Evolutionary TheoryEvolutionary Theory

A Theory to Explain Change Over TimeA Theory to Explain Change Over Time

Modern Evolutionary TheoryModern Evolutionary Theory

• A theory is: a broad range of data that has been scientifically tested and supported.

• Modern evolutionary theory began when Charles Darwin presented evidence that evolution happens and offered an explanation of how it happens.

Evolutionary Theory Keeps Changing Evolutionary Theory Keeps Changing & Expanding& Expanding

• Like most scientific theories, evolutionary theory keeps developing & expanding.

• Many scientists since Darwin have tested & added to his ideas.

Peter & Rosemary Grant-research Galapagos Finches

Darwin’s Ideas From ExperienceDarwin’s Ideas From Experience

• In Darwin’s time, most people did not think that living things had changed over time.

• In fact, many doubted that Earth itself had ever changed.

• But Darwin saw evidence of gradual change.

• Darwin’s experiences provided him with evidence of evolution at work.

The Voyage of the BeagleThe Voyage of the Beagle

Galapagos IslandsGalapagos Islands

• Darwin collected several different species of birds called finches.

• Each of the finches are very similar, but have different bills.

• Each bill is suited to a particular food available on the individual islands.

Galapagos Plants and AnimalsGalapagos Plants and Animals

• Darwin noticed that many of the islands’ plants and animals were similar but not identical to species in South America.

• He suggested they were descendants of S. American species with modifications.

Darwin’s original notes on descent with modification to explain the different species of birds, tortoises, etc. seen on the Galapagos.

Descent with modification = EvolutionDescent with modification = Evolution

• Each of the Hawaiian honeycreeper species has a bill specialized for eating certain foods. Scientists suggest that all 23 honeycreeper species apparently arose from a single species that migrated to Hawaii.

Years of ReflectionYears of Reflection• After returning from his

voyage, Darwin spent many years studying his data.

• He did not report his ideas for many years.

• He took time to gather more data and to form a strong explanation for how evolution happens.

Breeding & SelectionBreeding & Selection• Darwin bred exotic pigeons

and studied the work of others who bred dogs, orchids, crops, and other livestock.

• Breeders take advantage

of natural variation in traits within a species.

• This is artificial selection.

Artificial SelectionArtificial Selection – The human practice of – The human practice of breeding animals or plants that have certain breeding animals or plants that have certain

traits.traits.

Natural SelectionNatural Selection

• Darwin suggested that nature “selects” which organisms survive and reproduce.

• Survival of the Fittest

Darwin’s Ideas from OthersDarwin’s Ideas from Others

• In Darwin’s time, most people—including scientists—believed that each species was created once and stayed the same forever.

• But, this view could not explain fossils of organisms that no longer exist, such as dinosaurs.

Do Organisms Change over Time?Do Organisms Change over Time?

• Some scientists tried to explain such observations by saying that species could die out but never change.

• Others proposed various mechanisms to explain how species may change over time.

LamarckLamarck

• In 1809, the French scientist, Jean Baptiste Lamarck proposed an explanation for how organisms may change over generations.

• He noticed that organisms are usually well adapted to their environment.

……LamarckLamarck

• Lamarck proposed (like Darwin) that organisms change over time as they adapt to changing environments.

• The mechanism for that change, however, that was proposed by Lamarck was wrong.

• He proposed that changes due to use or disuse of a characteristic would be passed on to the offspring.

• Ex: building muscles would mean their baby would have bigger muscles.

Use and DisuseUse and Disuse• Lamarck proposed a

reason why giraffes have a long neck:

• He said that giraffes stretched their necks to reach leaves high in the trees. Their offspring, therefore would have longer necks.

The Theory of Evolution by The Theory of Evolution by Natural Selection Natural Selection (See page 381)(See page 381)

Natural SelectionNatural Selection

• The process by which individuals that are better adapted to their environment survive & reproduce more successfully than less well adapted individuals do.

4 Steps to Natural Selection4 Steps to Natural Selection

1. Overproduction – Every population is capable of producing more offspring than can possibly survive.

2. Variation – Variations exist within every population. Much of this variation is in the form of inherited traits.

3. Selection – In a given environment, having a particular trait can make individuals more likely to survive & have successful offspring than others.

4. Adaptation – Over time, those traits that improve survival & reproduction will become more common in a population.

Natural Selection AnimationNatural Selection Animation

• http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s1/evm2s1_6.htm