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Chapter 23: The Evolution of Populations
Things to Know:
1.
All those bold-faced words again
2.
The first part of the chapter, pgs 445-452, will be covered in a lab
situation. But basically these pages are asking and answering the
question, “How does the allelic frequency in a population change over
time?”
3.
The second part of the chapter emphasizes how the genetic variation or
the number of different alleles in a population produces a large format
upon which the environment can act in order to select the most fit
organisms.
4.
Hopefully the concept map will help.
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Figure 23.0 Shells
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Concept Map
EVOLUTION
may occur through
Mutation
Migration
may result in
Genetic Drift
Natural Selection
Speciation
occurs as
Genetic Variation
Over-reproduction
Competition for
Limited Resources
from
Mutations
acts on
populations
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Gene Pool vs. Genotype
Contrast the meanings of the terms “gene pool” and “genotype.”
•
Gene pool is the total genetic information within the population
(Campbell). It can also be defined as “is a collection of all the alleles
possessed by all the reproductive members of a population of sexually
producing organisms.
•
Genotype is the genetic makeup of an individual within that population.
So it includes only those alleles that the individual carries and thus
carries a maximum of two alleles at each gene locus.
•
So in a gene pool you have many more alleles at a given locus. Take
blood type alleles for example. If you are type A, then you have a
maximum of two possible alleles because you can be IAIA or Iai. But
within the population you could also have IB allele.
•
So imagine if you will a population where a few individuals have the
allele for a receptor that will prevent HIV from infecting cells. . .
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• A research team in 1996 looked at thousands of blood samples and
found that a few of the samples had cells with DNA that had a mutation
to the gene that makes the CCR5 receptor. This receptor is needed for
viral binding to the WBC of humans.
•
So people who carried two copies of this allele for the gene
made no CCR5 receptors and those with one copy made fewer
receptors than average.
•
So people carrying two copies of the mutant CCR5 gene almost
never became infected.
•
Who carries this mutation?
• Relatively common in Europe, 20% of population carry one
or two copies
• Most common in Sweden
• As you move towards southeast Europe, the frequency of
carriers decreases.
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• Only a small fraction of Greeks
• Even smaller number of Central Asians do.
• From the rest of the world, this particular mutation is missing
altogether.
• So why do we find the distribution pattern for CCR5 in this
fashion? Could “it” have been valuable to European ancestors in
some way and therefore favored by natural selection?
• Could the CCR4 receptor been selected for during an earlier
infectious condition where millions of others were killed and
those possessing the receptor were afforded some advantage?
• 700 years ago The Black Death, one of several bubonic
plague occurrences, killed over 25% of all Europeans between
1347 and 1350.
• Bubonic plague is caused by a bacterium Yersinia pestis,
which binds to white blood cells, injects toxin, depresses the
immune system, multiplies.
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• The hypothesis is: does this bacterium bind to the same
receptor? If so, Europeans who were born without CCR5, were
protected from the plague and their descendants are protected
from HIV.
• “The fact that the AIDS epidemic has been far more
destructive in Africa and Southeast Asia than in Europe or the
United States might be due in part to the different evolutionary
histories of the continents.”
• Extra Credit questions???
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Equilibrium in a Gene Pool?
Can complete equilibrium in a gene pool exist in real situations?
•
What does “equilibrium” or “genetic equilibrium” mean?
• H-W states that a population at equilibrium will have genotypic
frequencies remain the same from generation to population.
•
Of the necessary H-W conditions that need to be met:
• large populations can be met in real situations
• the absence of mutations can never be met because spontaneous
mutations always occur.
•
Most gene probably undergo spontaneous mutations once in
every 50,000 to 1,000,000 duplications and the rate of mutation
is probably different for different genes. But this rate is very
low and is usually insignificant in altering gene frequencies in a
large population.
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• Immigration and emigration of individuals would change the gene
pool in a population and a high percentage of natural populations do
experience some amount of migration.
• Random mating? An organism’s genotype does influence its mate
selection, the physical efficiency and frequency of mating, its fertility so
random mating just doesn’t exist!
• No natural selection. All alleles have equal chance of existing.
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•
So, since it is unlikely that all of H-W conditions will be met, there will
always be a continuous change to the genetic makeup of a population and
this is evolution.
•
So H-W is a mathematical model to determine if a population is at
equilibrium. It also resolves the issue of why recessive genes do not
disappear in a population over time because without the stated parameters,
there is nothing to change the genetic frequencies.
• What will change those frequencies?
•Natural selection
•Mutation
•Migration
•Genetic drift
•Nonrandom mating or sexual selection
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How can you determine whether or not a given population is in genetic
equilibrium?
•
Simply compare the genotypic frequencies between two generations
•
You are studying a population’s blood groups from one generation to the
next. You find:
Phenotype
Generation (G)
M
MN
N
Total
1
241
604
195
1040
2
183
460
154
797
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•
So recall that MN blood groups can be determined by two alleles, LM
and LN with some codominance. So we can have LMLM, LMLN, and
LNLN.
Frequency
G1
So:
G2
LMLM
241/1040 = 0.231
183 / 797 = 0.230
LMLN
604 / 1040 = 0.581
460 / 797 = 0.577
LNLN
195 / 1040 = 0.188
154 / 797 = 0.193
1.0
1.0
So the genotypic frequencies are found to be essentially the same.
You could also have proven this using allelic frequencies. Can you do t
this?
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Figure 23.3a The Hardy-Weinberg theorem
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Figure 23.3b The Hardy-Weinberg theorem
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Figure 23.4 Genetic drift
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Figure 23.5 The bottleneck effect: an analogy
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Genetic Variation
Genetic variations are the key to natural selection and they can occur:
•
Within a population
• What is meant by polymorphism?
•
All the genes in the gene pool allow for some variability but this
variability doesn’t prevent the organisms from interbreeding.
That is, they look different (different phenotype) but still
interbreed.
•
Examples: blood groups of A, B, AB and O and sickle-cell trait
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Figure 23x2 Polymorphism
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•
Balanced Polymorphism is when different forms of the
polymorphic genotype are in equilibrium in the population.
This is usually achieved through complex geneticenvironmental interactions.
•Heterozygote Superiority: So the Aa state has a survival
advantage over both the dominant and recessive homozygous
condition. So both alleles are maintained in the population.
Sickle cell anemia in a malarial environment.
AA: sickle cell anemia
aa: susceptible to malaria
Aa: sickle-cell trait but resistant to malaria.
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Figure 23.12x Normal and sickled cells
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Figure 23.10 Mapping malaria and the sickle-cell allele
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•How is balanced polymorphism important in evolution?
It maintains variability to permit continued adaptive changes.
But it seems like only a certain number of different forms
can increase the fitness of a population. Too many
different forms, lowers the fitness and this “genetic
load” reduces the overall fitness of the population.
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•
Between Populations
• Geographic Polymorphism or clines
•Here you simply have a variety of forms but each is specifically
adapted to its environmental conditions. This way the species
maintains itself in a variety of environmental conditions and allowing
it to become more widespread.
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Figure 23.8 Clinal variation in a plant
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Figure 23.9 Geographic variation between isolated populations of house mice
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What is “fitness?”
•
It seems to give us a mindset of “males battling it out” but may include
producing and releasing more sperm, better camouflage, more efficient
attractors of pollinators, more resistant to attack by a predator (a plant
may contain a toxic substance that tastes repulsive).
• So Campbell defines “fitness” as the contribution an individual, and
therefore its genotype, makes to the gene pool of the next generation as
compared to other individuals.” So if one individual, plant or animal is
able to reproduce more efficiently than other, its fitness is greater.
•So it certainly is a combination of surviving and reproducing.
The individual exposes itself through its phenotype to its
environment and survives based on that phenotype. Its reproductive
ability allows it pass on those “survivability traits” and this increases
its genes in the gene pool.
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Figure 23.12 Modes of selection
Natural selection can favor
a phenotype in three
fashions. All modes pass on
traits that represent
differential reproductive
success
To one
extreme
Phenotype favored is at
both extremes of the
range
Favored phenotype is
more intermediate
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Figure 23.13 Directional selection for beak size in a Galápagos population of the
medium ground finch
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Figure 23.14 Diversifying selection in a finch population
The beaks on these birds represent two extremes where the
smaller beaks on the left feed efficiently on soft seeds while
the larger- beaked birds feed efficiently on the hard seeds.
It’s easier to see why an intermediate sized beak would not
be able to crack the hard seeds but the soft seeds? Maybe
the intermediate beak has trouble getting the seed out of the
shell once it is cracked. I really don’t know.
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Figure 23.15 The two-fold disadvantage of sex
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Figure 23.16x1 Sexual selection and the evolution of male appearance
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Figure 23.16x2 Male peacock
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Figure 23.x1 Edaphic Races of Gaillardia pulchella
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Figure 23.2 Population distribution
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Figure 23.1 Individuals are selected, but populations evolve
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Figure 23.5x Cheetahs, the bottleneck effect
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Figure 23.6 Gene flow and human evolution
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Figure 23.7 A nonheritable difference within a population
•These butterflies differ in coloration
despite having the same gene at the
same locus for coloration.
•This coloration difference is simply
due to when the butterflies emerged
from their cocoons. Hormone levels
are thought to be the factor.
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Figure 23.11 Frequency-dependent selection in a host-parasite relationship
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