Chapter 11: The Evolution of
Populations
Mechanism for Evolution video
Darwin stated that in a population
there are variations
• Population:
– All the individuals of the same species in a given
area.
• Gene pool
– The combined alleles (different form of the gene
individuals in a population
– Individual members of a population contribute
their alleles to a common pool of genes.
On an island, there are 7 boars. 2 are pure for the long bristles gene (BB), 3 are
hybrids (Bb) for long bristles and 2 are pure recessive (bb) for short bristles.
What is the likelihood of either allele (B or b) showing up in the next generation?
• Allele frequency:
– Measure of how common an allele is in the
population
– The likelihood of that allele showing up in the
population.
– 2 BB (Homozygous long bristles)
– 3 Bb (Heterozygous long bristles)
– 2 bb (Homozygous short bristles)
1. Count the number of each allele, remembering that
each individual has 2 alleles for each trait
2.
“B” (Long allele)
“b” (Short allele)
2 BB
2x2=4
3 Bb
3
3
2 bb
2x2=4
7 “B” alleles
7 “b” alleles
3. 7/14 = 0.5 or 50 %
7/14 = 0.5 or 50%
Therefore the chance of a “B” or a “b” allele for bristle
length is 50/50 for this population
How about something closer to home?
Let’s look at the allele frequency for colorblindness in our
classroom.
# of “N” “n” alleles
# of colorblind individuals = _____
# of carrier females = ____
# of homozygous dominant females = ___
# of normal males = ___
____
____
____
____
Normal allele
____
____
Colorblind allele
____
Total # of alleles ____
Allele frequency Normal ____ Colorblind ____
____
How did the short bristle or colorblind allele
come about in the first place?
• Mutation
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A random change in DNA of a gene
May form a new allele for a gene
Gene mutations are inheritable if in a gamete
Spontaneous
• Recombination
– Due to meiosis (formation of gametes) new
combinations of chromosomes and therefore genes
may occur
– Shuffling of genes
Can the allele frequencies in a population change over time?
Brown is dominant to Green
What will happen to brown and green alleles in the gene pool
once the brown beetles join the population?
Before: Green alleles ____ Brown alleles ____
____
Frequencies:
_____
____
After: Green alleles ____ Brown alleles ____
____
Frequencies:
_____
____
The frequency of brown alleles in the gene pool will increase
while the frequency of the green alleles will decrease.
Changes in allele frequency within a gene pool can be
caused by a number of factors
1. Gene Flow
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The movement of alleles from one population to another
Migration of organisms into and out of the population
Increases the genetic variation of the receiving
population and decreases the variation of the other
population.
A lack of gene flow will lead to Speciation, the formation
of different species.
Did natural selection play a part in the loss of variation in
the beetle population or was there another factor not
related to natural selection?
2. Genetic Drift
•
In a small population (isolated one), a particular
allele may occur more or less frequently even though
it was purely due to chance.
•
Leads to loss of variation in a population
Among the Amish population, polydactylism is very common.
Why? Is it a favorable trait for working on farms & doing more
“hand work”?
Animation
Sicilian family
An Australian Aboriginal cave painting
What would happen to the population of frogs
illustrated below, in the next generation
3. Bottleneck effect
– Effect of a destructive event
that leaves a few survivors in
a population
– Form of Genetic drift
Why is a peacock have such impressive feathers?
4. Sexual Selection
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Certain traits increase the
likelihood of finding a mate
and therefore, passing on your
genes (making you fitter!)
Since females need to be
selective with their limited
number of eggs, she wants to
be certain that she selects the
correct male who will increase
the chances of having the
fittest offspring.
Why the peacock (male) is so
brightly colored
Compare the male to the
female cardinal
Mechanism for Evolution video
The story of the fruit flies
1.
2.
3.
4.
A bunch of flies were minding their own business feeding on bananas
Suddenly, a hurricane washed the bananas & flies onto another island
Since conditions & food are different on the new island, the flies evolve
separately from their mainland relatives
When some of the flies mix with the mainland relatives, they can no longer
produce viable offspring when they mate.
Speciation has occurred!!
New island
Mainland
Mainland
Look at the illustration above and discuss what might have happened.
The original population was beige in color. They were feed different types of
food, one high in starch and the other high in maltose which caused
variations in the coloration, resulting in light variations and the darker
variations of flies
The light flies only mated with the light colored flies and the darker flies only
mated with the darker flies, eventually resulting in two separate species
Reproductive isolation
Occurs when members of different populations can no longer mate and
produce successfully with each other.
Predict what will happen.
Geographic isolation
– A physical barrier such as a river, mountain,
valley, … separates two populations resulting
eventually in reproductive isolation with the
formation of new species.
– Most common form of isolation
– Isolated populations become genetically
different over time
Geographic Isolation
Animation
Watch the videos of mating dances for a ruffed
grouse, a peacock and a fiddler crab
• Ruffed Grouse courtship display. – YouTube
• https://www.youtube.com/watch?v=Hgh7nhG
zNUk
• https://www.youtube.com/watch?v=jTBHiZtn
CsA
• https://www.youtube.com/watch?v=yCn6g3p
Xc1s
Behavioral isolation
Isolation caused by differences in courtship or
mating behaviors
– Different chemical scents, dances, songs and
signals
If a shark and a dolphin both occupy the same niche, meaning they are top
fast swimming predators, would they become more or less alike in
appearance over time?
Convergent evolution
– Occurs when unrelated organisms occupy the same
niche and tend to have similar characteristics
– Analogous structures.
– Shark and dolphin
– Does not indicate common ancestry
What is the ancestor of the modern dog?
All the dog breeds (Canus familiaris) today share
common ancestry with the wolf (Canus familiarus
lupus)
Divergent evolution
– Closely related species evolved in different directions,
becoming increasingly different
Divergent evolution of the Galapagos finches
Adaptive Radiation
• Rapid evolution of many diverse species from
ancestral species
• Our beak activity showed this.
Coevolution
Two totally unrelated species evolve in response to
changes in each other over time.
Bees don’t see red, but do see yellow, blue, and UV. Thus,
bee-pollinated flowers are mostly yellow or blue with UV
nectar guides (landing patterns) to guide the bee
Birds, like hummingbirds have good eyes which can see red
Acacia tree and ants
How quickly does evolution occur?
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Gradualism – evolution is a constant process and
occurs at a steady rate
Punctuated equilibrium- Periods of equilibrium
followed by rapid periods of change.
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Change occurs w/ environmental pressures
Species arise abruptly then have long periods of little change
Gradualism versus Punctuated Equilibrium