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Modern Evolution
The modern theory of evolution
includes both Darwin’s ideas of
variation and natural selection and the
current knowledge of the sources of
variations.
1. Segregation and the recombination of
alleles during sexual reproduction.
2. Gene mutation occurs spontaneously or
may be caused by mutagenic (mutation
causing) chemicals.
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Natural Selection
• Natural selection involves the struggle of organisms
to survive and reproduce in a given environment
• Traits which are beneficial to the survival of an
organism tend to be retained and passed on, and
therefore, increase in frequency within a population
• Traits which have low survival value to an organism
tend to diminish in frequency from generation to
generation. These are selected against.
• If environmental conditions change, traits that have
low survival value may now have a greater survival
value. Therefore, traits that prove to be favorable
under new environmental conditions will increase in
frequency
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Geographic Isolation
1. Isolation of a population increases the
chances for speciation (the development of
a new species) by separating a small group
of organisms from the main population
with its large gene pool (inheritable traits)
2. Changes in gene frequency are more likely
to occur in small populations than in large
ones
3. Geographic isolation of a population is
caused by natural barriers like mountains,
large bodies of water, and deserts
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4. The evolution of an isolated population into a
new species may involve the following factors:
a) the gene frequency in the isolated
population may have been different than the
gene frequency in the main population to begin
with (founder effect)
b) different mutations occur in the isolated
population and in the main population
c) different environmental factors may also
have exerted different selection pressures on
each population (natural selection was
different)
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Reproductive Isolation
•If the isolated population becomes so
different from the main population that
members of the two cannot interbreed,
even if the geographic barriers are
removed.
•When two populations can no longer
interbreed and produce fertile offspring,
they have become two distinct species.
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Time Frame for Evolution
Gradualism- proposes that evolutionary change is
slow, gradual, and continuous
Punctuated Equilibrium- proposes that species
have long periods of stability (several million years)
interrupted by geologically brief periods of
significant change during which a new species may
evolve. This could be caused by drastic
environmental changes, such as global cooling or
warming. Environmental changes such as these
could also cause an extinction.
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Heterotroph Hypothesis
•This is one proposed
explanation for how life
arose and evolved on
primitive earth.
•According to the
Heterotroph Hypothesis,
the first life forms were
heterotrophic and had to
obtain their nutrients from
the environment.
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Primitive Earth
•Earth was very hot consisting of inorganic substances in
all states: solid, liquid, and gas. And the land was still
forming from cooling lava.
•The atmosphere consisted of water vapor, hydrogen,
methane gas, and ammonia (No Oxygen)
•As the earth cooled, water vapor condensed in the
atmosphere and rain fell forming seas described as “hot,
thin soup”, full of dissolved minerals from the land.
•In addition to the extreme heat, there was electrical energy
in the atmosphere causing lightning, radiation from the Sun
(because of the lack of an established atmosphere), and
radioactive gasses being given off from radioactive rocks
cooling.
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Synthesis of Organic Compounds
•The inorganic molecules in the seas chemically
bonded to form organic molecules, including
simple sugars and amino acids.
•These organic molecules became the building
blocks for the first life forms.
•Scientist Stanley Miller created an apparatus
that simulated the conditions on the early Earth.
His experiments showed that dissolved gasses
could combine to form simple organic
compounds, in the presence of heat and
electrical energy.
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• Some of the large complex molecules formed
groupings or clusters called aggregates
• These aggregates developed a surrounding
“membrane”
• It is believed that aggregates absorbed simple
organic molecules from the environment for food
• Therefore, they carried on a form of
heterotrophic nutrition
• Over time, the aggregates became more
complex and highly organized.
• Eventually they developed the ability to
reproduce and were therefore considered “living
cells”
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• It is thought that these early heterotrophic life
forms carried on a form of anaerobic respiration
known as fermentation
• As a result of the extended periods of
fermentation, carbon dioxide was added to the
atmosphere
• Eventually, as a result of evolution, some
heterotrophic forms developed the capacity to
use carbon dioxide from the atmosphere to form
organic compounds (food)
• These organisms were the first autotrophs
• Some bacteria are autotrophs, but most of the
autotrophs today are green plants and algae.
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Anaerobes to Aerobes
•Photosynthesis from the autotrophs added
oxygen molecules to the atmosphere. Eventually
the atmosphere was filled with O2.
•Over time, both autotrophs and heterotrophs
evolved the ability to use oxygen for respiration
(became aerobic, instead of anaerobic), because
it was more efficient than what they were doing.
•There are both heterotrophic and autotrophic
(plants) organisms on the Earth today.
•Presently, most life forms use aerobic respiration
today, although there are still some anaerobic
organisms around.
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3 Modern theories of continuous evolution
Divergent Evolution, Convergent Evolution, and Parallel
Evolution. All of them seem to occur at different times.
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Divergent evolution - Pattern of evolution in which two
closely related species gradually become more and more
dissimilar.
When people hear the word "evolution," they most
commonly think of divergent evolution, the evolutionary
pattern in which two species gradually become
increasingly different. This type of evolution often occurs
when closely related species diversify to new habitats. On
a large scale, divergent evolution is responsible for the
creation of the current diversity of life on earth from the
first living cells. On a smaller scale, it is responsible for the
evolution of humans and apes from a common primate
ancestor.
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Convergent Evolution
Convergent evolution causes difficulties in fields of
study such as comparative anatomy. Convergent
evolution takes place when species of different
ancestry begin to share analogous traits because of
a shared environment or other selection pressure.
For example, whales and fish have some similar
characteristics since both had to evolve methods of
moving through the same medium: water.
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Parallel Evolution
Parallel evolution occurs when two
species evolve independently of each
other, maintaining the same level of
similarity. Parallel evolution usually
occurs between unrelated species that
do not occupy the same or similar
niches in a given habitat.
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