swimmer”
trait.

19. The Role of Genes in Evolution
• Without variations, all members of a species
would have the same traits.
• Evolution by natural selection would not occur
because all individuals would have an equal
chance of surviving and reproducing.
• Variations can result from mutations in genes or
from the shuffling of alleles during meiosis.
• Only genes are passed from parents to their
offspring, because of this, only traits that are
inherited, or controlled by genes, can be acted
upon by natural selection.

20. Evolution in Action
• In finches beak size could change change very
quickly by natural selection.
– In 1977, scientists observed one of the Galapagos
Island, and the island had very little rain so most
of the plants died. Fewer of the seeds that the
finches usually ate were available. Instead, the
birds had to eat large seeds that were enclosed in
tough, thorny seed pods. Finches with larger and
stronger beaks were able to survive. The next year
many of the finches produced had larger and
stronger beaks.

21. How do New Species Form?
• Geographic Isolation is one of the main ways that
new species form.
– Isolation, or complete separation, occurs when some
members of a species become cut off from the rest of
the species.
– Sometimes a group is separated from the rest of its
species by a river, volcano, or mountain range. Even
an ocean wave can separate a few individuals from
the rest of their species by sweeping them out to sea
and later washing them ashore on an island.

22. • A new species can form when a group of
individuals remains separated from the rest of
its species long enough to evolve different
traits.
• The longer the group remains isolated from
the rest of the species, the more likely it is to
evolve into a new species.

23. Continental Drift
• Continental drift caused geographic isolation on a
world-wide scale.
• As Pangaea separated, species become isolated from
one another and began to evolve independently.
• One example of this is Australia, most mammals in
Australia belong to the group known as marsupials.
• Marsupials give birth to a very small young that
continue to develop in a pouch on the mother’s body.
• Only a few species of marsupials exist on other
continents.

24. The Fossil Record
• Fossil- is the preserved remains or traces of an
organism that lived in the past.
• A fossil can be formed from a bone, tooth,
shell, or other part of an organism.
• Other fossils can be traces of the organism,
such as footprints or worm burrows left in the
mud that later turned to stone.

25. How do Fossils Form
• Very few fossils are of complete organisms.
• Often when the animal dies, the soft parts of
its body either decay or are eaten before a
fossil can form.
• Usually only the hard parts of the animal, such
as the bones or shells, remain.
• The parts of plants that are most often
preserved as fossils include leaves, stems,
roots, and seeds.

26. • Most fossils form when organisms that die
become buried in sediments.
– Sediments are particles of soil and rock.
– Over millions of years, the layers of sediment
harden to become sedimentary rock.
• The formation of any fossil is a rare event.
• The conditions must be just right for a fossil to
form.

27. • Petrified fossils- is a fossil formed when
minerals replace all or part of the organism.
– Some remains that become buried in sediments
are actually changed to rock.
– Minerals dissolved in the water soak into the
buried remains.

28. • Molds and Casts
• Molds- a type of fossil formed when a shell or
other hard part of an organism dissolves, leaving
an empty space in the shape of the part.
– When the hard parts of the organism are gradually
dissolved.
• Casts- a type of fossil that forms when a mold
becomes filled in with minerals that then harden.
– An example is when you put gelatin dessert in a plastic
mold.

29. • Preserved Remains
– Organisms can also be preserved in substances other than
sediments.
– Organisms can be preserved in ice.
• Wooly Mammoths have had their entire bodies preserved in ice.
• The low temperatures preserved the mammoths’ soft parts.
– Bones and teeth have been preserved in tar pits.
• Tar pits were usually under water sources, so when the animals
came to drink water, they became stuck in the tar.
– Insects have been preserved in tree sap that when harden
becomes amber.
• The amber protects the organism’s body from decay.

30. Determining a Fossil’s Age
• Scientists can determine a fossil’s age in two
ways: relative dating and absolute dating.
• Relative Dating- a technique used to
determine which of two fossils is older.
– Relative dating can only be used when the rock
layers have been preserved in their original
sequence.
– Relative dating does not tell scientists the fossil’s
actual age.

31. • Absolute Dating- allows scientist to determine
the actual age of fossils.
– Radioactive Elements- an unstable particle that
breaks down into a different element.
– Half-life- the time it takes for half of the atoms in a
radioactive element to break down.
– Scientists can compare the amount of a radioactive
element in a sample to the amount of the element
into which it breaks down. This information can be
used to calculate the age of the rock, and thus the age
of the fossil.

32. What do Fossils Reveal?
• Fossil record- the millions of fossils that
scientists have collected.
– Despite gaps in the fossil record, it has given
scientists a lot of important information about
past life on earth.
– Fossil record provides clues about how and when
new groups of organisms evolved.
• Extinct- a species that does not have any living
members.

33. The Geologic Time Scale
• Using absolute dating, scientists have
calculated the ages of many different fossils
and rocks. From this information, scientists
have created a “calendar” of Earth’s history
that spans more than 4.6 billion years.
• Scientists have divided this large time period
into smaller units called eras and periods.

35. How Fast Does Evolution Occur?
• Gradualism- the theory that evolution occurs
slowly but steadily.
– Tiny changes in a species gradually add up to
major changes over very long periods of time.
– If this is correct, than fossil records would show
intermediate forms of all species should have
existed.
– However, the fossil record often shows no
intermediate forms for long periods of time.

36. • Punctuated Equilibria- the theory that species
evolve during short periods of rapid change.
– These periods of rapid change are separated by by
long periods of little or no change.
– According to this theory, species evolve quickly
when groups become isolated and adapt to new
environments.

37. Similarities in Body Structure
• Scientists compare body structures,
development before birth, and DNA
sequences to determine the evolutionary
relationships among organisms.
• Homologous structures- body parts that are
structurally similar in related species; provide
evidence that the structures were inherited
from a common ancestor.

38. Similarities in Early Development
• Scientists can also make inferences about
evolutionary relationships by comparing the
early development of different organisms.
• This gives scientists evidence that some
organisms are more closely related than other
organisms.

40. Similarities in DNA
• Scientists infer that the species inherited
many of the same genes from a common
ancestor.
• Scientists have begun comparing the genes of
different species to determine how closely
related the species are.
• Scientists have developed techniques that
allow them to extract, or remove DNA from
fossils.

41. Combining the Evidence
• Scientists have combined evidence from
fossils, body structures, early development,
and DNA and protein sequences to determine
the evolutionary relationships among species.
• Branching tree- is a diagram that shows how
scientists think different groups of organisms
are related.