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Chapter 15
Darwin's Theory of Evolution
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15–1 The Puzzle of Life's Diversity
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This variety of living things is called biological
diversity.
How did all these different organisms arise? How
are they related?
Evolution, or change over time, is the process by
which modern organisms have descended from
ancient organisms.
A scientific theory is a well-supported testable
explanation of phenomena that have occurred in
the natural world.
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Voyage of the Beagle
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Charles Darwin (born February 12, 1809 in
England)
1831 – set sail on the H.M.S. Beagle
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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.
That hypothesis, now supported by a huge
body of evidence, has become the theory
of evolution.
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On The Voyage
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Collected plants & animals on land
At sea – studied specimens, read,
recorded observations
Saw great diversity in the organisms –
enormous numbers of species!
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Patterns of Diversity
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Plants and animals seemed so
well suited to their
environments
Impressed by the many ways
organisms survive and
reproduce
Similar areas on different
continents were inhabited by
very different animals.
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Ex: why are there no rabbits on
the Australian grasslands? No
kangaroos in England?
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Living Organisms and Fossils
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Darwin also collected fossils
Some fossils looked like organisms that were
still alive
Other fossils looked completely different!
Darwin wondered:
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Why had so many of these species disappeared?
How were they related to living species?
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The Galápagos Islands
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1000 km west of South America (Ecuador)
Islands are close together but very different
climates
Darwin was fascinated in particular by the land
tortoises and marine iguanas
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The shape of a tortoise's shell could be used to
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identify which island a particular tortoise inhabited.
The Journey Home
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Darwin began to wonder if animals living on
different islands had once been members of the
same species.
these separate species would have evolved
from an original South American ancestor
species after becoming isolated from one
another
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15-2 Ideas that shaped
Darwin’s thinking
Geological Information
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Massive, rich fossil record – much older than
the current age of the earth beliefs.
James Hutton (1795) – old earth – great earth
changes take time
Charles Lyell (1831) – the processes shaping
the Earth now are the same ones that shaped
the earth in the past
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Could life change as earth has changed?
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Jean Baptiste Lamarck
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1809
“living things have changed over time”
Species descended from other species
Lamarck’s theory of evolution:
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Why was he wrong?
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Tendency toward perfection
Use and Disuse
Inheritance of Acquired Traits
Behavior doesn’t affect the DNA
He was WRONG.
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Thomas Malthus
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1798
Population growth (if gone unchecked) would
quickly over run the earth
Why don’t maple trees cover the earth since
they each release 1000s of seeds?
Darwin’s questions:
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What causes the deaths of so many?
What factors determine which survive and
reproduce?
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All species tend to produce more
offspring than they can support
Turtle eggs
Insect eggs
Frog eggs
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15-3 Darwin presents his case
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Worked for years but scared to publish
because of religious backlash
1858 – sent an essay by Alfred Russell
Wallace summarizing Darwin’s theory of
evolution
1859 – Darwin publishes “On the origin of
species by means of natural selection”
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What did Darwin say?
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Inherited Variation & Artificial Selection
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Artificial Selection = nature provides the
variation and humans select the variations they
find useful
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Members of a species vary
Examples: fat hog, fast horse, high milk producing
cow
Does something in nature select the same way?
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Variation within a species
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Artificial Selection
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Evolution by Natural Selection
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Compared nature to artificial selection
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Struggle for Existence
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Members of each species compete for food,
space, mates, and other necessities of life
Predators (that are better/faster) catch more
prey
Prey (faster, better camouflage) avoid being
caught
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Examples: fast cheetah, small rabbit, stronger lion,
more colorful butterfly
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Producing more offspring than can possibly
survive thus creating a struggle for existing
resources
Preying Mantis
sunflower
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Survival of the Fittest
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Fitness = being able to survive and reproduce
in its specific environment
Fitness results from adaptations
Adaptations = inherited characteristics that
improve an organisms chance of survival
Successful adaptation  better suited to the
environment  more fitness
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Adaptations –
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Anatomical/structural (quills on a porcupine)
Physiological (photosynthesis)
Behavioral (live/hunt in groups)
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Low Fitness  organisms die off
High Fitness  survive + reproduce 
Survival of the Fittest!
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Natural Selection of most fit organisms!
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Nature (the environment) will
determine which organism is the
fittest to Survive
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Natural Selection
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Results from NO human intervention, control,
or direction
Results in changes in the inherited
characteristics of a population
Increases a species’ fitness in the environment
(over time!)
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Descent with Modification
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Over large amounts of time… natural selection
produces organisms:
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with different structures
establishes different niches
occupies different habitats
organisms look different than their ancestors
 Descent with modification = each living thing
has descended with changes, from other
species over time.
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Common Ancestors
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Implies that organisms are all related to one
another.
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Tiger, panther, cheetah – related
Horses, dogs, cats – related farther back
Mammals, birds, reptiles, fish – related even farther
back
All living things – related!
Single Tree of Life!
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Evidence of Evolution
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Living things have been evolving on
Earth for millions of years…
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Evidence..
Fossil Record
 Geographic distribution of living species
 Homologous body structures
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body part with the same basic structure and embryonic
origin as that of another organism, though not necessarily
sharing the same function
Similarities in early development
(embryology)
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The Fossil Record
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Fossils = remains of ancient life
Different layers of rock were formed at
different times during Earth’s history.
Darwin proposed that organisms had
coming into being, lived, and vanished.
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Life had changed over time!
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Fossil Cephalopods
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Fossil record of the modern horse
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Fossil
record
of the
camel
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Since Darwin…
The number of known fossils has gone up
dramatically!
 Researchers have discovered many hundreds of
transitional fossils that document various
intermediate stages in the evolution of modern
species from organisms that are now extinct.
 Gaps remain in the fossil records
 These gaps do not indicate weakness in the
theory of evolution itself. Rather, they point out
uncertainties in our understanding of exactly
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how some species evolved.
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Geographic Distribution of Living
Species
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Darwin wondered about the finches he found in
the Galapagos Islands.
Where were they similar but still different?
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Also.. Slightly different from S. American mainland
species
Could the birds have changed as they adapted to
local environments?
=> descended with modification from a common
mainland ancestor
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Convergent Evolution
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Darwin found entirely different species of
animals on the continents of South America and
Australia.
Similar environments on those continents  he
sometimes saw different animals that had
similar anatomies and behaviors.
The existence of similar but unrelated species
was a puzzle to Darwin.
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Similar Species in
Similar Environments
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Convergent Evolution
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The adaptive
evolution of
superficially similar
structures, such as
the wings of birds
and insects, in
unrelated species
subjected to similar
environments.
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Unrelated organisms come
to resemble one another
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Analogous Structures
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Structures that serve the same function in different
species but they evolved independently
same function, different structure
Ex: bat wing, butterfly wing
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Divergent Evolution
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two or more related species becoming more
and more dissimilar
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Homologous Body Structures
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striking anatomical similarities among the body
parts of animals with backbones
of reptiles, birds, and mammals—arms, wings,
legs, and flippers—vary greatly in form and
function.
all constructed from the same basic bones
limbs has adapted in ways that enable organisms
to survive in different environments
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Homologous Body Structures
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structures that
have different
mature forms in
different
organisms but
develop from the
same embryonic
tissues
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Vestigial Organs
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organ that serves no useful function in an
organism
Why would an organism possess organs with
little or no function?
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One possibility = presence of a vestigial organ may
not affect an organism's ability to survive and
reproduce  natural selection would not cause the
elimination of that organ.
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The human appendix
has no function in
humans but is very
important in cows and
other animals
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Human tailbone is a vestigial
vertebrate tail
Human tail bone
consists of fused
vertebrae that no
longer function
as a tail
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Similarities in Embryology
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embryos of many animals with backbones are
very similar
many embryos look especially similar during
early stages of development
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chickens, turtles, and rats
Pharyngeal
slits
Dolphin
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Summary of Darwin's Theory
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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.
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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.
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