Chapter 15
Darwin’s Theory of Evolution
15-1 The Puzzle of Life’s Diversity
• Evolution – change over time
- the process by which modern organisms have descended
from ancient organisms
• Scientific Theory – a well-supported testable explanation of
phenomena that have occurred in the natural world.
Voyage of the Beagle
• Charles Darwin contributed more to our understanding of evolution
than anyone
• In 1831, Darwin set sail on a ship called the H.M.A. Beagle for a
voyage around the world. During his travels, Darwin made numerous
observations and collected evidence that led him to propose a very
important hypothesis about the way life changes over time.
• This hypothesis is the theory of evolution.
Darwin’s Observations
• Darwin saw a lot of diversity. For example, during one day in a
Brazilian forest, Darwin collected 68 different beetle species. He
realized that an enormous amount of species live on the Earth.
• Darwin was intrigued by the fact that so many plants and animals
were so well suited to whatever environment they lived in. He was
puzzled by the fact that different animals lived in different places.
Living Organisms and Fossils
• Darwin collected and preserved remains of ancient organisms, called
fossils
• Some fossils looked like animals that were still alive, and some he had
never seen before.
• Why had so many of these species disappeared?
How were they related to living species?
The Galapagos Islands
• A small group of islands called the Galapagos Islands influenced
Darwin the most. He noticed that even though these islands are close
together, they had very different climates. Some were hot and dry
with hardly any animals or plants. Some had greater rainfall and an
assortment of plants and animals.
• He saw that tortoises were different from island to island. He could
tell which island a tortoise belonged to by the shape of its shell.
Pinta Island tortoise
Isabela Island tortoise
Hood Island tortoise
• Darwin also found birds with different types of beaks.
• When Darwin returned home, he wondered if animals living on
different islands had once belonged to the same species.
15-2 Ideas That Shaped Darwin’s Thinking
In Darwin’s time, people believed that the Earth and all of its forms of
life had been created only a few thousand years ago. Therefore, they
believed that neither the planet nor any species had changed. Darwin
realized that what he discovered did not fit neatly into this view of
unchanging life.
An Ancient, Changing Earth
• James Hutton and Charles Lyell were two scientists who helped other
scientists realize 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.
Hutton and Geological Change
• Hutton proposed that layers of rock form very slowly. Also, some
rocks are moved up by forces beneath Earth’s surface. Some are
pushed up from the sea floor to form mountain ranges. The rocks are
then shaped by natural forces – rain, wind, heat, cold temperatures.
These geological processes happen very slowly, over millions of years.
So, Hutton knew that the Earth had to more than just a few thousand
years old.
Lyell’s Principles of Geology
• Lyell wrote a book called Principles of Geology which stressed that
scientists must explain past events in terms of processes that they can
actually observe, since processes that shaped the Earth millions of
years earlier continue in the present. For example, volcanoes release
hot lava and gases now, just as they did years ago. Erosions still carve
out canyons, just as they did years ago.
• Lyell’s work explained how huge geological features could be built up
or town down over long periods of time.
Lamarck’s Evolution Hypothesis
• Jean-Baptiste Lamarck was among the first scientists to recognize that
living things have changed over time and that all species were
descended from other species. He also realized that organisms were
somehow adapted to their environment.
• Lamarck proposed that when organisms stopped using certain organs
or used them more, organisms acquired or lost certain traits during
their lifetime. These traits could then be passed on to their offspring.
Over time, this process led to change in species.
• Lamarck published his hypothesis the year that Darwin was born;
1809.
Population Growth
• Thomas Malthus was another important influence of Darwin. In 1798
he published a book which said 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 not enough living space or food for everyone.
• Darwin realized that this applied even more to plants and animals
than to humans because humans do not produce as much offspring
as most other species do.
• This left Darwin wondering what causes the death of so many
individuals and what determines which ones survive and reproduce
and which one’s don’t.
15-3 Darwin Presents His Case
Publication of On the Origin of Species
• Darwin wrote a book about his findings called The Origins of Species.
In this book, he proposed that natural selection is the cause of
evolution. He presented evidence that evolution has been taking
place for millions of years, and continues.
• Some people felt his arguments were brilliant while others strongly
opposed the theory of evolution.
Inherited Variation and Artificial Selection
• One of Darwin’s most important insights was that members of each
species vary from one another in important ways. He argued that this
variation mattered (people is this day thought variation was very
unimportant).
• He knew that plant and animal breeders used genetic variation to
improve crops and livestock. They would breed only the largest hogs,
the fastest horses, or cows that produced the most milk. This is
artificial selection.
• Artificial selection – humans select genetic variations that they found
most useful
Evolution by Natural Selection
• Darwin was convinced that a process similar to artificial selection
happens in nature.
• Darwin realized that high birth rates and a shortage of life’s basic
needs would eventually force organisms into a competition for
resources
• Struggle for existence – members of each species compete regularly
to get food, living space, and other necessities of life.
• The predators that are faster or have a particular way of trapping
other organisms can catch more prey. Prey that are faster, better
camouflaged, or better protected can avoid being caught.
• The struggle for existence was central to Darwin’s theory of evolution.
• 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 enable organisms to become better suited to
their environment and thus better able to survive and reproduce.
• Survival of the Fittest – Individuals that are better suited to their
environment survive and reproduce most successfully. Individuals
with characteristics that are not well suited to their environment
either die or leave few offspring.
• Natural Selection – survival of the fittest – done by nature, takes
place without human control or direction
• Over time, natural selection results in changes in the inherited
characteristics of a population. These changes increase a species’
fitness in its environment.
• Descent with modification – each living species has descended, with
changes, from other species over time.
• -implied that all living organisms are related to each other
• Common descent – all species – living and extinct – came from
common ancestors. A single “tree of life” links all living things.
Evidence of Evolution
• Darwin argued that living things have been evolving on Earth for
millions of years. Evidence of this could be found in the fossil record,
the geographical distribution of living species, homologous structures
of living organisms, and similarities in early development
The Fossil Record
• Darwin saw fossils as a record of the history of life on Earth. Like
Lyell, Darwin proposed that Earth was many millions of years old.
During this long time, many species had come into being, lived for a
time, and then vanished. 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.
Geographic Distribution of Living Species
• Species living on different continents had each descended from
different ancestors. However, because some animals on each
continent were living under similar ecological conditions, they were
exposed to similar pressures of natural selection. Because of this,
different animals ending up evolving certain features that are the
same or very similar.
Homologous Body Structures
• Some living animals with backbone have similar body parts. For
example, the limbs of reptiles, birds, and mammals vary in form and
function, yet they are all constructed from the same basic bones.
• Homologous structures – structures that look different once fully
formed and may have different function, but have similar bone
structures
Homologous structures gives us strong evidence that all four-limbed
animals have descended from common ancestors
Vestigial organs – organs with little or no function
Similarities in Embryology
• The embryos of many animals with backbones are very similar.
• What do these similarities mean?
• The same group of embryonic cells develop in the same order and in
similar patterns to produce the tissues and organs of all vertebrates.
These common cells and tissues, growing in similar ways, produce
homologous structures.
Turtle embryo
Chicken embryo
Rat embryo
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Summary of Darwin’s Theory
Individual organisms differ, and some of this variation is heritable
Organisms produce more offspring than can survive, and many that do
survive do not reproduce
Because more organisms are produced than can survive, they compete for
limited resources
Each unique organism has different advantages and disadvantages in the
struggle for existence. Individuals best suited to their environment survive
and reproduce most successfully. These organisms pass their heritable
traits to their offspring. Other individuals die or leave fewer offspring. This
process of natural selection causes species to change over time.
Species alive today are descended with modification from ancestral species
that lived in the distant past. This process, by which diverse species
evolved from common ancestors, unites all organisms on Earth into a single
tree of life.
Strengths and Weaknesses of Evolutionary Theory
• Most of Darwin’s hypothesis have been confirmed and expanded
upon.
• Evolutionary theory continues to change as new information and new
ways of thinking come about.
• Researchers still debate such things as how new species arise and
why species become extinct. There is also uncertainty about how life
began.