Download Evolution - Harrison High School

To understand evolution you have to first have
an understanding of the age of the Earth.
The Earth is estimated to be approx. 4.6 billion
years old.
The first fossil record is believed to be about
3- 3.5 billion years old
This is a very long time, and Earth has changed
tremendously since its origin.
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What were some events that led to the development
of Darwin’s theory of evolution?
◦ Lamark- inheritance of acquired traits through use and
disuse, 1st theory of evolution
◦ Malthus- write an essay “Principles of Population” the idea
that people compete for a limited number of resources, and
population growth rates depend on this flux in resources
◦ Lyell- wrote “Principles of Geology” established that the
Earth has undergone tremendous changes, making it much
older than once believed
◦ Wallace- through his own research came to the same
conclusion as Darwin about Natural selection
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Who was Charles Darwin?
◦ Lived from 1809-1882
◦ Developed the theory of evolution by means of natural
selection
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Look for references to
◦ 1. Charles Darwin’s grandfather Erasmus Darwin,
and Darwin’s brother as an influence
◦ 2. Darwin’s religious beliefs and changes in his
belief system
◦ 3. Geological time, Lyell’s work on changes in
geological features
◦ 4. Common ancestor
◦ 5. Malthus’ writings about population
Define Evolution:
Cumulative change in groups of organisms through time
or
- descent with modification
- differential reproductive success
Principles behind evolution:
1. Individuals in a population show variation
among others in the same species
2. Variations are inherited
3. Animals have more young than can survive on
the available resources
4. Variations that increase reproductive success
will be more common in the next generation
The theory of evolution is supported with the following
evidence (see handouts for more explanation)
1.
Fossil record- using relative dating and carbon-14
dating to determine age of extinct and extant groups of
organisms.
2.
Biogeography- comparing differences in groups of
organisms in line with the migration of continents and
other changes in geography
3.
Comparative Anatomy- looking at common anatomical
structures (homologous or vestigial features)
4.
Comparative Embryology- looking at common tissue
development
5.
Molecular Biology- comparing the DNA and protein
sequencing of extant organisms and determining the
accumulation of mutations since they shared a common
ancestor (phylogeny- an evolutionary tree)
Darwin’s theory was based on Natural Selection
 Natural Selection: differential reproductive
success; the result of selective pressures
from the environment, that favor one
phenotype over another
 Artificial Selection: a man made selection of a
phenotype, like selective breeding of
agricultural plants and animals, horses, or
dogs. This is also the case with antibiotics or
antiviral medicines that change the body’s
internal environment.

Natural selection has three modes of action:
1. Stabilizing selection
2. Directional selection
3. Diversifying selection
Number
of
Individuals
Small
Size of individuals
Large
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Acts upon extremes and favors the
intermediate.
Number
of
Individuals
Small
Size of individuals
Large

Favors variants of one extreme.
Number
of
Individuals
Small
Size of individuals
Large

Favors variants of opposite extremes.
Number
of
Individuals
Small
Size of individuals
Large
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The evolution of new species due to a
barrier, either geographical or reproductive.
What is a species: organisms that can and
will mate to form a fertile offspring.
Geographical Barrier: Any geographical
formation that physically separates a
population. Ex river, mountain range, valley
Reproductive Barrier: Any mechanism that
impedes two species from producing fertile
and/or viable hybrid offspring.

Two type of reproductive barriers:
1. Pre-zygotic barriers- before fertilization
2. Post-zygotic barriers- after fertilization
a. Temporal isolation:
Breeding occurs at different times for
different species.
*Often controlled by hormones that are sensitive to
temperature and light availability.
b. Habitat isolation:
Species breed in different habitats.
*Habitats are dictated by the adaptations organisms
have for shelter, food, and protection.
c. Behavioral isolation:
Little or no sexual attraction between
species.
*Behaviors can be different without the organisms
physical features being different
d. Mechanical isolation:
Structural differences prevent gamete
exchange. This can be true also with the
pollinator that a plant employs.
e. Gametic isolation:
Gametes die before uniting with gametes
of other species, or gametes fail to
unite.
a. Hybrid inviability:
the Hybrid zygote fails to develop or
fails to reach sexual maturity.
b. Hybrid sterility:
the Hybrid fails to produce functional
gametes. Ex. Mule a hybrid of horse and
donkey, all mules are male and sterile
c. Hybrid breakdown:
Offspring of hybrids are weak or
infertile.
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Induced when the ancestral population
becomes separated by a geographical
barrier.
If separated for a long period of time they
will become reproductively isolated
Example:
Grand Canyon and ground squirrels


Result of a radical change in the genome
that produces a reproductively isolated
sub-population within the parent
population (rare).
Example: Plant evolution - polyploid
A species doubles it’s chromosome # to
become tetraploid.
Parent population
reproductive
sub-population

Two theories:
1. Gradualist Model (NeoDarwinian):
Slow gradual changes
accumulate in species
overtime.
2. Punctuated Equilibrium:
Evolution occurs in
spurts of relatively
rapid change followed
by a long period of no
change.
Gradualism
Punctuated
Equilibrium


Organisms separated either reproductively or
geographically are under different selective
pressures and evolve in different directions
This is often called Adaptive radiation
◦ Ex. Darwin’s Galapagos finches who descended
from a small group of mainland finches of South
America


Emergence of numerous species from a
common ancestor introduced to new and
diverse environments, where land
formations, food and predators may be
different.
Example:
Darwin’s Finches of the Galapagos
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Species from different evolutionary branches
may come to resemble one another if they
live in very similar environments.
These are analogous characteristics
Example:
Shark , Dolphin
Under similar environmental pressures these
two very different organisms have developed
similar body styles and predatory habits

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Evolutionary change, in which one species
act as a selective force on a second species,
inducing adaptations that in turn act as
selective force on the first species.
Example: symbiotic relationships are
everywhere. Think “Circle of Life”
1. Acacia ants and acacia trees
2. Humming birds and plants with flowers
with long tubes


Microevolution is change in the allele
frequencies of a population over generations.
This is on a small scale.
Allele frequencies refer to the actual number
of a particular allele within a population’s
gene pool ( all the alleles at all loci in all the
members of a population)
1.
2.
3.
4.
5.
Genetic Drift- loss of variation (allele frequencies)
due to a sudden environmental act that reduces the
population
Gene Flow – change in variation (allele frequencies)
due to immigration or emigration, movement of
individuals into or out of the population
Mutation- introduction of a new allele that becomes
established in the gene pool
Natural Selection- differential reproductive success,
due to environmental pressure on a favorable
phenotype
Non-Random mating -mate choice is no longer
based on equal chance or opportunity. Mate choice
has become selective and based on some
characteristic
Evolution occurs at the population level, population is defined as
a group of the same species that live in the same area and
interbreed, producing fertile offspring.
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Hardy Weinberg equilibrium theory states that a population’s
allele frequencies will remain unchanged generation after
generation, no evolution, if the following 5 conditions are held
constant:
Mutations do not change gene pool
Mating is random and each organism has equal
opportunity
No natural selection, no phenotype is more favorable
Population is large and contains variation
No gene flow (emigration, immigration in/out of
population)
p2 + 2pq + q2 = 1
p = dominant allele
q = recessive allele
Each letter represents the
frequency of a particular allele
in the population
p+q=1
p2= Homozygous dominant
genotype (AA)
2pq= Heterozygous genotype (Aa)
q2= Homozygous recessive
genotype (aa)
We can look at a population and identify
specific traits or phenotypes.
We can actually count the number of
individuals with those specific traits.
Ex. If there are 100 pigs 25 of them are black
and 75 of them are pink, or 25% is black and
75% is pink.
What if you knew the black allele was Dominant
and the pink allele was Recessive. Could you
determine which ones had which genotype?


If B= Black skin and b= pink skin in a pig
If 25% of the population were black and 75% were
pink, how many of them are
◦ Homzygous recessive bb
◦ Homzygous dominant BB
◦ Heterzygous Bb
• Remember that p is the dominant allele and q is the
recessive allele.
• What does bb, BB, and Bb look like?
• BB- black
Bb- black
bb- pink
• p2- black
2pq- black
q2= pink
• Can we calculate q? yes if we know q then we can find p
1. A randomly mating
population has an
established frequency
of 36% for organisms
homozygous recessive
for a given trait. What
is the frequency of the
recessive allele in the
gene pool ?
2. You have sampled a
population in which you
know that the percentage
of homozygous recessive
genotypes (aa) is 36%.
Calculate the following
a.
The frequency of the a
allele
b. The frequency of the A
allele
c.
Frequency of AA and Aa
q= 0.6
a.
b.
a.0.6, b. 0.4,
c. AA=.16 and Aa= .48