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Notes: Variation, Diversity, and Lethal Allele Maintenance
What have we learned about evolution so far?
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Evolution is change in the traits of a population over time
Natural selection is the mechanism of evolution
Evidence of evolution comes from the fossil record, DNA and proteins,
comparative anatomy, and embryology
Isolation can lead to speciation. The major types of isolation are
geographic, behavioral, temporal, and reproductive
What are we going to learn about evolution today?
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Variation within a species increases the probability that some members of
the species will survive
A great diversity of species increases the chance that at least some
organisms will survive great changes in the environment
Natural selection acts on the phenotype of an organism, not its genotype
How do lethal alleles stay in the gene pool?
Variation and Gene Pools
Genetic _______________________________ is studied in _______________________________ of
species. Because populations _______________________________ (mate with each other),
they share a gene pool.
A _______________________________ _______________________________ is made up of all the genes,
including all the different _______________________________, that are present in a
_______________________________.
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Allele Frequency
_______________________________ _______________________________ is
the _______________________________ of an allele in the gene
pool. For example, if a group of pea plants is 50%
_______________________________ (Bb), 30% homozygous
_______________________________ (BB), and 20% homozygous
_______________________________ (bb), the allele frequency of
the dominant allele would be _______________________________
% and the allele frequency of the recessive allele would
be _______________________________ %.
Calculating Allele Frequency
Here’s how I calculated that:
We start with the 30% of the total that is homozygous dominant (BB). We know
50% of the population is heterozygous (Bb). Only half of the alleles in the
heterozygotes are dominant, so we add half of 50%, or 25%, to 30%.
Here is the formula I used to calculate the allele frequency of dominant alleles:
% BB individuals+ half the % of the Bb individuals = allele frequency
30% + (½ x 50%) = allele frequency
30% + 25% = 55%
Then, to find the frequency of the recessive allele, just subtract 55% from 100%
100% - 55% = 45%
Another Definition of Evolution
Using genetic terms, _______________________________ can be
defined as any change in the allele frequency in a
population.
For example, in beetles, if the _______________________________
of the recessive green allele was 75%, but over time the frequency
_______________________________ to 71%, _______________________________ has occurred.
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Practice Problem
In pea plants, purple flowers (P) are dominant over white flowers (p). In one
population of pea plants, 15% of the plants are homozygous dominant, 60% are
heterozygous, and 25% are homozygous recessive.
What is the allele frequency of the dominant purple allele?
What is the allele frequency of the recessive white allele?
Variation Within a Species Increases Chance of Survival
Let’s say you are studying a bird that is very well
_______________________________ to its environment, but all of the individuals
are genetically very similar. If a deadly _______________________________
starts infecting these birds, they could all die because they are all
_______________________________ to the virus!
Now, if there was more genetic _______________________________
in this population of birds, there is a chance that some of
the birds might have a natural _______________________________
to the virus. These _______________________________ birds would
_______________________________ and be able to pass on their
genes.
Biodiversity Increases the Chance of Survival
We just looked at how variation _______________________________
species can increase the chance that some members of the
species will survive even if there is a change in the
environment.
Having a lot of _______________________________ (many different
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species present) increases the chances that some species will survive a
_______________________________ _______________________________ in the
_______________________________ .
For example, the _______________________________ that
scientists believe was the reason
_______________________________ went extinct did not
kill all of the life on earth because there was a
great _______________________________ of living
_______________________________ . If large dinosaurs
were the only species on earth at the time, all life
on earth might have been killed! But, since there
were many different species all adapted to
different _______________________________ and
_______________________________ , some animals were
able to survive and reproduce.
Natural Selection acts on the Phenotype of an
Organism, not its Genotype
Natural selection never acts directly on
_______________________________. _______________________________,
not genes, survive and reproduce or die without
reproducing, not genes. _______________________________
contribute to fitness, not _______________________________. For
example, a green beetle might be more
_______________________________ than a brown beetle, but if the
green color is _______________________________, the green
beetle could have the genotype GG or Gg and still be
equally fit. The green beetles that survive and reproduce
may pass on either the green allele (G) or the brown allele
(g).
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How are Lethal Alleles Maintained in the Gene Pool?
If there are alleles that can cause _______________________________
(deadly) genetic disorders, why are they still in the gene pool?
Wouldn’t they be removed because organisms that have them
die before they can reproduce?
Not necessarily! We are about to find out how!
Lethal Allele Maintenance
Example 1: Some lethal alleles are _______________________________, so an
organism would have to have _______________________________ of the alleles to
get the _______________________________ effect.
Example 2: Other lethal alleles are _______________________________, but are
only lethal in an individual that is homozygous dominant.
Heterozygotes can survive and pass on their genes. An example of this
is coat colors in mice. Mice with two dominant alleles for coat color die
before they are born, but heterozygotes survive.
Example 3: Huntington’s disease is a lethal, _______________________________
genetic disorder in humans. People with Huntington’s disease always
_______________________________ from the disease (unless something else
killed them first). However, the symptoms of Huntington’s disease
do not appear until the affected person is ________________ ________________ years old. Many people have already had children by
this age, so they have already had a chance to
_______________________________ _______________________________ the lethal
allele to their offspring.
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Lethal Allele Maintenance and the Heterozygote
Advantage
Sometimes, alleles that cause genetic disorders can give an
_______________________________ to _______________________________.
Sickle cell anemia is a genetic disorder that causes red blood
cells to be shaped like sickles. People who are
_______________________________ for the sickle cell allele have a
_______________________________ form of the disease and have a life
expectancy of about ________________ -________________ years with
treatment. Sickle shaped cells usually live about ________________
-________________ days. _______________________________ red blood cells live about
________________ days.
People who are _______________________________ for the sickle cell gene have
some red blood cells that are normal, and some that are sickle-shaped.
They do _______________________________ have a severe form of the disease.
_______________________________ is a disease that is caused by a parasite, and
it can be _______________________________. The malaria parasite infects red
blood cells. When the parasite infects normal red blood cells, it has enough time to
reproduce, creating many baby parasites that then burst out and infect more red
blood cells. When the parasite infects _______________________________ shaped cells, the
cells die before the parasite has a chance to reproduce.
This gives the _______________________________ an advantage: _______________________________
from malaria!
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