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Evolution

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Title: Evolution


1
Evolution
Satirical cartoon by Thomas Nast, from
Harpers Weekly, August 19, 1871.
2
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.

3
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.

4
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!
5
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.

6
Evolution defined
  • Evolution the change over time in the frequency
    of in-herited variations in a population (not
    individuals).

7
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.

8
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.

9
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..

10
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.

11
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.

12
Natural selection
  • How natural selection works
  • Individuals in a species vary.

13
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.

14
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.

15
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.

16
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.

17
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.
18
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.

19
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

20
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
21
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.

22
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.
23
Evolution
Satirical cartoon by Thomas Nast, from
Harpers Weekly, August 19, 1871.
24
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

25
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.

26
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.

27
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.
28
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.

29
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.
30
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.

31
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

32
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.
33
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.

34
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.

35
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.

36
Evidence for evolution
  • Biogeography
  • Plate tectonics the continents are on plates
    that glide over the surface of the earth
    carrying life with them.

37
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
38
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.

39
Evidence for evolution
  • Biogeography
  • Marsupials ori- ginated in SA 70
    million yrs ago then spread to
    Australia.

40
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.
41
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.

42
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.

43
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

44
Evidence for evolution
  • Selective breeding
  • Plants corn, wheat, potato, bean, cabbage, etc.
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