Charles Darwin
Darwinian Natural Selection
1. Individuals within populations are variable.
2. The variations among individuals are, at least in part,
passed from parents to offspring.
3. In every generation, some individuals are more
successful at surviving and reproducing than others.
4. The survival and reproduction of individuals are not
random. The individuals with the most favorable
variations, those who are better at surviving and
reproducing, are naturally selected.
Darwinian Fitness
• Darwinian Fitness is the ability of an individual to
survive and reproduce in its environment.
• Fitness of an organism is measured by how
many offspring it produces compared to other
individuals of its species.
• Adaptation refers to a trait that increases an
organisms fitness relative to individuals without
the trait (ie..Reverse Transcriptase).
Is Darwinian Natural Selection
Testable?
• Yes, each of the 4 postulates that make up
the theory are testable.
– Section 3.3 The evolution of flower color in an
experimental Snapdragon population.
– Section 3.4: The evolution of Beak Shape in
Galapagos Finches
The Evolution of Flower Color in
Snapdragons
• Question: Can Darwinian natural selection by
bumble bees influence the evolution of a floral
trait?
• Hypothesis: Yes.
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There is variation among individuals
The variation is heritable
Individuals vary in their reproductive success
Reproduction is nonrandom
The above will lead to evolution of the species.
Prediction
• Based upon the theory of Natural
Selection, We predict that If you allow wild
bumblebees to pollinate a population of
Snapdragon in a meadow, there will be a
difference in reproductive success among
the plants based on the different colors of
the flowers. The bees will preferably
pollinate one type of flower over the other
and the population will change over time.
Experiment and Results
Postulate 1: There is variation
among individuals
• ¾ of the plants had flowers that were almost
pure white, with just two spots of yellow on the
lower lip.
• ¼ of the population of plants had flowers that
were solid yellow.
• Therefore Postulate 1 is True; there is
variation within the population.
Postulate 2: Some of the
Variation is Heritable
• The difference in flower color for the population
of snapdragons was shown to be held at a
single locus.
• The dominant allele S expressed white flowers
with two yellow spots, while the recessive trait ss
is expressed as the yellow flower.
• Genotypes of the 48 plants were as follows
– SS = 12
– Ss = 24
– ss = 12
Postulate 3: Do individuals vary in
Their Success at Reproducing?
• Researchers measured reproductive success by
measuring the plants ability to export pollen and
making seeds.
– researchers tracked the number of times each flower
was visited by a bee.
– Researchers counted seeds produced from each fruit.
• Consistent with Darwin’s 3rd postulate, the plants
showed considerable variation in reproductive
success, both as pollen donors and as seed
producers.
Postulate 4:
Is Reproduction
nonrandom?
• White flowers attracted twice as many bees as yellow
flowers
• White flowers also produced slightly more seeds than
the yellow flowering plant.
• Therefore the data shows that reproductive success is
not random. The white flowers attract more bees.
– Could create a new hypothesis from the analysis. The yellow dots
on the white plants serve as a guide to pollinating bees.
Did the Population Evolve?
• Since bees selected white flowers more
often than yellow and color is carried on a
single gene the next generation should
have more white flower than the last.
• Results
– White flowers went from 75% to 77%
The Evolution of Beak Shape in
Galapagos Finches.
• Use the Galapagos Finch population to
demonstrate that Charles Darwin’s theory
of natural selection can be verified by the
scientific method.
3.5 The Nature of Natural
Selection
How selection does and does not
operate
Natural Selection Acts on Individuals, but
its Consequences Occur in Populations
• How did the finches on Daphne Major
react when a drought caused seeds to
change from small soft seeds to larger
harder seeds.
– Did birds with short shallow beaks start to
grow longer and deeper beaks in order to
survive?
– Why did beak size change over time?
Natural Selection Acts on Phenotypes, but Evolution
consists of changes in Allele Frequencies
• If beak size was not
genetically controlled but
rather a product of the
environment, would deep
beaks still have been
favored during the
drought?
– Would the phenotypic
frequencies in beak size
change after the drought?
• Evolution can only occur
when the traits are
genetically based.
• Evolution is measured by changes in Allele
frequency within a Population
Natural selection is Not Forward
Looking
• Natural selection adapts populations to
conditions that prevailed in the past, not
conditions that might occur in the future.
– The finch population became better adapted
to the drought only after the drought occurred.
– Following the drought in 1977, the finch
population was not better adapted to survive
the flooding that took place in 1983.
Can Natural Selection Create New
traits?
• Natural selection can
select only from the
variations that already
exist in a population.
– Example: Natural
selection cannot
instantly create a new
and optimal beak
• How then do new
unique traits come
into existence?
Evolution of New traits
1. Mutations produce new
alleles in all species
2. Meiosis and fertiliation
reomine existing alleses
to create new
genotypes
•
Natural selection acts
upon these new traits.
Corn oil
The Panda’s Thumb
• Natural selection is able
to “repurpose” existing
behaviors, structures or
genes.
• A trait that is used in a
new way and is
elaborated on by
selection into a
completely new structure
is called a preadaptation.
Is Natural Selection random?
Is Natural Selection progressive?
• Natural Selection is nonrandom. It is predictable
in that it increases a populations adaptation to
the environment.
• Natural Selection is not progressive. It does not
necessarily push organism toward more
complex forms of life.
– Examples of organisms becoming less complex
overtime?
– Are some animals considered higher than other
animals?
In nature, are there true altruistic
behaviors
• Altruistic behaviors
reduce the bearer’s
fitness and increases the
fitness of others.
• If such behavior is inherit
or genetic, it will be
strongly selected against.
• Individuals do not do
things for the good of the
species. They behave in
a way that maximizes
their individual fitness.
Altruistic behavior reduces the bearer’s
fitness and increases the fitness of others
Evolution of Darwinism
• Is the amount of variability in populations strictly
limited?
• If a new trait emerges in a large population,
won’t it be eventually be swallowed up in the
population no matter how advantageous it is?
• Was Lord Kelvin right about the age of the
earth?