Darwin’s Darwin’s TheoryTheory:: Natural Natural

SelectionSelection(14.1-14.3)(14.1-14.3)

The basics…The basics…

EvolutionEvolution:: process by which process by which modern organisms have modern organisms have descended from ancient descended from ancient organisms (change over time)organisms (change over time)

Scientific TheoryScientific Theory:: a well- a well-supported, testable explanation supported, testable explanation of phenomena that have occurred of phenomena that have occurred in the natural worldin the natural world

Voyage of the BeagleVoyage of the Beagle 1831: Darwin set sail 1831: Darwin set sail

from England aboard from England aboard the H.M.S. Beagle as a the H.M.S. Beagle as a NaturalistNaturalist

Throughout the voyage Throughout the voyage he went ashore and he went ashore and collected plant and collected plant and animal specimensanimal specimens

He also read scientific He also read scientific literature and kept literature and kept journalsjournals

The GalapagosThe Galapagos

Characteristics of plants and animals varied Characteristics of plants and animals varied noticeably among the different islands.noticeably among the different islands.

Darwin’s ObservationsDarwin’s Observations

Summary of ObservationsSummary of Observations

Many plants and animals are Many plants and animals are especially well-suited to the especially well-suited to the environments they inhabitenvironments they inhabit

Organisms tend to have many more Organisms tend to have many more offspring than are able to survive.offspring than are able to survive.

Influences on DarwinInfluences on DarwinHutton & Lyell:Hutton & Lyell: Geologic forces that Geologic forces that

shape the earth occur very slowly, so shape the earth occur very slowly, so earth must be more than a few thousand earth must be more than a few thousand years old; Geologic forces continue to act years old; Geologic forces continue to act on the earthon the earth

Malthus:Malthus: Population growth exceeds food Population growth exceeds food supply growth leading to struggle for supply growth leading to struggle for existenceexistence

** If the Earth has changed, life probably ** If the Earth has changed, life probably has too. If people experience struggle has too. If people experience struggle for existence, other organisms probably for existence, other organisms probably do as well. do as well.

LamarckLamarck:: Tendency for Tendency for

PerfectionPerfection Use and DisuseUse and Disuse Acquired TraitsAcquired Traits

Lamarck believed Lamarck believed changes acquired in changes acquired in a lifetime could be a lifetime could be passed on leading to passed on leading to a change in a a change in a species.species.

Applying Lamarck’s Applying Lamarck’s Hypothesis:Hypothesis:

Evaluating LamarckEvaluating Lamarck

Strength’sStrength’s• Species weren’t Species weren’t

static and changed static and changed over timeover time

• Species were related Species were related through common through common ancestryancestry

• Helped paved way Helped paved way for Darwin by for Darwin by bringing evolution to bringing evolution to the forefront of the forefront of scientific debatescientific debate

Weakness’sWeakness’s• Lamarck didn’t Lamarck didn’t

know about genes know about genes and how traits and how traits were inheritedwere inherited

• An organism’s An organism’s behavior has no behavior has no effect on its effect on its heritable heritable characteristicscharacteristics

On the Origin of Species Proposed a mechanism for Proposed a mechanism for

evolution called natural selectionevolution called natural selection Presented evidence that Presented evidence that

evolution has been taking place evolution has been taking place for millions of years and for millions of years and continues todaycontinues today

1858, Darwin received a 1858, Darwin received a manuscript from Alfred Russell manuscript from Alfred Russell Wallace proposing similar ideas, Wallace proposing similar ideas, he finally decided to publish. he finally decided to publish. Book was released in 1859Book was released in 1859

Artificial SelectionArtificial Selection• Selection by humans for Selection by humans for

breeding of useful traits breeding of useful traits from natural variation from natural variation among different among different organismsorganisms

• Natural selection works Natural selection works essentially the same essentially the same way, but the way, but the environment, not environment, not humans determines the humans determines the desirable traits. desirable traits.

Process of Natural SelectionProcess of Natural Selection(S.N.A.S.D.)(S.N.A.S.D.)

Struggle for Existence:Struggle for Existence: Organisms generally have more offspring than resources can support

Natural Variation:Natural Variation: Even within a species there will be distinct differences between individual organisms

Adaptation:Adaptation: Some variations provide individuals with an advantage that increases their chances of survival.

Survival of the Fittest:Survival of the Fittest: Those with the adaptations are more likely to survive and reproduce

Descent with Modification:Descent with Modification: Genes for the beneficial adaptations are passed on and the trait becomes increasingly more common in each new generation of the species

Evidence of Evolution IEvidence of Evolution IThe Fossil RecordThe Fossil Record

Homologous StructuresHomologous Structures

Geographic Species DistributionGeographic Species Distribution

Vestigial StructuresVestigial Structures: : organs that are so reduced organs that are so reduced

that they are just traces that they are just traces (vestiges) of homolgous (vestiges) of homolgous structures in other speciesstructures in other species

Evidence of Evolution IIEvidence of Evolution II

Similarities in DevelopmentSimilarities in Development Biogeochemical Factors Biogeochemical Factors (DNA)(DNA)