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Natural Selection


									    Natural Selection
The Darwin-Wallace theory of organic change
                over time.
  Thinking Question
• Some evidence exists that over
  several centuries, the number of
  people born with small wisdom teeth
  or no wisdom teeth has increased.
  Using your best understanding of
  Natural Selection, explain how
  selection could cause this.
 Darwin’s Evidence

• Darwin spent nearly 20 years
  analyzing evidence as he developed
  his ideas. He drew on:
 • His own field work during his
   voyage on board the Beagle.
 • His work in classifying barnacles,
   beetles, and other organisms.
 Darwin’s Evidence

• In addition, Darwin drew on evidence
 • Fossils
 • Comparative Anatomy
 • Comparative Embryology
• The fossil record shows that:
 • Things existed in the past that no
   longer exist today and
 • Things exist today that did not
   exist in the past.
• The fossil record is a record of
  change in the composition of the
  biosphere over time.
What is a fossil?
    Fossils are rare

• Most organisms are eaten or decay
  after death.
• Fossils only form in places where
  there is little erosion: lake bottoms,
  deep oceans, etc.
• We have little fossil record of beach
  organisms and alpine communities.
  Some fossils show
• Because fossils are rare, and change
  can be rapid, there are few “missing
  links” in the fossil record.
• However, some fossil sequences of
  marine organisms do show good
  evidence of transition from one form
  to another.
Comparative Anatomy

• Darwin looked at and described:
 • Homologous structures
 • Analogous structures
 • Vestigial structures
Homologous structures
 • Homologous structures are those that
   are shared between related
   organisms, but are slightly altered.
 • In modern terms, this indicates
   shared genes (similar structures), but
   also shows genetic differences led to
   divergent evolution (modifications of
   those structures.
Analogous Structures
• Analogous structures are similar
  solutions to the same environmental
  challenges seen in unrelated species
  living in similar environments.
• In modern terms, analogous
  structures represent convergent
  evolution. Rather than shared genes,
  the organisms are under similar
  selection pressures.

Insects and birds evolved flight independently.

Ancestral seals and penguins that had a streamlined
      body form were better able to survive.
Vestigial Structures

• Vestigial structures are those that
  were well-developed in an ancestor
  but are much reduced in a
• Vestigial structures show that
  different selection pressures shape
  organisms differently.
     Vestigial Structures

All higher vertebrates descend from ancestors with
  four limbs. What happened in the snake and the
Comparative Embryology
 • Like homology, embryology reveals
   shared traits between species,
   demonstrating shared genes.
 • As embryos develop, newer genes
   may shape the same structures into
   different parts: gill arches in fish
   become gills, but in mammals become
   parts of the face, jaw, and inner ear.
Comparative Embryology

  Can you tell these three mammals apart?
           Which one is human?
Molecular Comparisons
 • Shared traits give us some insights
   into shared genes. Today’s technolgy
   allows us to analyze genes directly.
 • Because small mutations accumulate
   in populations over time, more
   genetic differences between two
   different groups of organisms
   indicates more time since they
   separated from one another.
How Selection
  1. Variation Exists

• All populations vary as the result of
  the accumulation of small, random
  mutations over many generations.
2. Inheritance of traits

• Inheritable traits (those coded for by
  genes) are passed directly to the
  offspring from the parents through
  genetic information.
3. Differential Survival

 • More offspring are born than can
   survive. Many offspring die young.
   Those with traits best suited to the
   environment are more likely, though
   not guaranteed, to survive.
      4. Differential

• Some survivors fail to reproduce.
  Some have traits that better insure
  reproduction than others.
5. Differential Inheritance

  • Survivors that reproduce pass some
    of their traits on to their offspring.
    Those with favorable traits may pass
    those favorable traits on — or not.
 Natural Selection in

• Peppered Moth simulation:
Wisdom Teeth, Revisited
 • Now that we’ve gone through the
   process of selection, return to the
   paper on which you responded to the
   wisdom tooth question.
 • Draw a line under your original
   response, and write a second one,
   using what you just learned about the
   process of selection: Variation,
   Inheritance, Differential Survival,
   Differential Reproduction, Differential

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