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Natural Selection
Meiosis (sexual reproduction) and mutations
gives us genetic variation.
MUTATION: a change in a gene
If the population’s gene pool changes, then
the species evolves.
Two Types of Mutations:
(1) Changes in genes in somatic (body) cells:
can affect the organism itself, but cannot be
passed to sexually produced offspring.
(2) Changes in genes in gametes (egg and
sperm): do not affect the parent organism
itself, can be passed to sexually produced
offspring
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Mutations may result in a trait that improves
an organism’s chances for survival, so that
organism would be more likely to reproduce.
Then the favorable mutation would be passed
on to offspring.
This results in evolution.
Mutations occur very rarely (and are almost
always bad), so populations evolve slowly.
NOTE: EVEN THOUGH EVOLUTION GENERALLY
OCCURS SLOWLY…IT SOMETIMES CAN OCCUR
RAPIDLY
1) Directional Selection
 Selection AGAINST one extreme phenotype
(physical type)
 This causes one extreme phenotype to
become much more common (the population
is pushed in 1 DIRECTION)
 Ex. Peppered Moths
2) Disruptive Selection
 Selection AGAINST the normal phenotype
(physical type)
 This causes BOTH extreme phenotypes to
become much more common (the look of the
population is DISRUPTED)
 Ex. Dog breeding
3) Stabilizing Selection
 Selection AGAINST the extreme phenotypes
(physical type)
 This causes the NORMAL phenotype to
become much more common (the look of the
population is STABILIZED)
 Ex. Human weight at birth
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2) Genetic Drift – changes in gene pool
due to chance (has greater effect on smaller
populations)
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Examples
◦ 1) Bottleneck effect – drastic decrease in population
size may leave only certain alleles available. These
alleles will quickly be perpetuated.
◦ 2) Founder effect –When a few individuals colonize
a new area (same reasoning as bottleneck)
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Gene flow
Introduction or exit of alleles in a population
whenever individuals enter or leave.
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1) Molecular Biology
All living things use DNA and RNA.
The genetic code amongst all living things is
universally the same. (In other words, the fact
that the RNA codon AUG gives the amino acid
MET in humans is seen in every other living
thing as well)
This fact more than any other (at least to me)
indicates that all living things originated from
a common ancestor.
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Molecular Biology – Continued
The closer 2 different species DNA is to one
another, then the closer those 2 are related.
Which 2 species are closer related?
Species A: AACTGGCTTA
Species B: AACTAACCCG
Species C: TACTGGCTTA
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The remains or traces of organisms that have
once lived on Earth.
Fossil Record: the history of life on Earth,
based on fossils that have been discovered
The fossil record shows how organisms have
changed over time and shows that the Earth
is about 3.5 billion years old.
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Extinctions occur when there are major
changes in the environment and species do
not have the adaptations to survive.
The fossil record helps scientists to discover
relationships between different groups of
organisms and determine common ancestors.
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The Earth is divided into
layers called strata.
Generally speaking, the
lower the strata = the
older the layer of Earth
(each new layer
becomes stacked on the
older layer below it)
This means if two
fossils are found, the
fossil found in the
lowest layer would be
the oldest
There are also
techniques for
determining the
absolute (or relatively
exact date) of a fossil’s
age
Convergent evolution –
process by which different
species evolve similar traits
(SAME FUNCTION, DIFFERENT
ANCESTOR)
Example: Birds, bats, and
moths have wings, but they
did not evolve from a recent
common ancestor.
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Caused by:
◦ living in similar habits /
performing similar
functions
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Structures with closely related function but
do not come from the same ancestral
structure
◦ Same function, different structure
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Example: Birds, bats, and moths have wings,
but they did not evolve from each other.
Divergent evolution- build up of differences
between groups which can lead to the
development of a new species
 In other words, two different species that
evolved from the same ancestor.
 Caused by populations of the same species:
1. moving to two different environments or..
2. specializing in different areas of the same
environment
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Structures in different species that originated
from common ancestor
May have different function but similar
structure
Evidence for Evolution –
Comparative Anatomy
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Vestigial structures – no longer have a use,
but may have had a use in evolutionary
history.
Ex: human tail bone (coccyx) made of 4 fused
vertebrae that resemble the bones in an
animal’s tail.
Other examples: the appendix, and ear
muscles!
When two or more species have evolved
together, the situation is called coevolution.
Example:
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◦ Insects and Flowers
 Flowers provide food for insects. Insects take pollen
from one flower to the next so they can reproduce.