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APBiology 11
The Evolution of Populations – Chapter 23
Name ___KEY_____
What is the smallest unit of evolution?
______microevolution ________
What is a population? _______A group of organisms of the same
species who live in an area together/ habitat.
________________________________________________________________________
Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes
evolution possibleActivity: Genetic Variation from Sexual Recombination self study
The following diagram shows two populations of caribou which interbreed only
infrequently where their ranges overlap:
Fig 23.6 page 473
Chapter 23: Evolution of Populations key
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What is microevolution? ___________________________________________________
Change in allele frequencies in a population over generations. No new species. _
Population genetics is the study of how populations change genetically over time. It is a
synthesis of the ideas of ___Mendel____ and Darwin.
Here you can see an example of genetic variation between two geographically separated
populations of the house mouse on the island of Madeira, Portugal:
Refer to p. 470
Chapter 23: Evolution of Populations key
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A ‘cline’ is a graded change in a character along a geographic axis, as seen in the gradual
change in allele frequency in the mummichog fish population shown below. In which
type of environment does the Ldh-Bb allele seem to be most advantageous?
Ldh-Bb the allele for lactate dehydrogenase – B a metabolic enzyme that catalyses
better in cold water allows fish in cold water to swim faster than fish with the other allele.
Chapter 23: Evolution of Populations key
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The ultimate source of all variation in populations is ___mutation _.
In multicellular organisms, the mutations that are significant in an evolutionary sense are
those that happen in cells that lead to the production of __gametes_____. These cells are
called the ‘germ cells’.
How does a ‘point’ mutation differ from a ‘chromosomal’ mutation?
____Point mutation is in a small part of a chromosome, can only be one base.
Chromosomal mutations, rearrange, delete or change many loci and are often harmful.
What is an advantage of gene duplications with respect to evolution?
Can be neutral or helpful. Gene duplication is often helpful as with olfactory gene read
p.471________________________________________________________________
How has this concept lead to our ability to detect thousands of different odours?
________________________________________________________________________
________________________________________________________________________
What aspects of sexual reproduction increase variability within a population?
Crossing over, independent assortment during meiosis and
Random fertilisation.
Chapter 23: Evolution of Populations key
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What is the gene pool of a population?
_The total or complete set of alleles in a population
________________________________________________________________________
_______________________
This diagram illustrates the concept of allele frequencies in a population:
Chapter 23: Evolution of Populations key
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This diagram explains how the frequency of an allele in the gene pool of a population
will remain constant over many generations:
Hardy-Weinberg equilibrium states that, at a locus with two alleles, the three genotypes
will appear in the following proportions:
________________________________________________________________________
Chapter 23: Evolution of Populations key
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The Hardy-Weinberg Theorem
Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is
evolving
Investigation: How Can Frequency of Alleles Be Calculated?
The Hardy-Weinberg principle is used to describe a population that is not evolving.
State the Hardy-Weinberg Theorem: _________________________________________
________________________________________________________________________
________________________________________________________________________
See Campbell’s Activity 22
See Lab Bench 8
If the frequency of alleles in a population remains constant, the population is at HardyWeinberg equilibrium. There are five conditions for Hardy-Weinberg equilibrium. It is very
important for you to know these conditions, so enter them below.
CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM
1. _____________________________________________________________________
2. ______________________________________________________________________
3. ______________________________________________________________________
4. ______________________________________________________________________
5. _____________________________________________________________________
Chapter 23: Evolution of Populations key
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It is not very likely that all five of these conditions will occur, is it? Allelic
frequencies change. Populations evolve. This data can be tested by applying
the Hardy Weinberg equation. Let’s look at how to do this.
Equation for Hardy-Weinberg Equilibrium
p2 + 2pq + q2 = 1
Where p2 is equal to the frequency of the homozygous dominant in the
population, 2pq is equal to the frequency of all the heterozygotes in the
population, and q2 is equal to the frequency of the homozygous recessive
in the population.
Consider a gene locus that exists in two allelic forms, A and a, in a
population.
Let p = the frequency of A, the dominant allele
and q = the frequency of a, the recessive allele.
So,
p2 = AA,
q2 = aa,
2pq = Aa
If we know the frequency of one of the alleles, we can calculate the
frequency of the other allele:
p + q = 1, so
p=1–q
q=1–p
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So, here is a problem to try. Suppose in a plant population that red flowers (R) are dominant
to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype.
How many individuals would you expect to be
a) homozygous dominant and
b) heterozygous for this trait?
Let p = frequency of the dominant allele (R) and q = frequency of the recessive allele (r).
q2 = frequency of the homozygous recessive = 25% = 0.25. Since q2 = 0.25, q = 0.5.
Now, p + q =1, so p = 0.5.
a) Homozygous dominant individuals are RR or p2 = 0.25, and they will represent
(0.25)(500) = 125 individuals.
b) The heterozygous individuals are calculated from 2pq = (2)(0.5)(0.5) = 0.5, and in a
population of 500 individuals will be (0.5)(500) = 250 individuals.
In a population of plants, 64% exhibit the dominant flower color (red), and 36% of the plants
have white flowers. What is the frequency of the dominant allele? (There are a couple of
twists in this problem, so read and think carefully.)
This problem requires you to recognize that individuals with the dominant trait can be either
homozygous or heterozygous. Therefore, you cannot simply take the square root of 0.64 to
get p. For problems of this type, you must begin with the homozygous recessive group. So . .
.
Let p = frequency of the dominant allele (R) and q = frequency of the recessive allele (r)
1. q2 = frequency of the homozygous recessive = 36% = 0.36. Because q2 = 0.36, q = 0.6.
2. Now, p + q =1, so p = 0.4.
3. Notice that this problem asks for the frequency of the dominant allele (p), not the
frequency of the homozygous dominant individuals (p2). So, you are done . . . the frequency
of the dominant allele = 40%.
Chapter 23: Evolution of Populations key
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Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies
in a populationBioFlix: Mechanisms of EvolutionActivity: Causes of Evolutionary
ChangeBiology Labs On-Line: PopulationGeneticsLab
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GENETIC DRIFT
What is genetic drift? http://www.evotutor.org/EvoGen/EG1A.html
http://darwin.eeb.uconn.edu/simulations/drift.html
http://darwin.eeb.uconn.edu/simulations/jdk1.0/selection.html
_____________________
_____________________
_____________________
_____________________
_____________________
_____________________
_____________________
_____________________
Why is genetic drift often referred to as the bottleneck effect?
______________________________
____Example is the cheetah, black
rhino._________________________
______________________________
______________________________
______________________________
______________________________
_________________
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Here is an actual example to illustrate the bottleneck effect:
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What specific example of genetic drift is referred to as the founder effect?
__________________
Amish people in Pennsylvania,
________________________________________________________________________
________________________________________________________________________
______________________________________________________
Does genetic drift result in a population that is better adapted to its environment, or does
it have a random effect? __________________________________
Does natural selection result in a population that is better adapted to its environment, or
does it have a random effect? _________________________________
GENE FLOW
What is gene flow? __________________ movement of
individuals in and out of a population, immigration and
emigration, plants and animals. _____________________
Is gene flow a random or directed process?
____________________________________
http://www.evotutor.org/EvoGen/EG1A.html
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How can we use the concept of HardyWeinberg equilibrium to determine
allele frequencies and genotype
frequencies in a population?
1) First, recall that p represents the
allele frequency of the dominant
allele, and q the frequency of the
recessive allele. If there are only two
alleles at this locus, p + q = _____.
2) Using the analysis in the diagram at
the right, we can see that, from
generation to generation,
p2 + 2pq + q2 = _____.
3) If we know how many organisms are showing the dominant phenotype, can we
calculate p2 ? _____ Can we calculate 2pq? _____ Why or why not?
___________________________________________
_____________________________________________________________________
4) If we know how many organisms are showing the recessive phenotype, can we
calculate q2? _____ Why or why not? ____________________________________
_____________________________________________________________________
5) Once we know q2, we can calculate everything else. Try this sample problem, from
the end of Lab 8:
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In Drosophila the allele for normal-length wings is dominant over the allele for vestigial
wings (vestigial wings are stubby little curls that cannot be used for flight). In a
population of 1,000 individuals, 360 show the recessive phenotype. How many
individuals would you expect to be homozygous dominant and heterozygous for this
trait?
________________________________________________________________________
________________________________________________________________________
DO ALL THE SAMPLE PROBLEMS AT THE END OF LAB 8.
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A Closer Look at Natural Selection (p 479)
What do we mean by the Darwinian fitness of an organism?
__The ability of an organism to reproduce and pass it’s genes to the next generation.
______________________________________________________
What is relative fitness? The contribution that an individual makes to the gene pool of
the next generation, relative to the contribution of other individuals.
Here are two striking examples of how natural selection has resulted in adaptations to an
organism’s environment:
Chapter 23: Evolution of Populations key
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Natural selection is the only evolutionary mechanism that consistently leads to adaptive
radiation.
Figure 23.13 is important because it helps explain the three modes of selection. Label each
type of selection, and fill in the chart to explain what is occurring.
Type of selection
How it works
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Changes in beak size depending on climate fluctuations is an example of
________directional_________ selection
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Summarize the big idea from this section. Scan through the entire concept to pull out this
information. Three major factors alter allelic frequency and bring about evolutionary change.
List each factor, and give an explanation.
Factor
Explanation
Natural
selection
Genetic drift
Gene Flow
a. Which of the factors above results in a random, nonadaptive change in allelic
frequencies?
Genetic drift
b. Which of the factors above tends to reduce the genetic differences between
populations and make populations more similar?
Gene flow
c. Of the three factors you listed above, only one results in individuals that are better
suited to their environment. Which is it?
Natural Selection
What is often the result of sexual selection?
Sexual dimorphism – marked differences in secondary sexual characteristics which are not
directly associated with reproduction or survival.
Size, colour, ornamentation, behavior.
What is the difference between intrasexual selection and intersexual selection? Give an
example of each type of selection.
page 482
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The Preservation of Genetic Variation in Populations (p 483)
Why is it an advantage to maintain genetic variation in a population? _______________
____protection against population destroyed by virus or disease.
Protection against all of the members being preyed on/eaten.___
If there is a sudden change in the environment, some may survive and others may not.
Eg. Pepper moth in England.
More possible niches can be occupied. ____________
What are some mechanisms that help to preserve genetic variation in a population?
a) Diploidy _____________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
b) Balancing Selection (results in balanced polymorphism)
i) Heterozygote Advantage: How does the heterozygote advantage preserve variation in
some situations? (Understand example of sickle cell allele and malaria.)
_____Heterozygotes carry the dominant and the recessive gene. This gives tghem an
advantage in selection. For example people with sickle cell gene survive malaria where
others may not. People carrying the HIV
immune gene survive. This gene is related to the
gene for survival of the bubonic plague.
_______________________________________
_______________________________________
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________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
___________________________________________
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ii) Frequency-Dependent Selection _________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
c) Neutral variation ______________________________________________________
________________________________________________________________________
________________________________________________________________________
Finally, give four reasons why natural selection cannot produce perfect organisms.
Page 484
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