Evolution by Natural Selection

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Evolution by Natural Selection

• AP Biology
• Chapter 22

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Whos this Darwin guy anyway?

• Wrote On the Origin of Species by Means of
  Natural Selection
• Supported idea of evolution not so new, really.
• Proposed mechanism that explained how evolution
  could occur. This WAS new Natural Selection
• Said Natural Selection led to Evolutionary
  Adaptation

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Natural Selection Defined

• Natural Selection a population can change over
  generations if individuals that possess certain
  heritable traits leave more offspring than other
  individuals.

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Natural Selection leads to Evolutionary Adaptation

• Evolutionary Adaptation an accumulation of
  inherited characteristics that enhance organisms
  ability to survive and reproduce in specific
  environments.

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Evolution

• The processes that have transformed life on Earth
  from its earliest forms to the vast diversity of
  today.
• Change in kinds of species over time on Earth.
• Text a change over time in the genetic
  composition of a population

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Before Darwin

• Darwin was NOT the first person to suggest
  evolution of life on Earth
• Several Greek philosophers believed in a gradual
  evolution of life
• None explained HOW such evolution might occur
• However, Aristotle (whose ideas greatly
  influenced Western science) believed each life
  form to be perfect and permanent.

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The View in Darwins Time

• Earth was a few thousand years old
• Earth was populated by unchanging forms of life
  that had been individually made during a single
  week in which the Creator also formed the entire
  Universe.

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Laying the Groundwork for DarwinScientists and
Ideas BEFORE Natural Selection

• Linnaeus
• Father of Taxonomy
• Binomial system
• Developed system for grouping similar species
  into increasingly general categories
• Kingdom, phylum, class, order, etc.
• For Linnaeus, this grouping did not imply
  evolutionary relationships, but it became an
  important point in Darwins arguments later on.

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Study of fossils
• Remains or traces of organisms from the past
• Most found in sedimentary rock
• Paleontology study of fossils
• Largely developed by Georges Cuvier
• 1769-1832
• French

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Cuvier
• Developed the science of Paleontology
• Documented the succession of fossil species in
  the Paris Basin.
• Said each layer (stratum) of rock has unique
  fossil species
• Said that the deeper (older) the stratum, the
  more different the fossils are from modern life
• From stratum to stratum, new species appear and
  others disappear.

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Cuvier, however, was NOT an evolutionist
• He advocated catastrophism
• Each boundary between strata corresponded to some
  catastrophe such as a flood or drought
• The catastrophe destroyed species living in the
  area at the time
• Change in species in next strata was due to new,
  immigrating species repopulating the area after
  the catastrophe

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Hutton
• Founder of Modern Geology
• Said different landforms could be explained by
  processes we can currently observe operating in
  the world - Not necessarily epic catastrophes
• Example Canyons are formed by rivers cutting
  down through rocks
• Example sedimentary rocks with marine fossils
  were made in oceans as sediment and dead sea life
  fell to the bottom

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Hutton also said that the Earth was much older
  than was believed by most at the time
• Said the earth was many millions of years old
  rather than 6000 years old as was believed by
  most at the time.

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Hutton explained Earths features by the theory
  of gradualism
• The profound changes in Earth are the result of
  slow but continuous processes over a long period
  of time.
• In direct contrast with catastrophism

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Lyell
• The leading geologist during Darwins Era
• Incorporated Huttons ideas of gradualism into a
  theory called uniformitarianism
• Geological processes have not changed throughout
  Earths history.
• Example The forces that build and erode
  mountains are the same today as they were in the
  past.

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Charles Darwin was especially influenced by the
  work of Hutton and LyellEspecially
• Ancient Earth
• Dramatic effects of slow, subtle processes

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Lamarck
• NOT the first to come up with the idea that life
  had changed over time
• WAS the first to suggest how this might occur
• Unfortunately for him, his idea was wrong, and it
  is for this that he is rememberd.

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Lamarck
• Published his ideas about evolution in 1809 (the
  year Darwin was born)
• Most remembered for the mechanism he proposed for
  HOW evolution occurred
• It incorporated two ideas
• Use and disuse
• Inheritance of acquired characteristics

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Use and Disuse
• Parts of the body used extensively become larger
  and stronger Blacksmiths arm
• Inheritance of Acquired Characteristics
• Modifications an organisms acquires during its
  lifetime can be passed along to offspring
• Classic Example of Larmarckian evolution
• Giraffes long neck acquired by stretching to get
  highest leaves
• Slightly longer necks passed on to young and so
  on

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Laying the Groundwork for Darwin Scientists and
Ideas BEFORE Natural Selection

• Inheritance of acquired characteristics seems
  silly to us today, but this notion was actually
  generally accepted in Lamarcks day.
• Even so, in Lamarcks time, the question of a
  mechanism of evolution was mostly irrelevant
  because everyone believed that species were
  specially created 6000 years ago and were
  unchanging. NO theory of evolution was to be
  taken seriously.
• In fact, Lamarck was more visionary than he is
  credited for.
• Said evolution best explained the fossil record
  and current diversity of life on Earth
• Recognized the great age of the Earth
• Empahsized adaptation to the environment as the
  main product of evolution

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Charles Darwin

• 1809-1882
• British Naturalist
• Proposed the idea of evolution by natural
  selection
• Collected LOTS of clear evidence to support his
  ideas

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Voyage of the HMS Beagle

• 1831- 1836
• Travels around the world
• Makes many observations of the natural world
• Darwin is the ships naturalist
• In his 20s
• Main mission was to chart S. American Coast

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Voyage of HMS Beagle -

• Stopped in Galapagos Islands

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Galapagos Islands

• Islands of relatively recent volcanic origin
• Most animal species on Galapagos live nowhere
  else in the world
• BUT they DO resemble species living on mainland
  S. America.
• Made a big impression on Darwin

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Darwins Observation

• It was as though the islands had been colonized
  by plants and animals that had strayed from the
  South American mainland and then diversified on
  the different islands.

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Darwins Observation

• The fossils interested Darwin, too.
• Clearly not the same as living species
• But clearly South American in their resemblance
  to living species
• In Argentina, Darwin collected fossils of
  gigantic armor-plated beasts, megatheres, which
  were unlike anything else anywhere in the world
  nearly. Only the tank-like armadillos, which
  Darwin had also seen in South America, bore any
  resemblance to them.

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Example Darwins Finches

• Darwin observed 13 types of finches on the
  Galapagos that although they were quite similar,
  seemed to be different species
• Some were unique to individual islands
• Some were distributed on two or more islands that
  were close together.

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Other Examples
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Return from the Galapagos

• Darwin returned to Great Britain in 1836 and
  started reassessing his observations from the
  voyages.
• It occurred to Darwin that a new species could
  arise from a ancestral form by the gradual
  accumulation of adaptations to a different
  environment.

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The Origin of Species

• By the early 1840s, Darwin had worked out the
  major features of his theory of natural selection
  as the mechanism of evolution
• Still, he delayed in publishing his ideas,
  evidently anticipating the uproar it would cause.

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The Origin of Species

• In 1858, Darwin finally published his book, The
  Origin of Species, when it became apparent that a
  younger scientist was about to publish a similar
  idea.
• Alfred Wallace

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The Origin of Species

• Two Main Ideas
• Evolution explains lifes unity and diversity
• Natural selection is a cause of adaptive evolution

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Descent with Modification

• Term Darwin used in place of evolution
• Darwin perceived unity in life with all organisms
  related through descent from some unknown
  ancestor that lived in the remote past.
• As the descendants of that first organism spilled
  into various habitats over millions of years,
  they accumulated diverse modifications or
  adaptations that fit them to specific ways of
  life.

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Descent with Modification

• History of life is like a tree with multiple
  branching and rebranching from a common trunk all
  the way to the tips of the youngest twigs

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Descent with Modification

• At each fork, of the evolutionary tree is an
  ancestor common to all lines of evolution
  branching from that fork.
• Closely related species such as lions and tigers
  share many characteristics because their lineage
  of common descent extends to the smallest
  branches of the tree of life.

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Descent with Modification Darwins Finches
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Descent with Modification

• Most branches of evolution, even some major ones,
  are dead ends
• About 99 of all species that have ever lived are
  extinct.
• Explains why there are no living animals to fill
  the gap between the elephants and their nearest
  relatives today
• Hyrax and manatee

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Descent with Modification Descent of the
Elephant
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Descent with Modification

• Linnaeus unintentionally provided Darwin with a
  connection to evolution.
• To Darwin, the natural hierarchy of the Linnean
  scheme of classification (Kingdom, phylum, class,
  etc.) reflected the branching genealogy of the
  tree of life
• Organisms at the different taxonomic levels were
  related through descent from common ancestors

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Darwins focus in The Origin of Species

• Darwins focus in the his book was less on the
  actual creation of new species and more on how
  populations of species became better adapted to
  their environment through natural selection
• Ernst Mayr, a very important modern day
  evolutionary biologist, analyzed Darwins theory
  of evolution in this way

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Basic Ideas of Natural Selection

• OBSERVATIONS
• Size of a population would increase exponentially
  if all individuals that are born reproduced
  successfully
• OVERREPRODUCTION
• In reality, populations tend to remain more or
  less stable in size.
• Environmental resources are limited.
• INFERENCE
• Production of MORE individuals than the
  environment can support leads to competition with
  only a fraction of individuals surviving each
  generation

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Example

• This moss produces far more spores than can
  survive its the environment.
• New moss plants born from these spores will
  compete for resources with each other.

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Basic Ideas of Natural Selection

• OBSERVATIONS
• No two individuals are exactly alike. Variation
  exists in populations.
• Much of this variation is inheritable.
• INFERENCES
• Survival is not random, but depends partly on the
  hereditary make-up of surviving individuals.
  Those individuals whose inherited traits best fit
  them to their environment are more likely to
  leave offspring than less-fit individuals
• The unequal ability of individuals to survive and
  reproduce leads to a gradual change in a
  population with favorable characteristics
  accumulating over time.

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Example

• These are all the same species of beetle
• Some fairly dramatic color variations exist among
  its members
• These color variations are heritable and are
  passed on to offspring
• Those with coloration that best allows them to
  blend with their environment will survive to pass
  on their coloration genes to offspring.

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Summary of Darwins Main Ideas

• More organisms are made than can exist in a
  population
• Called OVERREPRODUCTION - Competition results
• Variation exists in populations
• Variation is often inherited genetically based.
• Natural selection occurs through interaction
  between
• Environment
• Variability among individuals in a population
• Natural selection is differential success in
  reproduction
• Better fit individuals leave more offspring
  than others
• They pass their better fit genes on to these
  offspring and so on
• The product of natural selection is adaptation of
  populations to their environment.

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Artificial Selection

• Darwin found evidence that natural selection
  might work by studying artificial selection
• Artificial selection - The breeding of
  domesticated plants and animals

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Artificial Selection

• Humans have modified plants and animals over many
  generations by selecting individuals with desired
  traits as breeding stock.
• As a result, the plants and animals we grow for
  food often bear little resemblance to their wild
  ancestors.

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Example of Artificial Selection

• The mustard plant is the ancestor of the common
  vegetables viewed in this photo

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Example of Artificial Selection

• The power of artificial selection can also be
  viewed in our pets.
• All domestic dogs were originally bred from wolves

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Artificial Selection - Another Example

• Insecticide resistance
• Works much like antibiotic resistance in bacteria
• Bugs susceptible to the insecticide die.
• Any bug that has a resistance gene will survive
  and reproduce
• It also will have no competition as it does this.

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Artificial Selection

• Darwin reasoned that if so much change can be
  achieved in such a relatively short time frame (a
  few generations), then natural selection should
  be capable of LOTS of change over hundreds or
  thousands of generations
• He argued that even if advantages of some
  variations were only slight, the advantageous
  variations would accumulate over time and natural
  selection would eliminate less favorable ones.

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Gradualism

• The importance of Lyells geologic gradualism is
  evident in Darwins work
• Darwin envisioned life evolving by gradual
  accumulation of very small changes.
• He proposed that natural selection operating over
  VAST spans of time could account for the entire
  diversity of life on Earth.

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Darwins Two Main Ideas

• Diverse life forms arise through Descent with
  Modification from ancestral species.
• Natural Selection over VAST spans of time is how
  it happens.

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Some Points to Keep in Mind

• A population is the smallest unit that can evolve
• Evolution can only be measured as changes in
  relative proportions of different variations in a
  population over generations
• INDIVIDUALS CANNOT EVOLVE
• ONE critter with a new trait does NOT mean a new
  species has evolved.

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Some Points to Keep in Mind

• Natural Selection can ONLY work with HERITABLE
  variations
• Variation acquired within a lifetime due to an
  organisms experiences cannot be inherited.
• Thus, natural selection cannot act on such
  variation

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Some Points to Keep in Mind

• Natural Selection is situational
• This means that environments vary
• What might be a groovy adaptation in one
  environment could be a real drag in some other
  environment.

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Example of Natural Selection Darwins Finches

• Darwin hypothesized that the different beaks
  found in the different Galapagos Finches are
  evolutionary adaptations to different food
  sources.

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Darwins Finches the Medium Ground Finch

• Medium Ground Finches on Daphne Major (a
  Galapagos Island) use their strong beaks to crush
  seeds.
• The seeds they like are small seeds that are
  produced in abundance in wet years.

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Darwins Finches the Medium Ground Finch

• In dry years, all seeds are in short supply
• Finches must resort to eating both small and
  large seeds.
• Larger seeds are more difficult to crush.

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Darwins Finches the Medium Ground Finch

• Scientists observed that the average beak depth
  (size) in the finch population changes over the
  years.
• During drought average size increases
• Size decreases again during wet years

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Darwins Finches the Medium Ground Finch

• Beak size is an inherited trait
• Scientists attribute changes in beak size to
• Big beaks are better at cracking big seeds in dry
  periods
• Small beaks are more efficient at working with
  little seeds in wet periods

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Darwins Finches the Medium Ground Finch

• The studies of the Medium Ground Finch reinforce
  that
• Natural selection is situational whats good in
  one environment may not be so good in another

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Darwins Finches the Medium Ground Finch

• Also important is that beak evolution of these
  finches does not result form the inheritance of
  an acquired characteristic
• The environment did not create these beaks
  (remember the blacksmith and his arm)
• Instead the environment acted on the different
  types of beaks present in the population at
  certain times.
• The proportion of thicker-beaked finches
  increased during dry periods because those
  individuals with thicker beaks transmitted their
  genes to more offspring than the thin-beaked
  birds.

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Evidence for Evolution

• Comes from 5 major areas
• Biogeography
• Fossil Record
• Comparative Anatomy
• Comparative Embryology
• Molecular Biology

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Biogeography

• The geographical distribution of species
• Islands have lots of species unique or native to
  the island itself
• However they are closely related to species of
  the nearest mainland or neighboring island
• This causes some questions to arise

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Biogeography

• Two islands that have very similar environments,
  but that are in very different parts of the world
  SHOULD be populated with closely related species,
  but they are not.
• Instead they are populated by species related to
  those on the nearest mainland where the
  environment is often quite different from the
  island.

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Biogeography

• Tropical animals of South America are more
  closely related to species found in the South
  American desert.
• Why arent they more closely related to other
  tropical animalssay in Africa? The environment
  is certainly more similar there

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Biogeography

• Why are there not very many placental mammals
  found in Australia? Is Australia inhospitable to
  placental mammals?
• No. Introduced placental mammals have exploded in
  number there
• The unique Australian fauna evolved on that
  continent in ISOLATION from places where
  placental mammals lived.

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Biogeography

• All of these biogeographical patterns make little
  sense if species were individually placed in
  suitable environments.
• These patterns make sense if we find modern
  species where they are because they evolved from
  ancestors that inhabited these regions or other
  regions at least somewhat nearby.

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The Fossil Record

• Shows fewer species and more simple species in
  most ancient rocks more species of increasing
  complexity in more recent rocks.
• This succession of fossil forms is in agreement
  with other evidence for evolution
• Biochemistry, molecular biology and cell biology
  all say that prokaryotes are the ancestors of all
  life.
• This means that prokaryotes should precede
  eukaryotes in the fossil record.
• They do. The oldest known fossils are
  prokaryotes.

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The Fossil Record

• More evidence The chronological appearance of
  the different classes of vertebrate animals in
  the fossil record.
• Fossil fishes predate all other vertebrates.
• Next in the fossil record we see amphibians,
  reptiles, mammals and birds
• This is consistent with the history of vertebrate
  descent as revealed by many other types of
  evidence (anatomy, molecular, etc.)

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The Fossil Record

• Vertebrate Fossil Record, cont.
• If all species had been created individually at
  the same time, all vertebrate classes would make
  their first appearance in the fossil record in
  rocks of about the same age.
• This is NOT what is actually observed.

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Vertebrate Evolution
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The Fossil Record

• Transitional Forms
• The Darwinian view predicts that evidence of
  transitional forms should be in the fossil record
• Transitional forms fossils that link older
  fossils to modern species
• Example a series of fossils documents the
  changes in skull shape and size that occurred as
  mammals evolved from reptiles.

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The Fossil Record Transitional forms continued

• In the past few years, researchers have found
  fossilized whales that link our modern aquatic
  mammals with their ancient land ancestors.

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The Fossil Record Transitional forms - Whale
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Comparative Anatomy

• Species grouped in the same taxonomic category
  (mammals, for example) have lots of anatomical
  similarities
• Many of the same bones make up the forelimbs of
  humans, cats, whales and bats.
• The forelimbs of ALL these organisms have vastly
  different functions
• It makes no sense that best way to design a bats
  limb for flying and whales fin for swimming would
  be to make them from the same bones
• The more likely explanation is that all these
  animals had some common ancestor with these same
  bones and they have become modified for different
  functions over years.

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Comparative Anatomy Vertebrate Forelimbs
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Comparative Anatomy

• Homologous structures
• Similar characteristics that are the result of
  common ancestry
• Bat wing and human forelimb (etc.)

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Comparative Anatomy

• Vestigial organs
• Structures of marginal, if any, importance to the
  organism.
• Historical remnants of structures that had
  important functions in ancestors.
• Example Vestigial hindlimbs and pelvis in
  whales.

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Comparative Anatomy

• Vestigial organs, cont.
• Vestigial organs are evidence of natural
  selection. It would be wasteful to continue to
  provide blood, nutrients, etc. to an organ that
  no longer had a major function i.e., hind limbs
  on totally aquatic animals.
• Natural selection would tend to favor individuals
  with a reduced form of such organs

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Comparative Embryology

• Closely related organisms go through similar
  stages in their embryonic development.
• Example All vertebrate embryos go through a
  stage in which they have gill pouches on the
  sides of their throats.

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Comparative Embryology

• At the earliest stages of development,
  similarities between fish, frogs, snakes, birds,
  humans and all vertebrates are much more evident
  than differences
• It is not until development progresses that the
  original structures begin to diverge to take on
  the characteristics of their classes.
• Comparative embryology reveals homologies not
  visible in adult forms

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Comparative Embryology

• Fish gill pouches develop into gills
• Terrestrial vertebrates gill pouches become
  other things
• Eustachian tubes that connect middle ear with
  throat in humans
• Comparative embryology is often able to establish
  homology in structures (like gill pouches) that
  become so altered in later development that their
  common origin is not apparent in the adult forms.

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Molecular Biology

• Perhaps the bottom line in the discussion of
  evidence of evolutionary relationships among
  organisms is DNA.
• Darwinian logic predicts that closely related
  organisms would share similar DNA (and proteins
  the products of genes).
• This premise is commonly used in DNA analysis in
  humans paternity, forensics, etc.

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Molecular Biology

• Darwin speculated that ALL forms of life are
  related to some extent through branching descent
  from the earliest organisms.
• DNA and protein analysis supports this claim.
• Even humans and bacteria share at least some
  proteins (hence genes) in common.
• Example cytochrome c is a protein found in the
  electron transport chain in cellular respiration
  of all aerobic organisms.

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Molecular Biology

• Hemoglobin analysis of several vertebrates
  confirms the relationships of these vertebrates
  to each other. The more closely related, the
  more amino acids in hemoglobin are the same.

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Molecular Biology

• A common genetic code is overwhelming evidence
  that all life is related.
• The language of the genetic code has been passed
  along through all branches of the tree of life
  ever since the codes inception in an early life
  form.