Science Explorer Chapter 5

Changes Over Time

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.

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.

• 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.

• 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.

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.

• 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.

• 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.

• 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.

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.

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.

• 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.

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.

• 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.

• 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.

• 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.

• 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.

• 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.

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.

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.

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.

• 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.

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.

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.

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.

• 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.

• 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.

• 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.

• 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.

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.

• 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.

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.

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.

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.

• 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.

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.

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.

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.

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.