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Chapter 15
Darwin
became a naturalist on the HMS
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Beagle in 1831 and
took aand
5 ayear journey
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around the world.
Darwin made many observations and collected
evidence that led him to his theory of
evolution.
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Darwin collected many plant and animal specimens.
He found many fossils of organisms once alive
that resembled organisms presently living.
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Fossils from the San
Francisco Bay region.
Mesozoic fossils are all
marine, (J) Jurassic
"clams" , (K) a
Cretaceous ammonite,
and (L) a Cretaceous
ichthyosaur. Land-animal
fossils are all Cenozoic
in age, such as (M) part
of the Miocene beaver
skull, (N) a Pleistocene
saber-toothed cat, and
(O) a Pleistocene
mammoth.
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Darwin observed that the characteristics of many
animals and plants varied noticeably among the
different Galapagos Islands.
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Saddle back shell Hood Island
Dome shaped shell Isabela Island
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Darwin observed the similarities and differences
amongst these finches and noticed how their beaks
varied and were suited for the type of food they ate.
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(1785) James Hutton - Earth is shaped by
geological forces over long periods of time.
(1798) Thomas Malthus - more offspring are
born than survive, limited by food and space.
(1809) Jean-Baptiste Lamarck - organism’s
acquire or lose traits by selective use or disuse
passing on the trait to the next generation.
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Lamarck’s reasoning
for his law of use
and disuse
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(1831) Charles Darwin - evidence from voyage
around the world laid the groundwork for his
theory of evolution.
(1833) Charles Lyell - processes occurring now
shaped Earth’s geological features over long
periods of time.
(1858) Alfred Wallace - he also speculated that
evolution by natural selection occurs. This
spurred Darwin on to publish his theory.
Origin of Species
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Darwin proposed that new species originate from
ancestral forms through the gradual accumulation of
adaptations over very long periods of time.
In his book, Darwin describes descent with
modification.
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Some heritable variations
in populations
are better suited
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offspring than the environment can support, competition for
resources favors those better suited. These individuals
reproduce and pass on their traits.
Darwin observed artificial selection in which nature
provided variation and humans selected variations they
found useful.
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Variation leading to
Natural Selection
Insect Resistance
Artificial Selection
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Darwin
proposed this
idea.
Descent with modification
implies a common ancestor
with a single tree of life linking
all living things.
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In humans: use of drugs selects for pathogens
that through chance mutations are resistant
to the drugs’ effects.
Bacteria and viruses evolve rapidly due to
rapid rates of mutation and reproductive,
posing a challenge to our society.
HIV Resistance to 3TC
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Fossil Record
Geographical distribution of living
species
Homologous structures
Similarities in embryological
development
Fossils
provide the
strongest
evidence of
evolution.
Sedimentary
strata reveal
the relative
ages of fossils.
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Absolute ages are determined by radiometric dating.
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During his voyage, Darwin observed
animals in similar environments on
different continents that had similar
anatomy and behavior.
All species of finch on
the Galapagos Islands
evolved from a
common ancestor
from Ecuador.
Differing selective
pressures on the each
Island was the driving
force for the
evolution of beaks.
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During the past few
decades scientists have
demonstrated that what is
now called South America
was part of a large land
mass called Gondwana,
which included Australia
and Antarctica.
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Horses have a complex evolutionary history. The earliest
horses evolved in North America; many lineages arose and
died out, and ancestors of several of these lineages
crossed into Asia over the Bering land bridge and into
South America over the Central America land bridge.
Organisms living in
different
geographical
locations under
similar ecological
conditions are
exposed to similar
pressures of
natural selection.
Evolution of
shared traits in
unrelated species
is convergent
evolution.
 Regardless
of the appearance of
organisms, two species are
closely related when they share
common ancestry.
Homologous structures have different functions but
evolved from common ancestries.
 Forelimbs of all mammals show the same
arrangement of bones with different functions.
Analogous structures have a similar function but are
anatomically different.
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Humans also have vestigial
features, evidence of our own
evolutionary history. The
appendix is believed to be a
remnant of a larger, plantdigesting structure found in our
ancestors
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Cave-dwelling tetra fish are blind; they have small vestigial
eyes that do not work. Why have them? Biologists have found
a possible answer: genetic mutations that hamper eye
development also may increase the number of taste buds.
Thus, mutations that happened to give the fish an advantage in
tasting and smelling—a huge benefit in a dark environment—
might also have inadvertently, and harmlessly, caused the
degeneration of their eyes.
.
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Pythons and boa constrictors have tiny hind leg bones
buried in muscles toward their tail ends. These are
vestigial. Vestigial legs are a clue that snakes descended
from lizards. Over 100 million years ago, some lizards
happened to be born with smaller legs, which, in certain
environments, helped them move about unencumbered. As
generation after generation survived and reproduced,
this new form flourished.
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All vertebrate embryos have a tail and pharyngeal
pouches.
This
indicates
a common
ancestor.
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Genes are not active at the same time.
Those that are active during early development are
less subject to change than genes that are active
later.
This is because mutations occurring early in
development have a far greater chance of being
lethal and would not be passed on to future
generations.
Mutations occurring later in development have more
limited effects, are less likely to be lethal and are
more likely to be passed on.
That’s why humans and chickens are so different at
later stages despite similarities as embryos.
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Comparing DNA sequences is the most direct
way to determine evolutionary relationships.
The sequence of nitrogenous bases is more
similar in closely related species than in
species that are not as closely related.
Scientists can use this information, with
knowledge of mutation rates, to estimate how
long it’s been since 2 species shared a common
ancestor.
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Organisms differ and these differences may be
inheritable.
Organisms produce more offspring than survive.
Organisms compete for limited resources.
Organisms best suited to their environment
survive and reproduce, passing on these traits.
This process of natural selection causes species to
change over time.
Species alive today descended with modification
from a common ancestor uniting all organisms in a
tree of life.