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Evolution and Natural Selection
Part One
Evolution
Evolution and Natural Selection
Natural selection is a major
mechanism of evolution.
Evidence of Evolution
Evolution—change in a population’s
genetic makeup over time.
According to Darwin’s theory of natural
selection, competition for limited
resources results in differential survival.
Individuals with more favorable
phenotypes are more likely to survive and
produce more offspring, thus passing on
their traits to future generations.
On the Origin of Species by Means of
Natural Selection by Charles Darwin
Darwin
hypothesized
that all life
descended from
a common
ancestor.
Natural Selection: Darwin’s proposed
mechanism for evolution
— a population can change over time if individuals with
more fit traits leave more offspring than less fit individuals
Darwin’s background
• Loved nature, studied to be a
clergyman
• Went on HMS Beagle for voyage
around the world
In the Galapagos Islands, Darwin saw animals that
were similar to the mainland but slightly different
on each island.
Darwin inferred that adaptation to environment and
origin of new species are related.
Summary of Darwin's theory:
OBSERVATION 1: If all individuals born reproduce
successfully, a population will increase exponentially.
OBSERVATION 2: But populations
remain stable.
OBSERVATION 3: Resources are
limited
INFERENCE 1: Production of more
individuals than can be supported by the
environment leads to a struggle for
existence, with only a fraction of offspring
surviving each generation
OBSERVATION 4: Members of a population
vary extensively
OBSERVATION 5: Variation is heritable
INFERENCE 2: Fitness: Individuals whose
inherited traits confer an advantage have a
better chance of surviving in a given
environment and will leave more offspring
INFERENCE 3:
Unequal fitness will
lead to gradual
change in a
population, with
favorable traits
accumulating over
generation
Over time, a population might eventually
accumulate enough change to become a new
species
***Evolutionary fitness is measured by
reproductive success.
Genetic variation and mutation play roles
in natural selection. A diverse gene pool is
important for the survival of a species in a
changing environment.
Environments can be more or less
stable or fluctuating, and this affects
evolutionary rate and direction.
An adaptation is a genetic variation that is
favored by selection and is manifested as a
trait that provides an advantage to an
organism in a particular environment.
In addition to natural selection, chance
and random events can influence the
evolutionary process, especially for
small populations.
Natural Selection Acts On Phenotype
Part Two
Natural selection acts on phenotypic
variations in populations.
Environments change and act as
selective mechanism on populations.
The environment does not directly cause
changes in DNA, but acts upon phenotypes
that occur through random changes in DNA
Example: Flowering time in relation to
global climate change
Crop production is sensitive to climate
change; temperature has a large impact on
the rate of plant development.
Warmer temperatures will mean reduced
crop yields.
Example: Peppered moth
The light phenotype was favored before
the Industrial Revolution. The color
blended with the tree bark.
After the Industrial Revolution, mostly dark
colored moths were seen. They had an
advantage on dark tree bark.
Phenotypic variations are not directed
by the environment but occur through
random changes in the DNA and
through new gene combinations.
effects) or result in a new phenotype.
Some phenotypic variations significantly
increase or decrease fitness of the
organism and the population.
Example: Sickle cell anemia and
Heterozygote Advantage
Sickle cell disease is caused by a single
base substitution mutation. It is
maintained in the population in a state
of balanced polymorphism because of
the protective effect against severe
forms of malaria conferred by the
heterozygous states.
Rock Pocket Mouse
Video and Worksheet
Example: DDT resistance in insects
Pesticide-Resistant Organisms:
• Super Rats that can consume up to five times
the lethal amount of rat poison
• Head lice resistant to treatment
• DDT no longer effective against disease
vectors such as mosquitoes
• Fruit flies resistant to malathion
• The Colorado potato beetle has evolved
resistance to 52 different compounds
belonging to all major insecticide classes
(multiple resistance)
Pesticide
Resistance
http://www.pbs.org/wgbh/evolution/library/10/1/image_pop/l_101_02.html
Humans impact variation in other species.
Example: Artificial selection
Example: Loss of genetic diversity
within a crop species
Example: Overuse of antibiotics leading to
increase in antibiotic resistant bacteria.
Biggest problem: overuse and improper use of
antibiotics, especially in livestock.
Some evidence that evolution continues to
occur:
• Increase in antibiotic-resistant bacteria
such as MRSA and Clostridium difficile
• Evolution of the SARS virus and other
emergent diseases
• Lactose tolerance in Europeans
• Butterflies in the South Pacific have evolved
resistance to a killer bacteria in a single
year
Stickleback
• Video and Worksheet
What is Population Genetics?
The study of changes in the genetic makeup of
populations.
An important concept in population genetics is the
Gene Pool:
All the alleles in all the individuals that make
up a population.
What was the result?
Population
Genetics
Remember, individuals do not evolve, populations do.
Natural Selection
+
Genetics
Population- a group of individuals of a single species
that live in a specific area.
• Five Fingers of Evolution
Microevolution- is evolution on the smallest scale –
a generation-to-generation change in the frequencies
of alleles within a population.
Ex: Peppered Moth
Macroevolution- major biological changes that are
clearly visible.
Ex: Development of an entire new species
Directional Selection is most common
when an environment changes. One
phenotype favored over another.
Directional Selection
If only large seeds were available, birds with
larger beaks would have an easier time feeding
and would be more successful in surviving and
passing on genes.
Stabilizing selection maintains the status
quo by favoring the mean phenotype.
Example: human birth weight
Stabilizing Selection
Very small and very large babies are less likely to
survive than average-sized individuals. The fitness of
these smaller or larger babies is therefore lower than
that of more average-sized individuals.
Distruptive selection occurs when the
extreme phenotypes are favored. May
lead to speciation.
Disruptive Selection
In an area where medium-sized seeds are less common,
birds with unusually small or large beaks would have
higher fitness. Therefore, the population might split into
two groups—one with smaller beaks and one with larger
beaks.
Example: Wood Frog and Leopard Frog
Wood Frog
Breeds in early April
Leopard Frog
Breeds in mid-April
geographic variation – difference in
variation between population subgroups in
different areas
Genetic Drift
Genetic drift occurs in small populations when an
allele becomes more or less common simply by
chance. Genetic drift is a random change in allele
frequency.
Bottleneck Effect
The bottleneck effect is a change in allele frequency
following a dramatic reduction in the size of a
population.
For example, a disaster may kill many individuals in a
population, and the surviving population’s gene pool
may contain different gene frequencies from the
original gene pool.
Founder Effect
The founder effect occurs when allele frequencies
change as a result of the migration of a small
subgroup of a population.
Founder Effect
Two groups from a large, diverse population could
produce new populations that differ from the
original group.
The Hardy-Weinberg Law:
If evolution can be defined as a change in gene (or
more appropriately, allele) frequencies, is it
conversely true that a population not undergoing
evolution should maintain a stable gene frequency
from generation to generation? This was the question
that Hardy and Weinberg answered independently.
The Hardy-Weinberg Law:
The condition in which Gene (allele) frequencies do not
change from one generation to the next.
The population is said to be in genetic equilibrium.
What conditions must be met for the HardyWeinberg Law to hold true?
The gene pool remains the same from generation to
generation.
Five conditions must be met……….
• 1) Large Populations
– In small populations alleles of low frequency
might be lost by the death of a few individuals
“Genetic Drift”
• 2) No Migration
– Individuals may not migrate into or out of the
population
• 3) No Mutations
– These will change the
frequency of the
alleles in the
population
• 4) No Natural
Selection
– Each member of the
population must
survive long enough to
have offspring
• 5) Random Mating
– Each member of the
population must have
an equal chance to
reproduce
Population Genetics
• The Hardy-Weinberg law does NOT apply to
situations in the real world!
• Mutations occur spontaneously
• Reproduction is NOT random
• Natural selection DOES occur
Population Genetics
• The failure of HardyWeinberg Law is a sign
that evolution is
occurring!
Hardy-Weinberg Equation:
p2 + 2pq + q2 = 1 and
p+q=1
p = frequency of the dominant allele in the population
q = frequency of the recessive allele in the population
p2 = percentage of homozygous dominant individuals
q2 = percentage of homozygous recessive individual
2pq = percentage of heterozygous individuals
Population Genetics
• Example
30% of a population of apple rose that show the
recessive phenotype of a yellow color as opposed
to the dominant phenotype of green color
What is the frequency recessive allele and what is
the frequency of the dominant allele?
• p+q=1
• Where p = dominant allele
•
q = recessive allele
•
q2 = .30
q = √.30 = .55
q = .55
p+q=1
P + .55 = 1
p = .45
Population Genetics
• What % of the apple roses that are
heterozygous for color?
now use:
p2 + 2pq + q2 = 1
Population Genetics
Heterozygous is 2pq
2 (.45 x .55 ) = .50
50% are heterozygous
PROBLEM #1.
You have sampled a population in which you know that the
percentage of the homozygous recessive genotype (aa) is 36%. Using
that 36%, calculate the following:
• The frequency of the "aa" genotype.
• The frequency of the "a" allele.
• The frequency of the "AA and Aa"
• The frequencies of the genotypes "AA" and "Aa."
PROBLEM #1.
•
•
The frequency of the "aa" genotype.
Answer: 36%, as given in the problem itself.
•
•
The frequency of the "a" allele.
Answer: Since the frequency of aa is 36%. which means that q2 = 0.36. If q2 =
0.36, then q = 0.6, again by definition. Since q equals the frequency of the a
allele, then the frequency is 60%.
•
•
The frequency of the "A" allele.
Answer: Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is by
definition equal to p, so the answer is 40%.
PROBLEM #1.
The frequencies of the genotypes "AA" and "Aa."
Answer: The frequency of AA is equal to p2, and the frequency of Aa is equal to
2pq. So, using the information above, the frequency of AA is 16% (i.e. p2 is 0.4 x
0.4 = 0.16) and Aa is 48% (2pq = 2 x 0.4 x 0.6 = 0.48).
• Bozeman on Natural Selection
• Bozeman on Genetic Drift