Chapter 15

Darwin’s Theory of Evolution

15 – 1 The Puzzle of Life’s Diversity

• There are A LOT of different organisms

• This variety of living things is called Biological Diversity

Q: What scientific explanation can account for the diversity of life?

A: A collection of scientific facts, observations, and hypotheses known as Evolutionary Theory

Evolution• Change over time

Theory• A well supported testable

explanation of phenomena that have occurred in the natural world

Charles Darwin• Born in England, February 12, 1809

• Ships naturalist on a trip around the world aboard the HMS Beagle

• During his travels, Darwin made numerous observations and collected evidence that lead him to propose a revolutionary hypothesis about the way life changes over time

Charles Darwin

The Voyage of the Beagle

Darwin’s Observations

• Darwin saw far more diversity during his travels than he saw in his native country

Patterns of Diversity

• Darwin was intrigued by the fact that so many plants and animals seemed remarkably well suited to whatever environment they inhabited

• He was also impressed by the ways in which organisms survived and produced offspring

• He was also puzzled by where different species lived and did not live

Fossil• Preserved remains of ancient

organisms

• Some looked like organisms that were still alive

• Others looked completely unlike any creature he had ever seen

• Darwin wondered, why had so many species disappeared, and how were they related to living species

The Galapagos Islands

• Of all the Beagles ports of call, the one that influenced Darwin the most was a group of islands off the coast of South America called the Galapagos Islands

• Although they were close together they all had different climates and differences between the species

Galapagos Tortoises

The Journey Home

• While heading home, Darwin spent a great deal of time thinking about his findings

• Darwin observed that the characteristics of many animals and plants varied noticeably among the different islands of the Galapagos

• Darwin began to wonder if animals living on different islands had once been members of the same species

15 – 2 Ideas that Shaped Darwin’s Thinking

• To understand how radical Darwin’s thoughts appeared, you must understand a few things about the world in which he lived

• Most Europeans belived the world was only a few thousand years old

• Nothing had changed since creation• Rocks and major geological features were

thought to have been produced suddenly by catastrophic events that humans rarely ever witnessed

• Slowly after studying many scientific theories of his time, Darwin began to change his thinking dramatically

Hutton and Lyell

• Geological forces that have shaped Earth form very slowly, often over millions of years

Hutton and Geological Change

• Geological forces that have shaped Earth form very slowly, often over millions of years

• Therefore Earth had to be much more than a few thousand years old

Lyell’s Principles of Geology

• Charles Lyell – author “Principles of Geology”

• Process that shaped the Earth millions of years earlier continue in the present

• Darwin Thought: If the earth could change over time, could life be changing as well?

Lamarck’s Evolution Hypothesis

• Jean – Baptiste Lamarck

• French naturalist

• One of the first scientists to recognize that living things have changed over time, and that all species were descended from other species

• Also realized, that organisms were somehow adapted to their environments

Lamarks theory had 3 aspects to it

1. Tendency toward perfection

• Organsims continually change and acquire features that help them live more successfully

Ex.) Birds wanted to fly, so their wings grew and became suited to flying

2. Use and Disuse

• Body parts that were used developed, and those that were not went away

Ex.) Reptiles stopped using their legs and evolved into snakes

3. Evolution of Acquired Traits

• Organsims that changed a characteristic in their lifetime, would pass that change on to its offspring

• This theory is WRONG!

• He will forever be known in this class as Lamark the Loser!

Population Growth• Thomas Malthus - English

economist

• Published an essay in which he noted that babies were being born faster than people were dying

Malthus reasoned• That if the human population

continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone

• Darwin realized that this applied even more to plants and animals than it did to humans because animals and plants had more offspring than humans did

15 -3 Darwin Presents His Case

Publication of On the Origin of Species

• Darwin did not rush to publish his thoughts

• Darwin was not only stunned by his discoveries, he was disturbed by them

• In 1858 Alfred Russel Wallace wrote to Darwin about a theory of evolution that was similar to his

• Suddenly Darwin had an incentive to publish his own work

Inherited Variation and Artificial Selection

• One of Darwin’s most important insights was that members of each new species vary from one another in important ways

• Ex.) some trees have bigger fruit, some cows give more milk

Artificial Selection• Selection by humans for breeding

of desired characteristics

Evolution by Natural Selection

• Darwin was convinced that a process like artificial selection occurs in nature

• High birth rates and a shortage for resources would force organisms into a competition for resources

Survival of the Fittest• A key factor in the struggle for

existence, was how well suited an organism is to its environment

• Darwin called the ability of an organism to survive and reproduce in its environment fitness

• Darwin purposed that fitness is the result of adaptations

Adaptation

• Any characteristic that increases an organisms chance of survival

• Adaptations can be

• Anatomical–Speed, protection, camouflage

• Physiological–The way it goes through

photosynthesis

• Behavioral–The way it hunts

• Individuals with low levels of fitness die and do not reproduce, and those with high levels of fitness survive and reproduce successfully

• He called this process Survival of the fittest

Natural Selection• Overtime, natural selection results

in changes in the inherited characteristics of a population. These changes increase a species fitness in its environment

Descent with Modification

• Darwin proposed that each living species has descended, with changes from other species over time

• He called this Descent with Modification

The Fossil Record

• By comparing fossils from older rock layers with fossils from younger layers, scientists could document the fact that life on Earth has changed over time

Geographical Distribution of Living Species

• Many species share similar characteristics because they inhabit similar environments

Homologous body structures

• Structures that have different forms but develop from the same embryonic tissue

Vestigial structures• Structures that are reduced in size

and seem to no longer have a function

Ex.) appendix, tailbone, wisdom teeth

• Scientists think that vestigial structures are parts that once functioned in an ancestor

Similarities in Embryology

• Many embryos look especially similar during early stages of development

• All vertebrates go through a stage in which they have gill pouches

• Similarities suggest an evolutionary relationship among all vertebrate species

Summary of Darwin’s Theory

1. Individual organisms in nature differ from one another. Some of this variation is inherited

2. Organisms in nature produce more offspring than can survive, and many of those that survive do not reproduce.

3. Because more organisms are produced than can survive, members of each species must compete for resources.

4. Because each organism is unique, each has different advantages and disadvantages in the struggle for existence.

Summary of Darwin’s Theory5. Individuals best suited to their environment survive and

reproduce most successfully. The characteristics that make them best suited to their environment are passed on to offspring. Individuals whose characteristics are not as well suited to their environment die or leave fewer offspring.

6. Species change over time. Over long periods, natural selection causes changes in the characteristics of a species, such as in size and form. New species arise, and other species disappear.

7. Species alive today have descended with modification from species that lived in the past.

8. All organisms on Earth are united into a single tree of life by common descent.