1. I: Evolution
If I have seen further it is by standing on the shoulders of
Giants.
-- Sir Isaac Newton
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Theories in Science
In the context of scientific inquiry, a theory is:
A conceptual framework supported by a large body of evidence
Broader in scope than a hypothesis. A theory ties information
together and leads to specific testable hypotheses
In other words, a theory is a big deal in science, NOT a
synonym for guessing
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(This used to be a joke, but I’m not laughing anymore.)

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Historical Overview
What can explain both the unity and diversity of life on Earth?
Organic evolution: genetically based change over time. It acts
on individuals in the present, but only manifests in the
population over generations.
Natural Selection: mechanism causing the match between
organisms and their environment (adaptive evolution =
adaptation)
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Traditional views involved unchanging and perfect species
inhabiting a young Earth (Old Testament, Linnaeus, etc.)
The emergence of paleontology and geology helped lay the
groundwork for Darwin’s contributions
Other areas of research also influenced his thinking, including
studies on human population growth
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Fig. 22-2
American Revolution
French Revolution
U.S. Civil War
1900
1850
1800
1750
1795
1809
1798
1830
1831–1836
1837
1859
1837
1844
1858
The Origin of Species is published.
Wallace sends his hypothesis to Darwin.
Darwin begins his notebooks.
Darwin writes essay on descent with modification.
Darwin travels around the world on HMS Beagle.
Malthus publishes “Essay on the Principle of Population.”
Lyell publishes Principles of Geology.
Lamarck publishes his hypothesis of evolution.
Hutton proposes his theory of gradualism.
Linnaeus (classification)
Cuvier (fossils, extinction)
Malthus (population limits)
Lamarck (species can change)
Hutton (gradual geologic change)

4. Lyell (modern geology)
Darwin (evolution, natural selection)
Wallace (evolution, natural selection)
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Younger stratum
with more recent
fossils
Layers of deposited
sediment
Older stratum
with older fossils
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Several 18th century naturalists (including Erasmus Darwin)
suggested life evolves as environments change
Jean-Baptiste Lamarck hypothesized that species evolve through
use and disuse of body parts and subsequent inheritance of
acquired characteristics
This mechanism is unsupported by evidence (e.g., even if you
and your mate lost the same finger, your children would still be
born with all ten), but it did refocus subsequent research

5. Lamarck’s Hypothesis
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The miniature phenotype of Bonsai trees is caused by
manipulations of a bonsai master, not genetics. Would the next
generation still be stunted if we planted their seeds and allowed
them to grow naturally?
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After first studying medicine, then theology at Cambridge,
Darwin took an unpaid position as naturalist for a 5-year voyage
around the world
During his travels on HMS Beagle, he collected thousands of
specimens of plants and animals
He also observed and noted adaptations of plants and animals
that inhabited diverse environments, including the renowned

6. Galápagos finches.
Darwin’s Research
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In 1844, Darwin wrote an essay on the origin of species and
natural selection but did not introduce his theory publicly,
anticipating an uproar
In 1858, Darwin received a manuscript from Alfred Russell
Wallace, who had developed a theory of natural selection
similar to Darwin’s
Darwin quickly finished The Origin of Species and published it
the next year
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NORTH
AMERICA
EUROPE
AFRICA
AUSTRALIA
GREAT
BRITAIN
SOUTH

7. AMERICA
ATLANTIC
OCEAN
PACIFIC
OCEAN
Cape of
Good Hope
Tierra del Fuego
Cape Horn
Tasmania
New
Zealand
Andes
Equator
The
Galápagos
Islands
Pinta
Marchena
Genovesa
Santiago
Daphne
Islands
Pinzón
Fernandina
Isabela
San
Cristobal
Santa
Fe
Santa

8. Cruz
Florenza
Española
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Darwin never used the word evolution in the first edition of The
Origin of Species
The phrase descent with modification summarized Darwin’s
perception of the unity and diversity of life: unity comes from
common descent and diversity comes from subsequent
modification
The history of life is like a tree with branches, where nodes
represent common ancestors
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The Origin of Species (1859)
Darwin developed two main ideas in his book:
Descent with modification (i.e., evolution) explains the unity

9. and diversity of life
Natural selection causes the match between organisms and their
environment (adaptation)
He reviewed the evidence for the first, and started the second
part by making some key observations, then drawing inferences:
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Observation 1: Members of a population often vary greatly in
their traits
Observation 2: Traits can be inherited from parents to offspring
Observation 3: All species are capable of producing more
offspring than the environment can support
Observation 4: Owing to lack of food or other resources, many
of these offspring do not survive
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Phenotypic variation in a single snail species
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10. This is a photo of a puffball fungus expelling millions of spores
as an example of great reproductive capacity.
Just one of these fungi could blanket the entire terrestrial
surface of Earth with its offspring if environmental conditions
did not prevent it.
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Inference #1: Individuals whose inherited traits give them a
higher probability of surviving and reproducing in a given
environment tend to leave more offspring than other individuals
Inference #2: This unequal ability of individuals to survive and
reproduce will lead to the accumulation of favorable traits in
the population over generations
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(b) A stick mantid
in Africa
(a) A flower mantid
in Malaysia
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If three conditions are met, natural selection WILL operate in
a population:
Phenotypic variation must:
1. exist among individuals
2. be genetically heritable
3. affect reproductive success (fitness)
(variability, heritability, and fitness effects)
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Pocket mice from the Tularosa Basin, NM
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From Natural Selection to Speciation
Individuals with certain heritable characteristics survive and
reproduce at a higher rate than other individuals
Natural selection increases the adaptation of organisms to their
environment over time at the population level
If an environment changes over time, organisms move, or a
barrier splits a population, then evolution by natural selection
can generate new species (next lecture)
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Evolution by natural selection is supported by an overwhelming
amount of evidence. Note that each of these is an entire
category that includes thousands of separate, peer reviewed
studies:
Artificial Selection
The Fossil Record
Comparative Anatomy
Convergent Evolution
Molecular Data
Natural Populations
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Artificial Selection
Darwin noted that humans have modified other species by
selecting and breeding individuals with desired traits, a practice
he dubbed artificial selection
Familiar examples include crop plants, livestock, and pets
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For millennia, humans have selected certain ancestors to modify
the traits of their descendants

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The Fossil Record
The fossil record provides evidence of the extinction of species,
the origin of new groups, and changes within groups over time
A Darwinian view of life predicts that evolutionary transitions
should leave signs in the fossil record
Paleontologists have discovered fossils of many such
transitional forms
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Trilobites evolving over millions of years at a single dig site

14. Trilobites and other fossils can be easily found in AZ,
especially the Grand Canyon and Mogollon Rim areas.
https://www.npr.org/2020/08/21/904943006/fallen-boulder-
reveals-313-million-year-old-fossil-footprints-at-grand-canyon
Bristolia insolens
Bristolia bristolensis
Bristolia harringtoni
Bristolia mohavensis
Latham Shale dig site, San
Bernardino County, California
Depth (meters)
0
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4
6
8
10
12
14
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1
2

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2
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These and many other “missing links” have been found in the
fossil record, just as evolutionary theory predicts.
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The first fossil of Archaeopteryx was coincidentally
discovered just as Darwin published his magnum opus, leading
to accusations that his publishing co. fabricated it as a publicity
stunt. Many more have been found around the world since then.
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Comparative Anatomy
Homology is similarity resulting from common ancestry
Homologous structures are anatomical similarities that represent
variations on a structural theme present in a common ancestor
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Fig. 22-17
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Whale
Cat
Bat

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Human embryo
Chick embryo
Pharyngeal
pouches
Post-anal
tail
Comparative embryology reveals anatomical homologies not
visible in adult organisms (yep, we all had tails and gills before
birth)
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Vestigial structures, such as the pelvis “leftovers” in marine
mammals, still resemble ancestral structures but no longer have
an apparent function.
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Convergent Evolution (Analogy)
Convergent evolution is the development of similar, or
analogous, features in distantly related groups
Analogous traits arise when groups independently adapt to
similar environments in similar ways
Does not provide insight into ancestry (in fact, often obscures
it), but does provide evidence of natural selection
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Many placental mammals have a marsupial counterpart that
evolved to fill the same niche “down under.”
Because they did not come from a common ancestor, such
convergences are analogous (seem the same), homologous not
(are the same).
Sugar
glider
Flying
squirrel
AUSTRALIA
NORTH
AMERICA
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Convergent body form among fast-swimming predators
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An exploding body of molecular data supports the conclusion
that evolution by natural selection has occurred and continues to
do so.
More similar DNA sequences in closely related organisms,
long-term genetic monitoring, selection on functional proteins,
endosymbioses, etc.
Molecular Data
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Thousands of studies have documented natural selection
occurring in nature, including DDT resistance in mosquitos,
antibiotic resistance in bacteria, and sexual selection in guppies
In addition to observations, countless manipulative experiments
have shown natural selection occurring
Kettlewell’s studies of peppered moths provide a classic
example of combining both approaches
Go to the following site, hit play, and become a virtual agent of
natural selection! https://askabiologist.asu.edu/peppered-moths-
game/
Natural Populations

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