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Variations in populations
Variations within a
population.
Where do the differences among
individuals come from?
What are the forces that affect variation?
What would the world be like without
variation?
Field of biology that studies the variations
in populations is called microevolution.
Microevolution is the change within a
population over time.
Key Ideas
Population - a group of interbreeding
organisms
Gene pool - consists of all the genes of a
local population.
Adaptation – a trait that is beneficial for
the survival of an organism
Sources of variation on a
genetic level
#1 Mutations (the ultimate source of
variation) any change in DNA
#2 Crossing-over during prophase of
meiosis – random assortment of alleles.
#3 Independent assortment during
meiosis – random assortment of
chromosomes.
Changes in Gene Pools
In a large population
1.Natural Selection - survival of the fittest
a) basic principles
i. Offspring have genetic variation
ii. Some of these variations provide an advantage
b) Ex. English peppered moth
Changes in Gene Pools
2. Gene flow - when individuals migrate
from one gene pool to another
emigration – (to migrate, to wander)
immigration (to take up residency)
• kudzu
Changes in Gene Pools
In a large population
3. Mutations - only if they are beneficial to
survival
Example of beneficial mutation – sickle cell
anemia
Changes in Gene Pool
In a small population all of the above
can have an effect plus:
4. Genetic drift – random change in allele
frequency
This has a big effect on small populations b/c
the sampling size of the gene pool is small
Example analogy: Coin toss
What is the main source of
variation?
A. Gene flow
B. Genetic drift
C. mutations
D. Independent assortment
The field of Boilogy that studies
the variations in populations is
called __.
A. macroevolution
B. microevolution
C. Genetic drift
D. populations
Crossing-over takes place
during the ____ of meiosis.
A. Prophase
B. Metaphase
C. Anaphase
D. Telophase
A group of interbreeding
organisms is called a ____.
A. gene pool
B. biome
C. community
D. population
Sickle cell anemia is an
example of ____.
A. Natural selection
B. Artificial selection
C. Gene flow
D. Genetic drift
What evidence do we have
that these changes have
occurred?
1. Fossil record




Physical remains of ancient organisms
Paleontology – the study of fossils
Often found in layers of sedimentary rock
Usually develop from the hard body parts of
an organism
 Teeth, shell, bones, woody stems
Fossil Record
Evidence that changes have occurred over
time
physical records of organisms not found on
Earth today.
Comparison of younger fossils w/ older fossils
help determine ancestral relationships of
extinct and living species.
What evidence do we have
that these changes have
occurred?
DNA analysis- The genetic code is nearly universal
 Scientists compare
amino acid sequences of the same proteins in
different species.
nucleotide sequences of the same genes in
different species
 Close similarities support the hypothesis that
vertebrates share a common ancestor.
What evidence do we have
that these changes have
occurred?
1. Homologous structures
 Similar features that originated in a shared
ancestor.
 Examples:


Galapagos Finches
Forelimbs of various organisms
Homologous Structures
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19
Analogous structures
* Features with identical functions but different
embryological development.
Ex. Wings of birds and the wings of insects
These two organisms evolved independantly of one another.
Embryonic development shows evidence of
a common ancestor.
Patterns of evolotion
* Coevolution – the change in two or more species in close
association with one another.
Ex. Plants and the insects that pollinate them
* Convergent evolution – the change in two species based
on their environment. Usually responsible for analogous
structures.
Ex. Whale and porpoise
* Divergent evolution – when two or more related
populations become more and more dissimilar.
Can
result in a new species.
Ex. Darwin's finches, domestic dogs
CLADOGRAM
a diagram that shows evolutionary relationships
among similar organisms.
 Assumptions in cladistics

Group of organisms studied is related by descent
from a common ancestor.
 Change in characteristics occurs in lineages over
time


Diagram shows
Primitive characteristics – shared by all or most of
the members of the group
 Derived characteristics – characteristics that differ
from those of the ancestor, unique modifications of
primitive features

Explanations of how
evolution works.
Are species still changing
today?
Antibiotic resistant bacteria
Pesticide resistant insects
Jean Baptiste de Lamarck
1744 – 1829
Proposed that:
 similar species descended from a common
ancestor.
acquired traits were passed on to offspring.
A trait could arises or dissappear as a
result of the organisms experience or
behavior.
Jean Baptiste de Lamarck
Example of acquired traits
Disappearance of tails
Lamarck’s hypothesis was disproved
by Weismann and his tailless mice
 This was an important forerunner of
modern evolutionary theory.
Charles Darwin
Charles Darwin
1809 – 1882
1831 sailed on the ship H.M.S. Beagle
was the ships naturalist
collected specimens and keep careful records
of his observations
Based on observations he hypothesized
that species changed over time as a result
of natural selection
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Charles Darwin
Natural selection
 organisms best suited to their environment
reproduce more successfully than other
organisms.
Published book called “On the origin of
species” in 1859 that discussed this
hypothesis
Darwin’s 5 points
Populations have variation
Some variations are favorable
More offspring are produced than survive
Those that survive have favorable traits
A population will change over time