Chapters 13

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Chapters 13 14Evolution
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Evolution and Lifes 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

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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
• Darwins observations during his 5 years at sea
  changed the way we think of ourselves and our
  world

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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

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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.
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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

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Darwins 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

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Darwins Mechanism for Evolution

• When Darwins 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

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Darwins 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

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Darwins Mechanism for Evolution

• Darwins views were soon accepted by biologists
  around the world
• Since the discovery of Mendels ideas about
  genetics in the early 1900s, genetic principles
  have been added to Darwins ideas, forming the
  modern theory of evolution

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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.
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Understanding the Fossil Record

• More than a century has passed since Darwins
  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

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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 Earths
  past with the help of a record in the rocks
  called the geologic time scale

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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
  lifes history

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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

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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

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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

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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

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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

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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

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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 Earths organisms
• Although incomplete, the fossil record has long
  inspired scientists

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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 Earths climate and geography
• As Earths environments changed over time, many
  species died out
• The fossil record shows that change followed
  change on Earth

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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

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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

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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
  organisms evolutionary past
• The whales pelvis is evidence of its evolution
  from four-legged, land-dwelling mammals

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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 four-legged 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

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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.
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DNA and Proteins Contain Evidence of Evolution

• Although complete fossil histories for living
  organisms are rare, an organisms 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

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What Homologies Tell Us

• The structural and biochemical similarities among
  living organisms are best explained by Darwins
  conclusion Living organisms evolved through
  gradual modification of earlier forms descent
  from a common ancestor

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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

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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

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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

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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

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The Peppered Moth Natural Selection in Action

• Why did the dark peppered moths become more
  common?
• Darwins 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

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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

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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

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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 organisms 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

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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

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How Species Form

• Because natural selection favors changes that
  increase an organisms 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

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Darwins 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

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Darwins 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

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Darwins 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?

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Darwins 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

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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.
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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

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Does Evolution Occur in Spurts?

• Following Darwins 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

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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

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