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Chapters 13 & 14:
Evolution
Evolution and Life’s Diversity
• Scientists have accumulated considerable
evidence to show that organisms alive today
have been produced by a long process of
change over time
• The process by which modern organisms have
descended from ancient organisms is called
evolution
• Charles Darwin contributed more to our
understanding of the process than anyone else
Voyage of the Beagle
• On December 27th, 1831, H.M.S. Beagle
sailed from England to survey the coast
of South America
• Charles Darwin was onboard
• Darwin’s observations during his 5
years at sea changed the way we think
of ourselves and our world
Voyage of the Beagle
• During his journey, Darwin often left the ship
to collect specimens of animals, plants, and
fossils
• He made careful observations and recorded
them in a journal
• What Darwin saw on his voyage led him to
doubt the idea that species were constant
Above you can see the course of
the Beagle, the ship in which
Darwin sailed around the world.
On this voyage, Darwin collected
thousands of specimens of plants,
animals, and fossils.
Fitness: To Survive and
Reproduce
• Darwin noted that most animals and plants
have body parts and behaviors that do
certain things very well
• The physical traits and behaviors that
enable organisms to survive and
reproduce in their environment give them
what Darwin called fitness
Darwin’s Mechanism for Evolution
• Darwin returned to England in 1836
• He got married and wrote several books
• For 20 years, Darwin gathered evidence
supporting his ideas about evolution, but he did
not publish them
• Then in 1858 another biologist, Alfred Russel
Wallace, sent Darwin an essay that presented
these same ideas
• This prompted Darwin to finally publish his
work
Darwin’s Mechanism for Evolution
• When Darwin’s book On the Origin of Species
by Means of Natural Selection appeared in
November of 1859, it stirred up great
controversy
• Darwin's conclusion that species changed over
time and gave rise to new species contradicted
the prevailing beliefs that God created all
species and that species did not change
Darwin’s Mechanism for Evolution
• In The Origin of Species, as the book is
commonly known, Darwin not only presented
much evidence that evolution occurred but also
proposed that natural selection was its
mechanism
• Organisms with traits well suited to an
environment are more likely to survive and
produce more offspring than organisms
without these favorable traits
• This process is called natural selection
Darwin’s Mechanism for Evolution
• Darwin’s views were soon accepted by
biologists around the world
• Since the discovery of Mendel’s ideas
about genetics in the early 1900s, genetic
principles have been added to Darwin’s
ideas, forming the modern theory of
evolution
In 1859 Darwin published his
famous book, The Origin of
Species. He accomplished much
of his work in his study at Down
House in Kent, England.
Understanding the Fossil Record
• More than a century has passed since Darwin’s
death in 1882
• During this period, a great deal of new
evidence has accumulated supporting the
theory of evolution
• This evidence has come from a variety of
sources, including studies of fossils,
comparisons of the structures of organisms,
and the rapidly expanding knowledge about
DNA and proteins
Fossils are any Traces of Dead Organisms
• Most people think of fossils as shells or old
bones
• Actually, fossils are any traces of dead
organisms
• Tracks of dinosaurs, footprints of human
ancestors, insects trapped in sticky tree sap,
impressions of leaves or skin, and animals
buried in tar are all fossils
• Both biologists and geologists date the Earth’s
past with the help of a record in the rocks
called the geologic time scale
Fossils are any Traces of Dead Organisms
• Relative dating is a technique used by scientists
to determine the age of fossils relative to other
fossils in different layers of rock
• For fossils to form, very special conditions are
necessary
• If a skeleton or shell is to fossilize, the dead
animal must be buried by sediment
• Calcium in the bone or in the shell is slowly
replaced by other, harder minerals
• The fossil record provides only a glimpse into
life’s history
How Fossils are Dated
• Since the late 1940s scientists have been able
to determine the ages of rocks and fossils by
measuring the amount of radioactive decay, or
breakdown, of radioactive atoms in the rock
• A radioactive atom contains an unstable
combination of protons and neutrons
• Since it is unstable, a radioactive atom will
eventually change into a more stable atom of
another element
How Fossils are Dated
• For example, carbon-14, a rare form of carbon
found in tiny amounts in living things, decays
into nitrogen
• The term half-life describes how long it takes
for one-half of the radioactive atoms in a
sample to decay
• Since carbon-14 decays relatively rapidly,
other isotopes with longer half-lives are more
often used to date fossils
How Fossils are Dated
• Because the rate of decay of a radioactive element
is constant, scientists can use the amount of
radioactive element remaining in a rock or fossil to
determine its age
• This technique is called radioactive dating
• Evolution can be a very slow process; the
transformation of one species into another by
natural selection requires thousands of years
• Using radioactive dating, scientists have
determined that the Earth is about 4.5 billion years
old
How Fossils are Dated
• By combining radioactive dating, relative
dating, and observations of important events
in the history of life on Earth, scientists
have divided the 4.5 billion years into large
units called eras
• Eras are further divided into periods, which
are in turn divided into epochs
The Fossil Record
• Most fossils are found in sedimentary rock
– Formed when exposure to rain, heat, and
cold breaks down existing rocks into small
particles of sand, silt, and clay
• The chancy process by which organisms are
fossilized means that the fossil record is not
as complete as we would like it to be
• For every organism that leaves a proper
fossil, many die and vanish without leaving
a trace
The Fossil Record
• The quality of fossil preservation also
varies
• Some fossils are preserved so perfectly
that we can see the microscopic structure
of tiny bones and feathers
• Other fossils are not preserved as well
and so raise fascinating questions about
their meaning and importance
What the Fossil Record Tells Us
• Scientists who study fossils are called
paleontologists
• Over the years, paleontologists have collected
millions of fossils to make up the fossil record
• The fossil record represents the preserved
collective history of the Earth’s organisms
• Although incomplete, the fossil record has
long inspired scientists
What the Fossil Record Tells Us
• Paleontologists have assembled good
evolutionary histories into many animal groups
• The fossil record also tell of major changes
that occurred in Earth’s climate and geography
• As Earth’s environments changed over time,
many species died out
• The fossil record shows that change
followed change on Earth
Similarities in Early Development
• Much of our evolutionary history can be seen in
the way human embryos develop
• Early in development, human embryos and
embryos of all other vertebrates are strikingly
similar
• In later stages of development, a human embryo
develops a coat of fine fur
• The similarity of these early developmental forms
strongly suggests that the process of development
has evolved
• New instructions on how to grow have been added
to old instructions inherited from ancestors
Comparing Organisms
• Comparing the way organisms are put together
provides important evidence for evolution
• Your arm appears quite different from the wing
of a bird or the front fin of a dolphin
• Yet you can see that the position and order of
bones in these limbs are very similar
• Biologists say that these three limbs are
homologous
• Homologous structures are structures that share
a common ancestry
• They are modified versions of structures that
occurred in a common ancestor
Vestigial Structures are Clues to
Evolutionary Origins
• Structures without function are found in living things
• A whale propels itself with its powerful tail and has
no need for hind limbs or the pelvis to which they
attach
• Nevertheless, whales still have a reduced pelvis that
serves no apparent function
• Structures with no function are known as vestigial
structures
• Vestigial structures are remnants of an organism’s
evolutionary past
• The whale’s pelvis is evidence of its evolution from
four-legged, land-dwelling mammals
Transitional Forms Link New
Species to Old
• Because new species form from existing species, Darwin
predicted that transitional forms, intermediate stages between
older and newer species, would be found in the fossil record
• There are now many good examples of evolutionary transitions
• For instance, modern whales are the descendants of fourlegged land animals that are also the ancestors or horses and
cows
• Fossil intermediates between modern whales and their 60
million year old ancestor reveal a history of slow
transformation
• Over time, the hind limbs became smaller and smaller, until
eventually they were lost entirely
Modern whales have forelimbs that are
flippers, no hind limbs, and a tiny,
nonfunctional pelvis.
Rodhocetus kasrani probably spent
little time on land. Its reduced hind
limbs could not have aided in walking
or swimming.
Ambulocetus natans apparently
walked on land in a manner similar to
modern sea lions. It swam by flexing
its backbone and paddling with its
hind limbs.
Mesonychids are the hypothesized link
between modern whales and hoofed
mammals.
DNA and Proteins Contain Evidence
of Evolution
• Although complete fossil histories for living
organisms are rare, an organism’s history is written
in the sequence of nucleotides making up its DNA
• If species have changed over time, their genes
should also have changed
• The theory of evolution predicts that genes will
accumulate more alterations in their nucleotide
sequence over time
• Thus, if we compare the genes of several species,
closely related species will show more similarities
in nucleotide sequences than will distantly related
species
What Homologies Tell Us
• The structural and biochemical
similarities among living organisms are
best explained by Darwin’s conclusion:
Living organisms evolved through
gradual modification of earlier forms –
descent from a common ancestor
How Natural Selection Causes
Evolution
• Darwin not only demonstrated that evolution
has occurred but also proposed its principal
mechanism – natural selection
• The key factor in natural selection is the
environment
• The environment presents challenges that
individuals with particular traits can better
overcome
• Thus, the environment “selects” which
organisms will survive and reproduce more
often
Evolution by Natural Selection
• Individuals whose characteristics are
well-suited to their environment survive
• Individuals whose characteristics are not
well-suited to their environment either
die of leave fewer offspring
– Survival of the fittest
The Peppered Moth: Natural
Selection in Action
• Over many generations natural selection
gradually changes a species in response to the
demands of its environment
• Adaptation is the process by which a species
becomes better suited to its environment
• Adaptation can also refer to any change in a
trait that increases the likelihood that an
organism will survive or reproduce
The Peppered Moth: Natural
Selection in Action
• Until the 1850s, dark gray peppered moths
were rare and were treasured by British
butterfly and moth collectors
• Almost all peppered moths were pale
• Around 1850, dark peppered moths started to
become more common, usually in heavily
industrialized areas
• By 1950, peppered moth populations living
near industrial centers consisted almost
entirely of dark individuals
The Peppered Moth: Natural
Selection in Action
• Why did the dark peppered moths become more
common?
• Darwin’s theory of evolution by natural selection
suggests a hypothesis
• The color change coincided with a great increase
in the number of factories in England
• Pale tree trunks were blackened by heavy
pollution from these factories
• Perhaps dark moths sitting on soot-darkened bark
escaped being eaten by birds because it was hard
for the birds to see them
The Peppered Moth: Natural
Selection in Action
• Light-colored moths, on the other hand,
would have stood out against a dark
background and would have been easily
spotted by hungry birds
Evolution as Genetic Change
• In order to describe the evolution of plants and animals,
modern evolutionary biologists study groups of organisms
called populations
• Because all members of a population can interbreed, they and
their offspring share a common group of genes, called a gene
pool
• The number of times an allele occurs in a gene pool
compared with the number of times other alleles for the same
gene occur is called the relative frequency of the allele
• Evolutionary change involves a change in the relative
frequencies of alleles in the gene pool of a population
The Development of New
Species
• Organisms, like members of a human community,
need to survive and acquire the necessities of life
• The combination of an organism’s job and the
place in which it lives is called its niche
• No two species can occupy the same niche in
the same location for a long period of time
• Chances are, one of the species will be more
efficient than the other
• The more efficient species will survive, reproduce,
and drive the less efficient species to extinction
The Process of Speciation
• New species evolving from old ones is called
speciation
• Individuals of the same species share a
common gene pool
• One of the most common ways in which new
species form is when populations are
separated
• The separation of populations so that they do
not interbreed is called reproductive isolation
How Species Form
• Because natural selection favors changes that
increase an organism’s chances of surviving
and reproducing, it will continuously shape a
species to improve the fitness between the
species and its environment
• When populations of a species are found in
several different kinds of environments, natural
selection will act to make each population
suited to its particular environment
Darwin’s Finches
• Darwin repeatedly saw patterns in how kinds of
animals and plants differed, patterns suggesting
that species changed over time and gave rise to
new species
• On the Galapagos Islands, 600 miles from the
coast of Ecuador, Darwin collected several species
of finches
• All of these species were similar, but each was
specialized to catch food in a different way
Darwin’s Finches
• Some species had thick, sturdy bills for
cracking open tough seeds
• Others had slender bills for catching insects
• All of the species of finches closely resembled
one species of South American finch
• In fact, all of the plants and animals of the
Galapagos Islands were very similar to those
of the nearby coast of South America
Darwin’s Finches
• If each one of these plants and animals
had been created to match the habitat of
the Galapagos Islands, why did they not
resemble the plants and animals of
islands with similar environments that
lie off the coast of Africa?
• Why did they instead resemble those of
the adjacent South American continent?
Darwin’s Finches
• Darwin felt that the simplest explanation
was that a few organisms from South
America must have migrated to the
Galapagos Islands in the past
• These few kinds of animals and plants
then changed over the years that they
lived in their new home, giving rise to
many new species
The woodpecker
finch captures
insects with its
grasping bill.
The crushing bill
of the large
ground finch
enables it to feed
on seeds.
The cactus finch
uses its probing
bill to feed on
cactuses.
Speciation and Adaptive
Radiation
• The process in which one species gives rise
to many species, as with the finches, is
called adaptive radiation or divergent
evolution
• During a period of adaptive radiation,
organisms evolve a variety of characteristics
that enable them to survive in different
niches
Does Evolution Occur in Spurts?
• Following Darwin’s lead, most biologists
have assumed that species formation is a
slow, gradual process that goes on all the
time
• The hypothesis that evolution occurs at a
slow, constant rate is known as gradualism
Does Evolution Occur in Spurts?
• Recently, some biologists have challenged
gradualism, arguing that species formation
occurs rapidly after major environmental
upheavals
• Short periods of rapid species formation have
been followed by long periods during which
little evolution occurred
• The hypothesis that evolution occurs at such
irregular rates is known as punctuated
equilibria