darwin’s theory

Darwin’s TheoryDarwin’s TheoryChapter 15Chapter 15

Section 3Section 3

Darwin Presents His CaseDarwin Presents His Case• The specimens Darwin brought back had

the scientific community in a buzz

• Observed that Galapagos species are found nowhere else in the world

• They looked similar to South American mainland species but were clearly different

Darwin’s BookDarwin’s Book

• Published his ideas about species diversity and evolution in The Origin of Species

• Proposed a mechanism for evolution called natural selection

• Many agreed with Darwin while others strongly opposed him

Natural VariationNatural Variation• Scientists used to believe species were

perfect and unchanging

• Darwin argued natural variation (differences among species) is found in all types of organisms– Some cows give more milk– Some plants give larger fruit

Artificial SelectionArtificial Selection

• Darwin also studied English farmers and breeders

• Used selective breeding to breed the “best” individuals– Largest hogs, fastest horses

• Darwin called this artificial selection

• Still used today!

Evolution by Natural SelectionEvolution by Natural Selection

• Darwin believed that evolution occurred through natural selection

• Natural selection = the process by which only the organisms best adapted to their environment will survive and reproduce

• 3 main parts to natural selection

1. The Struggle for Existence1. The Struggle for Existence

• There are more living things than resources

• Results in competition

• Competition = the struggle among living things to get their needs for life

2. Survival of the Fittest2. Survival of the Fittest• Individuals that are better suited to their

environments survive (high fitness)

• Individuals not suited to their environment will die (low fitness)

• Fitness – the ability of an individual to survive and reproduce in its environment– Is a result of

adaptations

2. Survival of the Fittest2. Survival of the Fittest

• Adaptation – any inherited characteristic that increases an organism’s chance for survival– Long necks in giraffes– Long toes / webbed feet

in birds– Coloring / Camouflage

3. Descent with Modification3. Descent with Modification

• Each living species has descended, with changes, from other species over time

• Implies all organisms are related to one another– Known as common

descent

FinchesFinches * Darwin studied different

types of finches found on different Islands

* Each finch seemed perfectly fitted for it’s island environment

* Especially with its beak

Darwin’s FinchesDarwin’s Finches

Evidence for EvolutionEvidence for Evolution

• Fossil Evidence– When living things from the past are

compared to living things today, we can see that change has occurred

Evidence for EvolutionEvidence for Evolution

• Geographic distribution of living things– Species are located in

different parts of the world but share similarities

– Beavers and capybara are similar

– Muskrat and coypu are similar

Beaver

NORTH AMERICA

Muskrat

Capybara SOUTH AMERICA

Beaver

Muskrat

Beaver andMuskrat

Coypu

Capybara

Coypu andCapybara

Coypu

Evidence for EvolutionEvidence for Evolution• Homologous body structures

– Limbs (arms, wings, legs, flippers) have different form and function but are constructed from the same bones

Turtle Alligator Bird Mammals

Typical primitive fish

Evidence for EvolutionEvidence for Evolution

• Vestigial structures– Body parts that no longer have

functions– i.e. Pink lump in corner of eye

• Remnant of a third eyelid • Frogs and turtles have third eyelid

to see underwater

– Appendix• In many mammals the appendix is

used to help digest food

Evidence for EvolutionEvidence for Evolution

• Embryology– Studying

embryonic development to look for similarities and differences

EmbryologyEmbryology

Summary of Darwin’s TheorySummary of Darwin’s Theory

• Evolution by Natural Selection

_______________ - The process by which organisms with favorable variations produce at higher rates because they are better adapted than those lacking increased fitness. Species change over time as new species arise and others disappear. (Survival of Fittest)

Natural Selection

Evolution by Natural SelectionEvolution by Natural Selection

1.1. Over ProductionOver Production

All organisms produce more offspring than can actually survive.

Evolution by Natural SelectionEvolution by Natural Selection

2.2. Genetic Diversity (Natural Variation)Genetic Diversity (Natural Variation)

All species have genetic variation. No two organisms are exactly alike. Some of this variation is inherited. If variation is limited, the species is less able to adapt.

Evolution by Natural SelectionEvolution by Natural Selection

3.3. Struggle for ExistenceStruggle for Existence

Every organism faces a constant struggle to survive. Not all organisms survive, many that do, don’t reproduce.

Living things face many challenges. Without challenges all alleles are equal.

Evolution by Natural SelectionEvolution by Natural Selection

4.4. CompetitionCompetition

Members of each species compete for limited resources…food, water, mate, sunlight, space, shelter, etc.

Evolution by Natural SelectionEvolution by Natural Selection

5.5. Environmental ChangeEnvironmental Change

Ecosystems are constantly changing due to natural succession, changes in climate, natural disasters, man’s impact, etc.

Evolution by Natural SelectionEvolution by Natural Selection

6. Fittness6. Fittness Because each organism is unique, each

has different advantages and disadvantages in the struggle for existence. How well suited an organism is to its environmentwill determine its chance of surviving.

Evolution by Natural SelectionEvolution by Natural Selection

7. Adaptation7. Adaptation

Some inherited characteristics increase an organisms chance of survival in the struggle for existence in a particular environment.

Evolution by Natural SelectionEvolution by Natural Selection

8. Survival of the Fittest8. Survival of the Fittest

Organisms that are best suited to their environment, are better able to survive, reproduce, and pass their favorablecharacteristics on to their offspring.

Evolution by Natural SelectionEvolution by Natural Selection

9. Inheritance9. Inheritance

Individuals that survive pass theirtraits on to their offspring. The characteristics of the individuals best suited to a particular environment are inherited and tend to increase in a population over time.

Evolution by Natural SelectionEvolution by Natural Selection

10. Descent with Modifications10. Descent with Modifications

Species alive today are modifications of species that lived in the past. They have descended, with changes, from other species over time.

Evolution by Natural SelectionEvolution by Natural Selection

11. Common Descent11. Common Descent All organisms on Earth

(living and extinct) are related to one another. They have a common ancestor and are united into a single tree of life.

How does color affect survival?How does color affect survival?• A group of mice lives in an area that has

dark soil.

• Owls eat mice in the area but because dark mice blend well with the soil, owls cannot see them easily

How does color affect survival?How does color affect survival?

• Dark mice are better protected because they blend in

• Color is an adaptation because it helps them survive

How does color affect survival?How does color affect survival?

• Dark mice sometimes have light colored offspring which are easy to spot

• Light colored mice are usually eaten first thus remain low in number

How does color affect survival?How does color affect survival?

• What do you think would happen if a chemical change took place and the soil changed to a light color?

• Owls will spot dark mice easier and as a result will eat more dark than light mice

• Dark mice are no longer adapted to the new surroundings

• Light mice are now better adapted

How does color affect survival?How does color affect survival?

• How will this change affect the balance of light and dark mice?

• More light mice will survive, reproduce, and grow

• Dark mice will get eaten more and will reduce in number

Natural SelectionNatural Selection

• What determined which mouse was better adapted to its surroundings?

• Soil color and the owls– When soil was dark,

dark mice were not eaten and survived

– When soil was light, light mice were noteaten and survived

Natural SelectionNatural Selection

• What determines which mouse is better adapted to its surroundings? – Light fur evolved to help beach mice survive

on white sand dunes, while darker fur help mainland mice survive in fields.

color

Natural SelectionNatural Selection

• Natural selection is the process by which only the organisms best adapted to their environment will survive and reproduce

• Something in nature does the selecting (i.e. owls chose light or dark mice)

• The change in mouse color was a result of natural selection