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T he Change of Populations over Time
Part 1:
Charles Darwin
Charles Darwin
Darwin (1809-1882) was
the father of evolution,
contributing more to our
understanding of evolution
than any other individual. He
wrote On the Origin of
Species, among other works.
HMS Beagle
In 1831, Darwin set sail on the HMS Beagle,
from England, to tour the world and collect
plant and animal samples. During his voyage,
he stopped at the Galapagos islands, where he
gathered some of the most important evidence
to support his new theory, evolution. Darwin
noticed how creatures in the isles were adapted
to the specific environment on their island.
Other Scientists
Jean-Baptiste Lamarck
Darwin’s Inspirations
Jean-Baptiste Lamarck was the first
scientist to propose a theory of how
organisms change over long periods of
time. He hypothesized that all organisms
have a tendency to complexity and
perfection, that body parts could be
changed through use or disuse, and that
these changes could be passed on to
offspring. Lamarck’s theory, while flawed,
paved the way for later biologists.
Hutton: Earth was shaped by
geological forces over time. Earth
was millions of years old.
Malthus: He wrote about the
growth of the human population.
Darwin’s Peers
Lyell: He argued that Earth’s surface
was shaped gradually by processes over
many years.
Darwin thought that the same gradual
change must apply to organisms as well.
Wallace: He proposed a theory of
evolution similar to Darwin’s.
Evidence for Evolution
The Fossil Record
Scientists could document the way
organisms have changed over time by
comparing living creatures to extinct ones
preserved in fossils. Fossils in older (and
lower) rock layers predated fossils closer to
the surface, the principle of Stratigraphy.
Geographic Distribution
Darwin observed that many organisms with physical similarities were distributed
closely geographically, as well. In areas spread apart (such as South America and
Australia) with similar environmental conditions, organisms were wildly distinct.
Evidence for Evolution
Homologous Body Structures
The skeletons of many living vertebrates, while used for varying body parts
and purposes, are all constructed out of similar bones (observe the forelimbs
of reptiles, birds, and mammals). Compare to Analogous body structures,
where body parts with different origins share the same purpose and so
resemble each other.
Similarities in Embryology
The embryonic developmental
stages of many vertebrates closely
resemble each other, and develop
similarly to make similar cells,
tissues, and body parts.
Part 2:
Natural Selection
Natural Selection
Natural selection is a process that affects the distribution
of phenotypes in a given population. Organisms that
carry certain phenotypes are more or less likely to survive
than others, and these organisms pass on genes to their
offspring. Natural selection acts only on the phenotype of
the organism, but not the genotype.
Natural Selection
Directional Natural Selection
Directional natural selection consists of selection against one extreme in the spectrum of
variance. For examples, giraffes with shorter necks must have been selected against in times
of drought, because their shorter necks would have prevented them from effectively
seeking out food.
Natural Selection
Disruptive Natural Selection
Disruptive natural selection is the selection against the mean, where the ‘middle’ segment
in the spectrum is culled, possibly leading to eventual speciation. An example is beak size
in some exotic birds. Smaller, thinner beaks would be able to better consume nectar, whilst
larger, thicker, beaks would be able to better crack and eat nuts. The middle-sized beaks,
which would be able to do none of these things well, would not benefit their owners.
Natural Selection
Stabilizing Natural Selection
Stabilizing natural selection is the selection of both extremes, where only a small, middle,
section in the continuum of variance is able to survive easily. This leads to loss of
variation in the population. An example is human babies, where infants of abnormally
large or small size are unlikely to survive. Infants of medium size are the most viable.
Part 3:
Population Genetics
The Gene Pool
The gene pool is the combined genetic information of
all members of a particular population.
It consists of all genes, including all the different
alleles present in a population.
The relative frequency of an allele is the number of times that the allele occurs in a
gene pool, compared with the number of times other alleles for the same gene
Evolution is any change in the relative frequency of alleles in a population.
Genetic Equilibrium
Factors Required for Equilibrium:
There can be no mutations
There must be random mating
There can be no natural selection
The population must be very large
There can be no movement into and out of
the population
Population Bottleneck
A population bottleneck is the drastic
reduction of the size of a population (at
least 50%) after an event (fire, flood,
human interference).
A population bottleneck
sometimes causes destruction of
genetic variation, leading to an
even bigger decrease in the
population as the lack of
variation allows many of the
same species to die to a single
Genetic Drift
Genetic drift is the random
change in allele frequencies that
occurs in small populations as a
chance event.
Genetic drift is one of the basic mechanisms of evolution that
relies solely on chance.
Some individuals may just happen to leave behind more
descendants than others, therefore the genes of the survivors are
passed on. Over time, repeated series of this kind of occurrence
can cause an allele to become common in a population.
Isolation and Speciation
Types of Isolation:
Allopatric: Fancy name for “geographic isolation”. This is
when two populations are separated by geographic barriers such as
rivers, mountains, or bodies of water. The result of this isolation is
the formation of two different gene pools and the lack of genetic
Sympatric: Sympatric speciation does not require large-scale
geographic distance to reduce gene flow between parts of a
Founder Effect
The Founder
Effect is the
change in allele
frequencies as a
result of the
migration of a
small subgroup of
a population.
As a result, the new population may have reduced genetic
variation, as there is a smaller number of individuals to
contribute to the gene pool.
Part 4:
1) What is the fossil record?
A. The history of fossils found in human history
B. The number of fossils found for each species that
have been discovered by mankind
C. The evidence of life on Earth that shows how
organisms change over time
D. How many burgers James ate on Saturday
2) What is a gene pool?
The number of a single allele in a population
A swimming pool full of genes
The total number of all genes in a population
How often a gene has a mutation in a population
3) What are the two main sources of
genetic variation?
Mutation, genetic engineering
Sexual reproduction, mutation
Asexual reproduction, Mutations
Crossbreeding, Sexual reproduction
4) What are the three ways natural selection affects
the distributions of phenotypes?
All of the above
A and B
A and C
5) In directional selection, what is the end
One extreme is favored
Both extremes are favored
The median of the traits is favored
Everything dies
6) List the 5 conditions required to
maintain genetic equilibrium.
7) What is a homologous structure?
A. A structure that does not have current function
but may have been useful to an ancestor
B. Parts that do not have common evolutionary
origins but are similar in function
C. Structural features with a common evolutionary
8) What is an analogous structure?
A. A structure that does not have current function
but may have been useful to an ancestor
B. Parts that do not have common evolutionary
origins but are similar in function
C. Structural features with a common evolutionary
9) What is a vestigial structure?
A. A structure that does not have current function
but may have been useful to an ancestor
B. Parts that do not have common evolutionary
origins but are similar in function
C. Structural features with a common evolutionary
10) Which type of isolation leads to organisms
looking alike but being different?
A. Geographic Isolation
B. Reproductive Isolation
C. Behavioral Isolation
• There must be random mating
• The population must be very large
• There can be no movement into or out
of the population
• No mutations
• No natural selection
7. C
8. B
9. A
10. B
Written By:
James Huang
John DeVito
Aria Wiedmann
Rachel Liu
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