Evolution

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Evolution
Satirical cartoon by Thomas Nast, from
Harpers Weekly, August 19, 1871.
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Objectives

• Define evolution.
• Outline the evidence for evolution.
• State that populations tend to produce more
  offspring than the environment can support.
• Explain that a consequence of over-reproduction
  of offspring is a struggle for survival.
• State that members of a species show variation.
• Explain how sexual reproduction promotes
  variation in a species.
• Explain how natural selection leads to evolution.
• Explain two examples of evolution in response to
  environmental change.

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Charles Darwin

• British lived from 1809 to 1882.
• Began to develop the theory of evolution on a
  trip around the world aboard the HMS Beagle.

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Charles Darwin

• Came to realize that the earth had changed
  over a long period of time, which caused some
  crea- tures to go extinct, opening up
  space for new creatures to appear.

The history of life on earth was similar to a
branching tree, tracing back to some common
ancestor. Nature selected which creatures
survived and passed on their characteristics.
Species change!
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Evolution defined

• Evolution the change over time in the
  frequency of
  inherited variations in a
  population over generations.
• Natural selection amplifies or diminishes
  inherited (not acquired)
  variations.

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Evolution defined

• Evolution the change over time in the frequency
  of in-herited variations in a population (not
  individuals).

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Evolution defined

• Lamarckian vs. Darwinian evolution change over
  time
• Lamarck was a Frenchman who lived before Darwin
  and first proposed creatures change over time
  (evolve).
• Example giraffes have a long neck because one
  stretched its neck to reach high branches and
  passed the trait on.

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Evolution defined

• Lamarckian vs. Darwinian evolution change over
  time
• Darwin would say that giraffes necks come in
  various sizes. Those with a longer neck can
  reach higher branches and survive better during
  hard times and pass the trait on to their
  offspring.

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Evolution defined

• Lamarckian vs. Darwinian evolution change over
  time
• Common errors
• Potato beetles evolved resistance to DDT in
  order to survive.
• There is no intention to evolve.
• The beetles either had or didnt have the genes
  needed to survive the chemical.
• Offspring of the survivors shifted the
  population toward a greater likeli- hood
  of resistance overall.
• Peppered moths come in various colors, white
  to black. Population colors shift as
  colors of tree trunks change over time, and
  pre- dators choose different colored moths..

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Natural selection

• Natural selection the differential success in
  the reproduction of different phenotypes
  resulting from the interaction of organisms with
  their environment.
• Charles Darwins mechanism of evolution.
• Developed from 5 observations
• 1) Species are so fertile that populations would
  rise exponen- tially if all
  individuals reproduced successfully.
• 2) Populations tend to remain stable in size,
  except for seasonal fluctuations.
• 3) Environmental resources are limited.
• 4) Individuals of a population vary no two
  individuals are exactly alike.
• 5) Much of this variation is heritable.

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Natural selection

• How natural selection works
• Populations tend to produce more offspring than
  the environment can support.
• As a consequence, overproduction of offspring
  leads to a struggle for existence, with only a
  fraction of the offspring surviving each
  generation.
• A generation of mice lasts 45
  days, then there are 4 7 babies. After 45
  days there are perhaps 25
  mice, then after another
  45 days there
  could be 125, then 625.

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Natural selection

• How natural selection works
• Individuals in a species vary.

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Natural selection

• How natural selection works
• Sexual reproduction promotes variation in a
  species.
• Genes are redistributed and mixed among
  individuals.
• Asexual reproduction would produce no differences.

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Natural selection

• Natural selection leads to evolution
• Survival is not random, but depends in part on
  the hereditary make-up of the individuals.
• Individuals more fit in their environment are
  likely to leave more offspring than those less
  fit.
• Preferential survival reproduction leads to a
  gradual change in a population, with favorable
  characteristics accumulating over the generations.

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Examples of natural selection

• Evolution of resistance to in-
  secticides in insect species.
• Effectiveness of insecticide decreases with
  time.
• Each generation there are more resistant
  insects.
• Natural selection edits
• existing variation.
• Natural selection favors
• characteristics that
• fit the current, local
• environment.

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Examples of natural selection

• Drug-resistant strains of HIV evolve rapidly in
  the viral population infecting any particular
  patient.
• Like pesticide resistance the drug 3TC
  interferes with HIV replication in human cells.
• Resistant strains become 100 of the population
  in just a few weeks.
• Bacterial resistance to antibiotic drugs
  works in the same way.

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Genetic drift

• Genetic drift is a 2nd mechanism of evolution.
• In each generation, some individuals may, just by
  chance, leave behind a few more descendents (and
  their genes) than other individuals. The genes
  of the next generation will be the genes of the
  lucky individuals, not necessarily the
  healthier or better individuals.

The percentage of green genes drops
accidentally.
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Genetic drift

• Genetic drift is a 2nd mechanism of evolution.
• The bottleneck effect A few creatures are lucky
  enough to escape some disaster that kills
  randomly.
• Northern elephant seals were hunted nearly to
  extinction (20 survived), leading to a loss of
  genetic diversity.

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Genetic drift

• Genetic drift is a 2nd mechanism of evolution.
• The founder effect colonization by a small
  subset of a population (little genetic variety)
• Ex Polydactyly in the Amish in
  Pennsylvania, a community that
  was founded by a small group of
  Germans in the
  1700s.
• Ex Huntingtons disease (nerve
  cells in brain degenerate) in the
  Afrikaner population of
  Dutch settlers in South Africa.
• Source of Asian eye characteristics

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Genetic drift

• Genetic drift is a 2nd mechanism of evolution.
• 8 of Chinese and Mongolian men descend from
  Genghis Khan! His sons had many children.

Spread of Genghis Y chromosome
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Migration

• Migration is a 3rd mechanism of evolution.
• Some individuals from a population of brown
  beetles might have joined a population of green
  beetles. That would make the genes for brown
  beetles more frequent in the green beetle
  population.

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Mutation

• Mutation is a 4th mechanism of evolution.
• A mutation could cause parents with genes for
  bright green coloration to have offspring with a
  gene for brown coloration. That would make the
  genes for brown beetles more frequent in the
  population.
• Ex a 6-toed cat.

Blond and red hair are human mutations from
the original brown.
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Evolution
Satirical cartoon by Thomas Nast, from
Harpers Weekly, August 19, 1871.
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Evidence for evolution

• Evidence of evolution is everywhere in biology.
• Fossil record
• Homologous structures
• Biogeography
• Embryonic development
• Molecular biology
• Selective breeding
• Natural selection within human lifespan

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Evidence for evolution

• Fossil record
• Fossils are any traces of dead organisms
  bones, tracks (foot-prints), leaf impressions,
  excrement, actual organisms frozen in ice, in
  amber, or in tarpits.

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Evidence for evolution

• Fossil record
• Most fossils are found in
  sedimentary rock where
  deeper rock is older, formed
  from sand or clay deposits.
• Stratigraphy dating fossils by charting the
  rock layers.
• Since the late 1940s, fossils are
  dated by the decay of radioactive
  isotopes.
• This is called radiometric dating.

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Evidence for evolution

• Fossil record
• The fossil record in rocks
  provides relative ages.
• Radiometric dating can
  determine absolute ages.
• Organisms accumulate radio- active isotopes
  when alive.
• Isotopes decline after death - they decay
  (transform) into another element.

Most carbon is 12C, but there is a small yet
constant amount of 14C in the air, and therefore
in our living bodies 1 part per trillion. This
amount declines after death.
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Evidence for evolution

• Half-life time for ½ of the isotope atoms to
  decay.
• Use 40K to date old rocks half-life 1.3
  billion yr.
• Use 235U for early vertebrates half-life 700
  mil. yr.
• Use 14C for recent fossils half-life 5,730
  years.

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Evidence for evolution

• Using 14C dating to determine the age of organic
  materials. Half-life of 14C is 5,730 years.

Work backward from the amount present today
to a time when there was maximum 14C, 1 part per
trillion.
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Evidence for evolution

• Fossil record
• Fossils give evidence about the major branches of
  descent in the tree of life.
• Order established
• Ex fossil fishes predate other
  vertebrates amphibians are
  next, followed by reptiles, then
  mammals and birds.

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Evidence for evolution

• Fossil record
• Transitional forms link old
  new species.
• Evolution of horses hooves from 5 toes.
• Evolution of whale from horse-like animal
  whale retains a pelvis where hind legs
  once attached forelegs now flippers

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Evidence for evolution

• Homologous structures
• Features of new species are altered versions of
  ancestral features. Similarity in
  characteristics resulting from common ancestry
  is known as homology.

All cats have a common ancestor.
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Evidence for evolution

• Homologous structures
• For example, the forelimbs of human, cats,
  whales, and bats share the same skeletal
  elements, but different functions because they
  diverged from the ancestral tetrapod forelimb.
• They are homologous structures.

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Evidence for evolution

• Vestigial organs are homologous structures that
  have marginal, if any, importance to a current
  organism, but which had important
  functions in ancestors.
• Skeletons of some snakes fossil whales retain
  vestiges of pelvis and leg bones of walking
  ancestors.
• In humans - wisdom teeth, tailbone, appendix.

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Evidence for evolution

• Biogeography - the study of the distribution of
  life forms over geographical areas.
• If evolution is true, then we should expect to
  find related species living near each other
• Except in cases of great mobility (like sea
  animals, birds, and animals distributed by
  humans) or over long periods of time (due to
  plate tectonics).
• If, however, we find that species are distributed
  in a random geographic manner, with closely
  related species no more likely to be found close
  to each other than unrelated species, then this
  would be strong evidence against evolution and
  common descent.

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Evidence for evolution

• Biogeography
• Plate tectonics the continents are on plates
  that glide over the surface of the earth
  carrying life with them.

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Evidence for evolution

• Biogeography
• Identical fossils in parts of the world now
  widely separated indicate that the continents
  were once joined.

The southern part of Pangaea
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Evidence for evolution

• Biogeography
• Australian example marsupials vary
  widely but are more closely
  relat- ed to each other than to
  similarly- appearing animals on other
  continents.
• All have a pouch!
• Placental predators
  out-competed them on
  other continents, and they disappeared. Australia
  (with no placentals) was isolated.

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Evidence for evolution

• Biogeography
• Marsupials ori- ginated in SA 70
  million yrs ago then spread to
  Australia.

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Evidence for evolution

• Biogeography
• Species tend to be more closely related to other
  species from the same area than to other species
  with the same way of life, but living in
  different areas.

The sugar glider from Australia is more closely
related to other marsupial mammals in
Australia than to the flying squirrel, a
pla- cental mammal of North America. This is an
example of convergent evolution.
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Evidence for evolution

• Embryonic development
• Genes for embryonic development are conserved in
  many different species making the embryos
  similar.
• All vertebrate embryos have structures called
  pharyngeal pouches in their throat at some stage
  in their development. These develop into
  different, but still homologous, adult
  structures gills of fish or Eustachian tubes in
  mammals.

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Evidence for evolution

• Molecular biology corroborates
  evolutionary trees.
• Evolutionary relationships
  among species are documented
  in their DNA and proteins.
• Ex the Cytochrome c protein is more similar
  when crea- tures are closely related
• Human chimp have the same
  104 amino acids, dog
  has 13 differences, rattle- snake
  has 20 changes.

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Evidence for evolution

• Selective breeding
• Humans have domes-
  ticated many animals,
  giving them new char-
  acteristics over time
  (they evolved).
• Dogs domesticated from wolves
  about 15,000 years ago.
• Sheep, cattle, horses goats, pigs, chickens

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Evidence for evolution

• Selective breeding
• Plants corn, wheat, potato, bean, cabbage, etc.