CHAPTER 15
CHAPTER 15
DARWIN’S THEORY OF EVOLUTION
SECTION 1
THE PUZZLE OF LIFE’S DIVERSITY
Key Concept Questions
• What was Charles Darwin's contribution to science?
• What pattern did Darwin observe among organisms of the Galápagos Islands?
Nature presents scientists with a puzzle.
Humans share the Earth with millions of other kinds of organisms of every imaginable shape, size, and habitat.
This variety of living things is called biological diversity.
• How did all these different organisms arise?
• How are they related?
• What scientific explanation can account for the diversity of life?
The answer is a collection of scientific facts, observations, and hypotheses known as evolutionary theory.
• EVOLUTION or change over time, is the process by which modern organisms have descended from ancient organisms.
• A THEORY is a well-supported testable explanation of phenomena that have occurred in the natural world.
What do we know?
• There are many different creatures on Earth
• All creatures have common characteristics
• The Earth is very old
• Creatures have changed over time
What has the fossil record shown us?
• Many creatures that lived in the past don’t exist today
What individual has contributed the largest understanding of evolution?
[pic]
• CHARLES DARWIN
o 1809 - 1882
o 1831 sailed on H.M.S. Beagle around world
o Proposed a way how evolution works
o How did creatures change over time?
• by natural selection
o Collected a lot of evidence to support his ideas
o British naturalist
o Voyage of the HMS Beagle
o Stopped in Galapagos Islands
[pic]
Galapagos Islands
• Recently formed volcanic islands. Most of animals on the Galápagos live nowhere else in world, but they look like species living on South American mainland.
• located 1000 km west of South America
• Darwin noted that although they were close together, the islands had very different climates.
• The smallest, lowest islands were hot, dry, and nearly barren. Hood Island, for example, had sparse vegetation.
• The higher islands had greater rainfall and a different assortment of plants and animals. Isabela Island had rich vegetation that tortoises could easily reach
FOSSIL is a preserved remain of ancient organisms
• Darwin noticed that some fossils resembled organisms that were still alive and other fossils did not resemble any living creatures
Why had so many of these species disappeared?
How are they related to living species?
Galápagos birds
• 22 of the 29 species of birds on the Galapagos are endemic
• found only on these islands
• collected specimens of all
• One particular group…
o at first, he paid little note to a series of small but distinctive birds
o some were woodpecker-like, some warbler-like, & some finch-like
Darwin’s finches
[pic]
• Finches with beak differences that allowed them to…
• successfully feed
• successfully compete
• successfully reproduce
• pass successful traits onto their offspring
Darwin’s conclusions
• variations in beaks
o differences in beaks in the original flock
o adaptations to foods available on islands
• natural selection for most fit
o over many generations, the finches were selected for specific beaks & behaviors
• offspring inherit successful traits
o accumulation of winning traits:
both beaks & behaviors
• separate into different species
What exactly was Darwin’s contribution to science?
• During his travels, Darwin made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time.
• Darwin was intrigued by the fact that so many plants and animals seemed remarkably well suited to whatever environment they inhabited.
• He was impressed by the many ways in which organisms survived and produced offspring.
o He wondered if there was some process that led to such a variety of ways of reproducing.
• Darwin was also puzzled by where different species lived—and did not live.
o He visited Argentina and Australia, for example, which had similar grassland ecosystems.
▪ Yet, those grasslands were inhabited by very different animals.
▪ Also, neither Argentina nor Australia was home to the sorts of animals that lived in European grasslands.
• For Darwin, these patterns posed challenging questions.
o Why were there no rabbits in Australia, despite the presence of habitats that seemed perfect for them?
o Similarly, why were there no kangaroos in England?
Key Concept Questions
• What was Charles Darwin's contribution to science?
o During his travels, Darwin made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time.
• What pattern did Darwin observe among organisms of the Galápagos Islands?
o Darwin was intrigued by the fact that so many plants and animals seemed remarkably well suited to whatever environment they inhabited.
SECTION 2
IDEAS THAT SHAPED DARWIN’S THINKING
Key Concept Questions
• How did Hutton and Lyell describe geological change?
• According to Lamarck, how did species evolve?
• What was Malthus's theory of population growth?
[pic]
In historical context
• Geologic theories
o Charles Lyell (1797-1875)
▪ studied the Temple of Scrapis (Sicily)
▪ built on land & used until 200AD
▪ high tide now above temple floor
▪ holes in columns well above high tide
▪ Meaning:
• In less than 2000 years, temple sunk well below sea level, and then was raised up again — natural processes and immense periods of time could produce great changes.
▪ Lyell stressed that scientists must explain past events in terms of processes that they can actually observe, since processes that shaped Earth millions of years earlier continue in the present
▪ Volcanoes release hot lava and gases now, just as they did on an ancient Earth.
▪ Erosion continues to carve out canyons, just as it did in the past.
▪ Lyell’s work explained how awesome geological features could be built up or torn down over long periods of time.
o James Hutton
▪ proposed that layers of rock, such as those that make up the distinct layers of sandstone, form very slowly
▪ some rocks are moved up by forces beneath Earth’s surface
▪ others are buried, and still others are pushed up from the sea floor to form mountain ranges. The resulting rocks, mountains, and valleys are then shaped by a variety of natural forces—including rain, heat, and cold temperatures
▪ Most of these geological processes operate extremely slowly, often over millions of years.
▪ Hutton, therefore, proposed that Earth had to be much more than a few thousand years old.
How did Hutton and Lyell describe geological change?
• Hutton and Lyell helped scientists recognize that Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present
LaMarck
• Organisms were adapted to their environments
o adaptation is a result of change caused by environmental pressures
• Use & Disuse
o organisms lost parts because they did not use them — like the missing eyes & digestive system of the tapeworm
• Perfection with use & need
o the constant use of an organ leads that organ to increase in size — like the muscles of a blacksmith or the large ears of a night-flying bat
• evolution of acquired characteristics
• example
o in reaching higher leaves giraffes stretch their necks & give the acquired longer neck to offspring
• not accepted as valid
Darwin’s view of Evolution
• giraffes that already have long necks survive better
• leave more offspring who inherit their long necks
• variation
• selection & survival
• reproduction & inheritance of more fit traits
LaMarckian vs. Darwinian view
• LaMarck
o giraffe’s “need’ to reach higher vegetation stretches their neck & stimulates transmission of longer neck to offspring
• Darwin
o long-necked giraffes survive better & leave more offspring with genes for long necks
Thomas Malthus – Studied human populations
• In 1798, Malthus observed 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 applied these ideas to all organisms
o The overwhelming majority of a species’ offspring die. Further, only a few of those offspring that survive succeed in reproducing.
Key Concepts Questions
• How did Hutton and Lyell describe geological change?
o Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present
• According to Lamarck, how did species evolve?
o By acquired characteristics
• What was Malthus's theory of population growth?
o If the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone.
SECTION 3
DARWIN PRESENTS HIS CASE
Key Concept Questions
• How is natural variation used in artificial selection?
• How is natural selection related to a species' fitness?
• What evidence of evolution did Darwin present?
A Reluctant Revolutionary
• Returned to England in 1836
• wrote papers describing his collections & observations
• draft of his theory of species formation in 1844
• instructed his wife to publish this essay upon his death
• reluctant to publish but didn’t want ideas to die with him
• Then, in 1858, Darwin received a letter that changed everything…
• Alfred Russel Wallace
o a young naturalist working in the East Indies, had written a short paper with a new theory. He asked Darwin to evaluate his theory and pass it along for publication.
o Darwin wrote to Lyell:
▪ "Your words have come true with a vengeance... I never saw a more striking coincidence... so all my originality, whatever it may amount to, will be smashed."
• November 24, 1859, he published the results of his work: On the Origin of Species.
• In his book, he proposed a mechanism for evolution that he called natural selection.
• He then presented evidence demonstrating that the process of evolution has been taking place for millions of years—and continues in all living things.
Darwin’s explanation of evolution:
- Several key insights led to Darwin’s idea for natural selection.
- Darwin noticed a lot of natural variation in domesticated plants and animals.
- Artificial selection is the process by which humans select traits through breeding.
• How is natural variation used in artificial selection?
o In artificial selection, nature provided the variation among different organisms, and humans selected those variations that they found useful
• Artificial Selection gone bad!
• Unexpected consequences of artificial selection
• Insecticide resistance
• Spray the field, but…insecticide didn’t kill all individuals
o Variation
▪ resistant survivors reproduce
▪ resistance is inherited
▪ insecticide becomes less & less effective
[pic]
STRUGGLE FOR EXISTENCE – members of each species compete regularly to obtain food, living space, and other necessities of life
• In this struggle, the predators that are faster or have a particular way of ensnaring other organisms can catch more prey. Those prey that are faster, better camouflaged, or better protected can avoid being caught.
FITNESS – the ability of an individual to survive and reproduce in its specific environment
ADAPTATION – any inherited characteristic that increases an organism’s chance of survival
• Successful adaptations, Darwin concluded, enable organisms to be better suited to their environment and thus better able to survive and reproduce.
• Adaptations can be physical characteristics, such as a porcupine’s sharp quills, or more complex features, such as behavior in which some animals live and hunt in groups
• Individuals with characteristics that are not well suited to their environment—that is, with low levels of fitness—either die or leave few offspring.
• Individuals that are better suited to their environment—that is, with high levels of fitness—survive and reproduce most successfully.
• This was a process that Darwin called SURVIVAL OF THE FITTEST
NATURAL SELECTION – process by which individuals that are better suited to their environment survive and reproduce most successfully
* Video clip – adaptation and natural selection
* Darwin and His Work – Mcgdougall video
How is natural selection related to a species’ fitness?
• Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species’ fitness in its environment
Natural selection explains how evolution can occur.
- There are four main principles to the theory of natural selection.
o Variation
o Overproduction
o Adaptation
o descent with modification
o Fitness is the measure of survival ability and ability to produce more offspring.
Natural Selection can take several forms
• Predation Selection
o camouflage (mimicry)
o speed
o behaviors & habits
o defenses (physical & chemical)
Natural Selection
• Selection acts on any trait that affects survival or reproduction
o predation selection
o physiological selection
o sexual selection
▪ Predation Selection
• Acting on predator & prey
o behaviors & habits
o camouflage (mimicry)
o speed
o defenses (physical & chemical)
▪ Physiological Selection
• Acting on body functions
o fitness (food-gathering)
o physiology efficiency (oxygen, food, water)
o disease resistance
o protection from injury
o biochemical versatility
▪ Sexual Selection
• attractiveness to potential mate
• fertility of gametes
• “ultimately, differential reproductive success”
o What’s a lion’s mane for?
• Sexual selection acts in all sexually-reproducing species
o it influences morphology & behavior
o it acts on both males and females
• Darwin proposed that over long periods, natural selection produces organisms that have different structures, establish different niches, or occupy different habitats.
• As a result, species today look different from their ancestors.
• Each living species has descended, with changes, from other species over time.
• He referred to this principle DESCENT WITH MODIFICATION
• Descent with modification also implies something else: that all living organisms are related to one another.
• Look back in time, and you will find common ancestors shared by tigers, panthers, and cheetahs
• If we look far enough back, the logic concludes, we could find the common ancestors of all living things.
• This is the principle known as COMMON DESCENT.
• According to this principle, all species—living and extinct—were derived from common ancestors
What evidence of evolution did Darwin present?
• Darwin argued that living things have been evolving on Earth for millions of years.
• Evidence for this process could be found in the fossil record, the geographical distribution of living species, homologous structures of living organisms, similarities in early development, and molecular similarities
Evidence for evolution in Darwin’s time came from several sources.
▪ Fossils provide evidence of evolution.
▪ Fossils in older layers are more primitive than those in the upper layers.
The study of geography provides evidence of evolution.
- island species most closely resemble nearest mainland species
- populations can show variation from one island to another
Homologous structures
• Structures that come from the same origin
o homo- = same
o -logous = information
o Forelimbs of human, cats, whales, & bats
• same structure
o on the inside
• same development in embryo
• different functions
o on the outside
• evidence of common ancestor
[pic]
• Compare the bones
o The same bones under the skin
▪ limbs that perform different functions are built from the same bones
The study of anatomy provides evidence of evolution.
- Homologous structures are similar in structure but different in function.
- Homologous structures are evidence of a common ancestor.
The study of anatomy provides evidence of evolution.
- Analogous structures have a similar function.
- Analogous structures are not evidence of a common ancestor.
Structural patterns are clues to the history of a species.
▪ Vestigial structures are remnants of organs or structures that had a function in an early ancestor.
▪ Ostrich wings are examples of vestigial structures.
Vestigial organs
• Structures on modern animals that have no function
• remains of structures that were functional in ancestors
• evidence of change over time
o some snakes & whales have pelvis bones & leg bones of walking ancestors
o eyes on blind cave fish
o human tail bone
o Hind leg bones on whale fossils
Embryology provides evidence of evolution.
- identical larvae, different adult body forms
- similar embryos, diverse organisms
Comparative embryology
• Development of embryo tells an evolutionary story
o similar structures during development
[pic]
Molecular and genetic evidence support fossil and anatomical evidence.
▪ Two closely-related organisms will have similar DNA sequences.
[pic]
Molecular record
• Comparing DNA & protein structure
• everyone uses the same genetic code!
• DNA
[pic]
Essence of Darwin’s ideas
His theory was simple…
1. Variation exists in natural populations
2. Many more offspring are born each season than can possibly survive to maturity
3. As a result, there is a struggle for existence
4. Characteristics beneficial in the struggle for existence will tend to become more common in the population, changing the characteristics of a species
5. Over time, and given a steady input of new variation into a population, these processes lead to the emergence of new species
Key Concept Questions
• How is natural variation used in artificial selection?
• In artificial selection, nature provided the variation among different organisms, and humans selected those variations that they found useful
• How is natural selection related to a species' fitness?
• The more fit an organism is, the more likely it will survive and reproduce.
• What evidence of evolution did Darwin present?
• fossil record, the geographical distribution of living species, homologous structures of living organisms, similarities in early development, molecular record
Questions for discussion:
DDT is an insecticide that was first used in the 1940s to kill mosquitoes and stop the spread of malaria. At first, it was very effective. However, over a period of years, people began to notice that it was becoming less and less effective. A possible explanation for this was that the insects were becoming resistant to the DDT. Explain how the resistance may have evolved.
Questions for discussion:
Is protecting an endangered species upsetting the process of natural selection? Explain your answer.
Questions for discussion:
Although wild turkeys can fly, domesticated turkeys cannot. Suppose that a population of domesticated turkeys escaped from a farm into a new environment. Give examples of environmental conditions that might determine whether that population would survive over time.
-----------------------
Ground finches
Tree finches
Warbler
finch
Cactus
eater
Seed eaters
Bud eater
Insect eaters
Large
ground
finch
Medium
ground finch
Small
ground
finch
Sharp-beaked finch
Cactus finch
Vegetarian
tree finch
Large
insectivorous
tree finch
Small
insectivorous
tree finch
Woodpecker finch
Warbler finch
125
67
45
8
32
Human
Macaque
Dog
Bird
Frog
Lamprey
120
110
100
90
80
70
60
50
40
30
20
0
10
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