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Descent with Modification: A
Darwinian View of Life
Chapter 22
History of Evolution
• Prior to November 24, 1859 and Charles Darwin:
• The view of nature was determined by deep-seated
beliefs held to be intractable truths rather than
experimentation and observation
• Biologists had slowly begun to accept various ideas of
evolution (species change through time)
• Evolution is the unifying principle of biology
• Explains the unity and diversity of life
History of Evolution
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Aristole- scale of nature
Linneaus- taxonomy
Cuvier- catastrophism
Hutton- gradualism
Lamarck- inheritance of acquired characteristics
Lyell- uniformitarianism
Darwin- natural selection
Lamarck’s Explanation of
Evolution
Jean Baptiste Lamarck (1744-1829)
• Proposed that similar species descended from a common
ancestor
• Hypothesized that acquired traits were passed on to offspring
• you would gain or lose features if you overused or didn't use
them, and you could pass these new traits onto your offspring.
• Not determined by your genes! (we know this is wrong)
• Became known as the Inheritance of Acquired Characteristics
Charles Darwin
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1809-1882 (British)
Attended medical school and studied to be a clergy.
Became interested in natural history
In 1831, set sail on the Beagle
• A 5 year mapping and collection expedition to South America and
the South Pacific.
• Posed as the ships naturalist where he collected and recorded
observations of specimens
Voyage of the HMS Beagle
• Invited to travel around the world
• 1831-1836 (22 years old!)
• makes many observations of
nature
• main mission of the
Beagle was to chart
South American coastline
• Collected and observed
many specimens of South
America
Findings
• Observed massive geological changes
• Concluded Earth must be VERY old
• Went along with Hutton and Lyell’s findings
Why should extinct
armadillo-like species
& living armadillos be
found on the same
continent?
“This wonderful relationship
in the same continent between
the dead and the living will…throw more light
on the appearance of organic beings on our earth,
and their disappearance from it,
than any other class of facts.”
Endemic organisms
• Organisms that only live in one location
Galapagos Islands
• Darwin noted that there existed many finches on the islands,
but while they had similarities, each was adapted to eating a
particular type of island food
Darwin’s Finches
• Found 14 different birds, thought they were all different birds
• Amazed to find out they were all finches But there is only
one species of finch on the mainland!
Descendant
species
Ancestral
species
Seed
eaters
Flower
eaters
Insect
eaters
Darwin’s Finches
• Darwin’s conclusions
• small populations of original South American finches landed on
islands
• variation in beaks enabled individuals to gather food successfully in
the different environments
• over many generations, the populations of finches changed
anatomically & behaviorally
• accumulation of advantageous traits in population
• emergence of different species
A Reluctant Revolutionary
• Returned to England in 1836
• wrote papers describing his collections & observations
• draft of his theory of
species formation in 1844
• instructed his wife to
publish this essay upon
his death
• reluctant to publish but
didn’t want ideas to die
with him
• Then, in 1858, Darwin received a letter that changed
everything…
Alfred Russel Wallace
a young naturalist working in the East
Indies, had written a short paper with a
new idea. He asked Darwin to evaluate his
ideas and pass it along for publication.
To Lyell—
Your words
have come true
with a vengeance…
I never saw a more striking
coincidence…so all my originality,
whatever it may amount to,
will be smashed.
The Origin of Species
Darwin published this work to explain
the variety of species that exist on the
planet
• On November 24, 1859, published after 21 years of refining
• On the Origin of Species by Means of Natural Selection
• Two main ideas:
• Evolution explains life’s unity and diversity
• Natural selection is a cause of adaptive evolution
Descent with Modifications
• Populations of organisms exhibit changes in characteristics
that are passes on through inheritable means.
• Summarized Darwin’s perceptions of the unity of life
• States that all organisms are related through descent from an
ancestor that lived in the remote past
Darwin’s View of History
• Darwin viewed the history of life like a tree,
• With multiple branches from a common trunk to the tips of the
youngest twigs that represent the diversity of living organisms
(1) Variation exists in natural populations
(2) Many more offspring are born each season than can possibly
survive to maturity
(3) As a result, there is a struggle for existence
- competition
(4) Characteristics beneficial in the struggle
for existence will tend to become more common in the
population, changing the average characteristics of the
population
- adaptations
5) Over long periods of time, and given a steady input of new
variation into a population, these processes lead to the
emergence of new species
Natural Selection
• Darwin concluded that evolution occurs by Natural selection.
• Organisms best suited to their environment reproduce more
successfully than other organisms.
• This occurs over time and the environment “selects” the traits
that will increase a population
• Can not occur without genetic variations among species.
4 Main Principles
1. Variation exists among individuals in a species.
2. Individuals of species will compete for resources (food and
space)
3. Some competition would lead to the death of some
individuals while others would survive
4. Individuals that had advantageous variations are more likely
to survive and reproduce.
Natural Selection in Action
• In humans, the use of drugs selects for pathogens that
through chance mutations are resistant to the drugs’ effects
• Natural selection is a cause of adaptive evolution
• Researchers have developed numerous drugs to combat HIV
but using these medications selects of viruses resistant to the
drugs
• The ability of bacteria and viruses to evolve rapidly poses a
challenge to our society
Evidence: Homology, Biogeography, and
the Fossil Record
Homology
• Used to explain why certain characteristics in related species have
an underlying similarity even though they may have VERY
different functions.
• Anatomical Homologies
• Homologous structures
• Vestigial organs
• seemingly functionless parts, snakes have tiny pelvic and limb
bones, humans have a tail bone
• Molecular Homologies
• When comparing the DNA of one species to another, more
similarities are found in species that are more closely
• Amino acid sequence in hemoglobin is one different between
gorillas and humans
• Humans and frogs differ by 67 A.A
Evidence: Homology, Biogeography, and
the Fossil Record
• Biogeography
• Biogeography is the study of the range and distribution
of plants and animals throughout the world
• distributions are consistent with the hypothesis that
related forms of life evolved in one locale and then
spread to accessible regions
• A different mix of plants and animals would be
expected whenever geography separates continents,
islands, seas, etc.
Sugar glider, Petaurus breviceps,
is a tree-dweller and resembles
the placental flying squirrel.
The Australian wombat, Vombatus, Kangaroo, Macropus, is an herbivore
is nocturnal and lives in burrows. It that inhabits plains and forests. It
resembles the placental woodchuck. resembles the placental Patagonian
cavy of South America.
Evidence: Homology, Biogeography, and
the Fossil Record
• Fossil Record
• Fossils are the remains and traces of past life or any
other direct evidence of past life such as trails,
footprints, or preserved droppings
• Fossils record the history of life from the past
• Document a succession of life forms from the simple
to the more complex
• Sometimes the fossil record is complete enough to
show descent from an ancestor