Chapter 22 Descent with Modification

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Chapter 22 Descent with Modification

• A Darwinian View of Life

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Charles Darwin

• The man responsible for brining the idea of
  evolution and natural selection to the forefront
  of science.
• His book, The Origin of Species, presented his
  idea of descent with modification.
• Although parts were wrong, or erroneous, the
  majority of it is correct. The errors were due
  mostly to the limitations of science at the time.

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Darwins 2 Key Points

• 1. He presented evidence that many species of
  organisms on Earth are descendents of ancestral
  species which were different from modern species.
• 2. He proposed a mechanism for this
  change--natural selection. The crux of natural
  selection is that populations will change over
  time as individuals become more fit for their
  environment.

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Our Current Understanding

• Our current definition of evolution is referred
  to as a change over time in the genetic
  composition of a population.
• Eventually, enough change will accumulate in a
  population that it will constitute a new species.

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The Uproar

• What made the idea of Darwinism so abrasive at
  the time is that it challenged traditional
  beliefs of Western culture
• 1. The Earth is only a few thousand years old.
• 2. All species which now exist have been here
  since the beginning.
• Darwinism challenges worldview.

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Greek Philosophers

• Many Greek philosophers suggested that life may
  have evolved gradually.
• However, one philosopher, Aristotle, had a great
  influence over Western science. His belief was
  that although different forms of life contained
  many similarities, they could be organized like
  rungs on a ladder in an order of increasing
  complexity--scala naturae. Each life on a rung
  was fixed and permanent.

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Greek Philosophers

• These philosophers ideas were congruent with the
  Old Testaments view of life and its
  creation--each species was designed by God and
  was perfect.
• Recognizing this, many scientists wanted to
  support it. Taxonomy, for example (Carolus
  Linnaeus) was founded for the greater glory of
  God.
• His binomial system of nomenclature is still used
  today.

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Georges Cuvier

• Developed the science of paleontology while he
  studied layers of rock around Paris.
• He noted that the deeper layers of rock contained
  fossils that were more dissimilar from current
  life.
• From one stratum to the next, many
  characteristics disappeared and others formed.
  Thus, he concluded, extinctions must have been
  common throughout the history of life.

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Georges Cuvier

• Cuvier was staunchly opposed to the idea of
  gradualism. Rather, he advocated
  catastrophism--the idea that a local catastrophe
  such as a flood or drought was responsible for
  the changes observed in species--their
  extinction. As these organisms were wiped out,
  new species repopulated the region.

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James Hutton

• In contrast to catastrophism, Hutton proposed the
  idea of gradualism---slow, cumulative, continuous
  change results in profound change.
• His examination of the geological features of
  Earth allowed him to propose parallels between
  the Earth and life.

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Charles Lyell

• Lyell, a great geologist, incorporated Huttons
  ideas into a theory called uniformitarianism--whic
  h states that the same processes of the past are
  taking place today at the same rate.
• Hutton was a horrible writer.
• He wrote The Principles of Geology.

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The Effect on Darwin

• Lyell and Hutton had a great impact on Darwin.
  Darwin felt that if these two were right, then
  the Earth is much older than 6,000 years as
  theologians argued.
• Also, he presumed, if these process could act on
  the Earth, then they could also act on living
  organisms producing slow and gradual change.

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Jean Baptist de Lamarck

• Lamarck is often remembered for his erroneous
  interpretation for how evolution
  occurs--inheritance of acquired characteristics.
• However, he has made many useful contributions to
  science.
• For example, he was the first scientist to
  propose the idea that evolutionary change can
  explain the fossil record.

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Jean Baptist de Lamarck

• Lamarck compared current species with fossil
  forms and chronicled an older to younger line of
  descent from fossil to living species.
• His 2 principles
• Use and disuse.
• Inheritance of acquired characteristics.

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Jean Baptist de Lamarck

• Lamarck also held the belief that an innate drive
  to be more complex was responsible for the
  observed change in organisms.
• This idea was rejected by science in favor of
  Darwins idea of natural selection. However,
  Darwin, too, believed in the inheritance of
  acquired characteristics--later refuted by
  science.

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Darwins Ride on the HMS Beagle

• In 1831, the boat left England to chart the
  poorly known coastlines of S. America.
• When the crew was charting the coastline, Darwin
  was ashore collecting specimens and making
  observations.
• His observations most of the plants and animals
  of S. America were different from those of
  Europe. The animals of the S. American temperate
  regions were more similar to those in the
  tropical regions of S. America than they were to
  temperate regions of Europe.

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Darwins Ride on the HMS Beagle

• Darwin, recall, was deeply impressed by the work
  of Lyell. His belief in Lyells principle of
  uniformitarianism was solidified when he
  witnessed a huge earthquake while in S. America.
  This firsthand witnessing of Chilean coastline
  rising several feet as a result of the quake got
  him thinking.

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Darwins Ride on the HMS Beagle

• Further exploration and his finding of marine
  fossils high up in the Andes mountains enabled
  him to understand how similar earthquakes over a
  long period of time could result in such change.
• He was now realizing that the physical evidence
  wasnt supporting traditional view.

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Darwins Ride on the HMS Beagle

• These ideas really sparked Darwins interest and
  when he arrived at the Galapagos he was intrigued
  by what he saw. Many of the species of animals
  he observed existed no where else in the world.
• He did not notice the significance of any of
  these observations until he returned home to
  England.

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Back Home in England

• Darwin started to notice changes as he examined
  his specimens after the trip.
• For instance, he noticed that many of the birds
  of the Galapagos resembled birds from the
  mainland of S. America.
• His hypothesis was that somehow a bird from the
  mainland must have colonized the Galapagos, and
  through gradual change diversified into the many
  forms now seen on the island.

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Back Home in England

• Darwin also began outlining the major features of
  the theory of natural selection.
• He was reluctant to publish these because of the
  outrage he knew it would cause.
• Lyell urged him to publish because he was afraid
  someone else working on the same thing would beat
  him to it.

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Alfred Russel Wallace

• A young British naturalist named Alfred Russel
  Wallace did beat him to it.
• While studying in the East Indies, Wallace sent a
  paper to Darwin for review and the paper
  basically said the same thing as Darwins did.
• Fortunately for Darwin, Wallace greatly admired
  him and gave him full credit for the development
  of the theory of natural selection.

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Charles Darwin

• Unlike his predecessors, Darwin was able to
  convince the biological community his theory is
  correct because of his reasoning, logic, and the
  overwhelming amount of supporting evidence.
• Darwins publication presented 2 main ideas
• 1. Evolution explains the unity and diversity of
  life.
• 2. Natural selection is the cause of adaptive
  evolution.

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Natural Selection

• The main ideas of natural selection are
• 1. Natural selection is the differential
  reproductive success of individuals and their
  traits.
• 2. Over time, natural selection increases the
  adaptations of organisms to their environment.
• 3. If an environment changes over time, or an
  individual moves, natural selection may result in
  adaptations to new conditions and may give rise
  to new a species as a result.

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Main Points of Natural Selection

1. Natural selection occurs through individuals and
   their environment.
2. Individuals dont evolve.
3. A population is the smallest unit that can
   evolve.
4. Evolution is measured only as changes in the
   relative proportions of heritable variation in a
   population over multiple generations.
5. Natural selection only amplifies inheritable
   traits, not acquired characteristics.
6. Natural selection is always operating, but which
   traits are favored depends on the environment.

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An Example of Natural SelectionEvolution in
Populations

• Differential predation and Guppy populations
• Observation Significant differences exist
  between populations in average age and size where
  a guppy reaches sexual maturity.
• There is a correlation between the population and
  the predator that preys upon it.
• Correlation does not prove causation.

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An Example of Natural SelectionEvolution in
Populations

• In some pools, the main predator is a small
  killfish which preys on juvenile guppies.
• In other pools, the main predator is the
  pike-cichlid which is large and preys upon
  sexually mature individuals.
• Guppies reproduce at a younger age and are
  generally smaller at maturity than guppies preyed
  on by kill fish.

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An Example of Natural SelectionEvolution in
Populations

• To test the correlation, Reznik and Endler put
  guppies from the pike-cichlid pool into the
  Killfish pools with no guppies.
• The experiment lasted 11 years (30-60
  generations).
• The researchers found that the guppies were an
  average of 14 heavier at maturity compared to
  non-transplanted guppies.
• The average age of maturity was also up.
• The results support the hypothesis that natural
  selection caused the differences seen between the
  two populations in nature.

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An Example of Natural SelectionEvolution in
Populations

• Conclusion I Because pike-cichlids prey mainly
  on reproductively mature adults, the chance that
  a mature guppy will survive long enough to
  produce several broods is relatively low.
• Thus the guppies with the most reproductive
  success are the ones who mature at a young age
  when they are small in size.

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An Example of Natural SelectionEvolution in
Populations

• Conclusion II In ponds with killfish, the
  guppies that survive early can grow more slowly
  and produce many broods.
• The sexually mature adults are larger at maturity
  than those seen in the pike-cichlid pools.

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Key Points Arising from Such Experiments

1. Natural selection is an editing process rather
   than a creative mechanism.
2. Natural selection depends on time and place. It
   favors characteristics in a genetically variable
   population that increases the fitness in the
   current environment.

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Homologies

• Homologies and the tree of life are important in
  explaining ancestry. Homologies shared by a
  greater number of species are likely to have
  evolved earlier on in history and are further
  down on the tree of life.

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Homologies

• Homologies that have evolved more recently are
  seen on only small branches of the tree.
• Example tetrapods--the vertebrate branch
  consisting of birds, amphibians, mammals and
  reptiles.
• They all have the same 5-digit limb structure
  which indicates these mammals all share a common
  ancestor.

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Homologies

• Similarities which result from common ancestry
  are known as homologies.
• Forelimbs of all mammals show the same
  arrangement of bones from shoulder to finger tip.
• These occur even if the structure is used for a
  completely different function.
• These anatomical differences would not have
  arisen in a new species if they didnt share a
  common ancestor.

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Homologies

• Such structures are said to be homologous
  structures--which are variations on a common
  theme that was present in the common ancestor.

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Homologies

• Molecular homologies exist as well--the genetic
  machinery of DNA and RNA also points to a common
  ancestor the existence of Hox genes in
  development.

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Other Similarities Revealed

• Comparative embryology
• Compares structures seen early on in
  development--pharyngeal pouches for example.
• Vestigial organs
• Organs with little or no use remaining from
  ancestral lineages--some snakes have vestiges of
  legs bones and a pelvis.

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Biogeography

• Biogeography is the study of the geographic
  distribution of species and helped Darwin form an
  important part of his theory of evolution.
• He showed that closely related species tend to be
  found in the same geographic region.
• The same ecological niches were occupied by
  vastly different species in different regions.

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A Biogeographical Example

• Eutherians (placental) vs. Marsupials
• Both of these are a type of flying squirrel, but
  are unrelated and distinctly different from one
  another.
• Marsupials are born as embryos and develop in the
  mothers pouch.
• Eutherians develop completely within the uterus
  of the mother.
• Although the two mammals look similar and occupy
  similar niches, the resemblance is not
  homologous--its analogous and the result of
  convergent evolution.

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Darwin and the Galapagos

• This is why the islands of this archipelago are
  so intriguing to Darwin. Although the islands
  are very similar to many other different islands
  elsewhere in the world and have the same
  climates, geography, etc., their organisms are
  not related to any of them. They only resemble
  those on the nearest mainland--S. America.