Chapter 7-1 Darwin’s Theory

Science 7

7-1 ObjectivesDescribe important observations Darwin

made on his voyage.State how Darwin explained differences

between similar species.Explain how natural selection leads to

evolution.

The Voyage of the BeagleDarwin set sail on the HMS Beagle (a ship) in

December 1831.22 years old.Was originally employed as the captain’s

companion, but eventually became the ship’s naturalist (someone who studies the natural world).

Darwin’s ObservationsDarwin’s most important observations

included the diversity of organisms, the remains of ancient organisms, and the characteristics of organisms on the Galápagos Islands.

DiversityDarwin was amazed by the tremendous

diversity of living things that he saw.In Brazil: insects that looked like flowers and

army ants.In Argentina: slothsCurrently, over 1.7 million species have been

identified by scientists.Species is a group of similar organisms that

can mate with each other and produce fertile offspring.

FossilsFossils were among one of Darwin’s

discoveries.A fossil is the preserved remains or traces of

an organism that lived in the past.Darwin discovered fossil bones that

resembled the bones of living animals (such as sloths and capybaras), only the fossil bones were much larger.

Darwin also discovered fossilized seashells on cliffs, far above the ocean.

Galápagos OrganismsDarwin reached the Galápagos in 1835.Darwin observed many unusual species such

as giant tortoises, Galápagos finches, marine iguanas, and blue-footed boobies.

Comparison to South American OrganismsDarwin found many similarities between

Galápagos organisms (plants and animals) and those in South America.

Darwin hypothesized that a small number of different plant and animal species had come to the islands from the mainland (via storms or set adrift on the ocean).

Eventually the islands organisms’ offspring became different from the mainland relatives.

Comparisons Among the IslandsDarwin notices difference in the animals on

different islands in the Galápagos. For example: tortoises on one island had

dome shaped shells on those on another had saddle-shaped shells.

AdaptationsFinches (birds) in the Galápagos were

different from one island to the other.Their beaks varied in sizes and shapes to suit

the life it leads (type of food it eats).Adaptation is a trait that helps an organism

survive and reproduce in its environment.

EvolutionUpon returning to England, Darwin spent 20

years consulting with other scientists.Darwin reasoned that plants or animals

that arrived on the Galápagos Islands faced environmental factors that were different from those on the mainland. Perhaps, Darwin hypothesized, the species gradually changed over many generations and became better adapted to the new environment.

The gradual change in a species is called evolution.

Why Just a Theory?Darwin’s ideas are often referred to as the

theory of evolution.A scientific theory is a well-tested concept

that explains a wide range of observations. A well-established theory is generally considered to be true by the scientific community.

Selective BreedingDarwin studied selective breeding in order to

understand how evolution might occur.

Natural Selection1859: Darwin published a book called On the

Origin of Species. Darwin proposed that evolution occurs in

nature through a mechanism called natural selection.

In Natural selection individuals that are better adapted to their environment are more likely to survive and reproduce that other member of the same species.

OverproductionDarwin was influence by Thomas Malthus.Malthus argued that organisms produce a lot

of offspring. They over-reproduce in order to make sure

that some of the offspring will survive to adulthood.

VariationsA difference between individuals of the same

species is called a genetic variation.

CompetitionMembers of a species compete with each

other for food and other resources in order to survive.

SelectionSome variations make individuals better

adapted to their environment. These individuals are better able to survive until adulthood and reproduce.

Darwin proposed that, over a long time, natural selection can lead to change. Helpful variations may gradually accumulate in a species, while unfavorable ones may disappear.