Morehouse College
BIO 112 General Biology
Peer-Lead Team-Learning (PLTL) Workshop
Evolution and Natural Selection
Introduction
Biological evolution and the process of natural selection are among the most fundamental
and essential concepts in biology. In this workshop, you will review the features of
genetics, population biology, and ecology that are required for understanding evolution
and natural selection. Common misconceptions will be challenged by the workshop
activities and you will consider the implications of life that has been molded by the
process of natural selection.
Pre-Workshop Assignment
Prior to your laboratory class meeting you should complete the questions given below
and bring your completed work to your laboratory class meeting.
Activity A. Concept and Relationships.
1. Write a brief definition for the following terms, in complete sentences:
Phenotype
Genotype
Environment
Phenotypic variation
Differential survival and reproduction
Heritability
Hardy-Weinberg population
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BIO 112 General Biology
Natural selection
Stabilizing selection
Adaptation
Mutation
Genetic Drift
Migration
Darwin’s Challenge
Inclusive Fitness Effects
Activity B. Variation in a Population
Despite our tendency to think of individuals within a given species as uniform, because
we describe just a few key characteristics that make species different from each other,
individual differences characterize all living things. This individual variation
(differences between individuals within one species, within one population) is in
biologically significant traits and such variation is always present.
What are the causes for differences between individuals in their characteristics?
What is responsible for the constant production of variation in a population?
Imagine a population of rabbits that vary in their reaction time to the scent of a coyote.
Some individuals immediately run to their burrow when they smell this predator, while
others are slower in their response. Draw a frequency histogram (on the graph below)
showing this response variation assuming a normal distribution in the response times of
rabbits. Label the graph axes and draw a smooth curve of the response frequencies.
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Activity C. Evolution is Change Over Time
We sometimes casually use the terms “evolution” and “adaptation” to refer to very shortterm changes that occur in the lifetime of an individual (an evolving personality or an
adaptable salesman). However, these casual uses are not biologically accurate and do
not correctly reflect the biological meaning of evolution and the adaptations that are the
consequence of evolutionary change. Biological evolution does not result from learning
or physiological changes that can be transferred directly from one generation to the next.
What is the time scale for biological evolution?
What must change for biological evolution to have occurred?
Biologically, what is actually transferred from parent to offspring, from one generation to
another?
How do we measure evolutionary change?
Is it ever possible for individuals to evolve?
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PLTL Workshop Problems
Activity 1. Causes for Natural Selection
The cause for natural selection can be any factor that results in some individuals
experiencing poorer survival and reproduction than other individuals in a population.
Charles Darwin called the causes for natural selection “The Hostile Forces of Nature”,
because they are the factors that prevent populations from growing infinitely large and
individuals must constantly confront these factors as they attempt to live and reproduce.
Ecologists often describe the causes for natural selection limiting factors, limiting
conditions and limiting resources.
a. Name and describe Darwin’s Hostile Forces of Nature (there are five):
b. Define the following:
Environmental Limiting Factor
Environmental Condition
Environmental Resource
c. Conduct a round-robin in your group identifying each potential cause for natural
selection as either an environmental condition or a resource (and justify your
answer):
Sunlight
Water
Calcium
The opposite sex
Air temperature
Wind velocity
Glucose
Carbon dioxide gas
Oxygen gas
pH of water
shelter
food
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BIO 112 General Biology
d. Can a given factor be limiting in one environment and non-limiting in another?
Each member of your group should choose one of the following factors and
describe for the other members in the group two situations or examples. Each
member of your group will pull an organism card from the “Species” Box and
both situations or examples must involve this organism. In one example, this
factor will be limiting and a second it will be non-limiting:
Sunlight
Water
Calcium
The opposite sex
Air temperature
Wind velocity
Activity 2. Natural Selection
It is convenient to assume that variation in a population (phenotypic variation) has a
normal distribution (as in the frequency histogram below), but any distribution pattern of
variation is sufficient for natural selection to occur.
a. List and describe the requirements for natural selection? (Let’s not be concerned
about whether natural selection results in evolutionary change at this point.)
b. Work in pairs on this section and present your results to the other members of your
group. Each student pair should choose one case below. In each case, label the
axes and mark the a frequency histogram showing which individuals experience
the negative effects of natural selection, then redraw the frequency histogram
showing how the population changed (evolve) as a consequence of selection.
Case 1. Penicillin is an antibiotic (an anti-bacterial) that can be used to treat the sexually
transmitted disease known as Gonorrhea, caused by the bacterium Neisseria
gonorrhoeae. There is natural variation in the resistance of Neisseria strains to penicillin
and prior to 1970, strains of Neisseria that were resistant to this drug were rare. During
the 1970s penicillin was distributed to sex workers in the Phillipines and SE Asia
(locations of US military R&R during the Vietnam War) and used as a gonorrhea
prophylactic treatment. The frequency of penicillin resistant gonorrhea infections
increased during the 1970s and is as high as 40% in some Asian countries today.
Case 2. The application of air quality regulations in much of the industrial world has
dramatically reduced the emissions of sulfur dioxide and soot (dust) in both the United
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States and Western Europe. In the 1950s and 1960s, species of peppered moths (genus
Biston) on both sides of the Atlantic were predominantly dark winged individuals (95%
melanic individuals) that were well camouflaged from their predators (birds) when
perched on the dark branches of trees in polluted environments. The uncontrolled
combustion of fossil fuels (mainly coal) releases sulfur dioxide that kills lichens that
would normally grown on tree trunks and branches. Light winged peppered moths are
best camouflaged when perched on trees covered with light colored lichens. Starting in
the mid-1960s, serious efforts to reduce the emission of soot and sulfur dioxide were
initiated. The population frequencies of dark winged peppered moths decreased as
pollution levels decreased. In the 1990s, the frequency of dark winged moths was only
30% of individuals.
Case 3. Average human birth weight is 7.0 pounds. In the recent past (1950), the
probability of early childhood mortality was approximately 1.5% for babies weighing 7 –
7.5 lbs. Newborns weighing less than 7.0 lbs had an increasing risk of death with smaller
birth weight (due to premature birth). Early mortality was 5% for newborns weighing 5.0
lbs, 20% for those weighing 4.0 lbs and 60% for those weighing 3.0 lbs. Newborns
weighing more than 7.5 lbs also experienced increasing risk of death with greater birth
weight (due to increased birth trauma). Early mortality was 3% for newborns weighing
9.0 lbs, 6% for those weighing 10.0 lbs and 15% for those weighing 11.0 lbs.
Initial Frequency Histogram
Population Mean
Future Frequency Histogram
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Activity 3. When Natural Selection Causes Evolution
Natural Selection occurs whenever there is phenotypic variation and a cause for selection
that “acts on” those differences between individuals. Are there any differences between
isogenic and polygenic population in the process of natural selection?
Natural selection does not necessarily cause evolution. Why?
Each pair of students who worked on a particular case in Activity 2 should now explain
to your group the requirements for natural selection to cause the evolutionary change that
was observed. Draw a “Future Frequency Histogram” for each case if the population
were isogenic.
What causes genetic differences between individuals in a population? When are these
causes actively inducing genetic variation?
Do the causes for natural selection cause genetic change in an individual during their
lifetime?
What is the relationship between mutation and natural selection? What is heritability and
what makes it important in the process of evolution by natural selection?
What are the three types of natural selection and resulting evolutionary change?
Work in pairs. Assign each pair one of the types of natural selection and draw “before”
and “after” frequency histograms for that type of selection and evolution. Mark the
“before” histogram in red showing that part of the population experiencing the negative
effects of selection and mark the same histogram in green to indicate that part of the
population experiencing the positive effects of selection. Each pair should present their
histograms to the group and explain the process and any assumption made.
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We define adaptations as products of evolution by natural selection. Again, for each of
the cases in Activity 2, describe the adaptation that resulted from evolution by natural
selection.
For a given species, is it possible for a particular trait (bill length in a bird species) to be
an adaptation in one environment but not in another environment?
What is the purpose of an adaptation?
Activity 4. Causes for Evolution
Natural selection is the most potent cause for evolutionary change but it is not the only
cause for evolution. What do we mean when we say that a population has evolved?
What would we measure to determine whether evolution had occurred?
What are the potential causes for evolutionary change in addition to natural selection?
How do these other causes for evolution differ from natural selection?
Activity 5. Are Phenotypes Altruistic or Selfish?
Charles Darwin recognized that his mechanism for evolutionary change had logical limits
such that some phenotypes could not evolve as a consequence natural selection.
Darwin’s statement of this logical limitation is often called Darwin’s Challenge: "If it
could be proved that any part of the structure of any one species had been formed for the
exclusive good of another species, it would annihilate my theory, for such could not have
been produced through natural selection." (Charles Darwin, 1859, The Origin of Species)
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BIO 112 General Biology
Define the following terms (in a round-robin) with respect to Darwin’s Challenge:
Selfish phenotype
Altruistic phenotype
Inclusive Fitness Effects
Mutualism
Kin Selection
Reciprocal altruism
Each member of your group should pull a card from the “Mutualism” box which
lists an example of a mutualistic interaction. Take turns describing the nature of
this interaction by listing the assistance each of the two species in the interaction
provides to the other species.
Within a given species, individuals may assist each other, but this assistance is
typically directed to help individuals who carry genes identical by descent. Often
an evaluation of the coefficient of relationship between two individuals is enough to
understand the benefit an aid giver receives by assisting another individual.
On the pedigree diagram below, take turns estimating the coefficient of relationship
between individuals assuming sexual reproduction and both males and females
having diploid genomes. Describe all the relationships that are possible in this
pedigree.
Relatives
Parent-offspring
Full siblings
Grandparent-grandchild
Aunt (uncle)-niece (nephew)
Cousins
Coefficient of Relationship (r)
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What factors, other than the coefficient of relationship, influences the evolution of
inclusive fitness effects?
Activity 6. What is Natural Selection Actually Selecting?
The level of organization at which natural selection occurs is not limited to that of
the individual organism and in fact may occur at multiple levels simultaneously.
None-the-less, it is important to identify the level at which selection is most potent
because that will determine the function of evolutionary changes and thus the
function of adaptations.
In your small groups, list the units of organization of life (on file cards) and
organize the cards to create a hierarchy of these units. You need not go down to the
level of atoms, but include all units that can reproduce, exhibit variation, and
heritability.
After your group has prepared a hierarchy of the units of life, complete the table
below indicating the relative values for variability, reproduction rate and
heritability.
Units of Life
Variability
Reproduction Rate
Heritability
How is the rate of evolutionary change influenced by variability, reproduction rate
and heritability?
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Explain how group selection (at the level of population or species) can occur but is
unlikely to be as potent as natural selection at the level of the individual organism.
Activity 7. Sexual Selection
Sexual selection is a special form of natural selection in which selection is different in
one sex than the other, and consequently evolution is different between the sexes. Sexual
selection may occur as a consequence of either male-male competition or as a
consequence of female choice between potential mates.
Male-male competition can result in selection for characteristics in males that improve
their ability to compete with other males for access to reproductive females. Examples
sexual selection by male-male competition are larger body size in male than females and
male weapons that are absent or reduced in females (such as horns and antlers).
Female choice results in some male being more successful in reproduction than others
because they had characteristics that were favored by females. Bright coloration in
males, and elaborated structures used in courtship displays by males are examples of
sexual selection by female choice.
The requirements for evolution by sexual selection are the same as the requirements for
evolution by natural selection. In your small group, list and explain the requirements for
evolution by sexual selection.
Work in pairs, work on one case, then report to your group. Draw a frequency histogram
of the trait in question and show how selection and evolution would be expected to occur.
Is sexual selection is occurring? What additional information do you need to determine
whether sexual selection is occurring? What is the cause of sexual selection in each case?
Case 1. In a species of stream fish, the mottled sculpin (Cottus bairdi), males build nests
each spring under rocks and females visit males and deposit eggs on the ceiling of their
rock nests. There is considerable variation in the body size of males that are found in
rock nests and the largest males in a given population receive the majority of the eggs
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that are deposited in nests. Although there is overlap in the size distributions of males
and females, the largest males are always larger than the largest females.
Case 2. The northern elephant seals, Mirounga angustirostris, is a marine mammal that
only comes on land to reproduce. Adult females form a group on a sandy beach and are
guarded by one male who vigorously and violently attempts to exclude other males from
his harem. The largest males are the most successful in excluding other males and
successfully copulating with many females. In fact, small young males may have little or
no reproductive opportunities. Adult males can be as large as 20 feet long and weigh
8000 lbs. while adult females are only as large as 11 feet long and weigh 2000 lbs.
Case 3. Red-winged blackbirds, Agelaius phoeniceus, are freshwater marsh birds. Males
are black and have bright orange-red and yellow patches on each shoulder. This is in
strong contrast to females and immature birds that are smaller than mature males and are
a streaked reddish-brown color.
Case 4. Sage grouse, Centrocercus urophasianus, is a pheasant-size bird of the western
United States. Males are larger than females and have a black throat and bib not seen in
females. Each year males and females visit a courtship site, a lek, where males perform a
distinctive courtship dance with elaborate tail feather displays and inflate bright yellow
breast air sac. Females watch males display before mating. Some males are much more
successful in attracting females than others.
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BIO 112 General Biology
Post-Workshop Activities
Review your understanding of evolution and natural selection by working the following
problems:
1. How do mutation and recombination yield phenotypic variation in organisms?
2. Does mutation only occur when natural selection has a negative effect on a
population? How about recombination?
3. Explain how truly altruistic phenotypes would be expected to disappear from a
population as a consequence of natural selection?
4. Does the absence of true altruism in nature mean that contributing to a hurricane
relief effort is unnatural? How about helping your brother with his homework?
5. Describing the thick winter fur of mountain sheep as “good for the species” may
be true but it is likely to be a misinterpretation of how natural selection has
yielded thick fur in this species. Explain.
6. The use of antibiotic drugs in animal feeds is used to minimize disease and
increase growth rates. How might this use of antibiotics backfire?
7. What determines how rapidly evolutionary change may occur?
8. What would happen to a population in which there was very strong negative
selection so on average each individual produced only one-half an offspring?
9. A population of dandelions, Taraxicum officinale, varies in the height of the
flowering stalk. Rabbits eat these flowers but prefer to consume those on the
tallest stalks. Is natural selection occurring? Is evolution occurring? Explain.
10. Plans to stockpile the antiviral medications Tamiflu as a fail-safe against the
possibility that H5N1 bird flu becomes pandemic in humans have been criticized
by some scientists. Some rare strains of H5N1 bird flu are resistant to Tamiflu.
Biologists predict that H5N1 will “become invincible” to Tamiflu and more
common with exposure to this drug. Explain the logic behind these predictions.
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Resources
Evolution Library. 2001. WGBH Educational Foundation and Clear Blue Sky
Productions, Inc. http://www.pbs.org/wgbh/evolution/library/01/index.html
Evolution Resources From the National Academies, National Academy of Sciences.
2005. http://nationalacademies.org/evolution/?referrer=google#research
Understanding Evolution. 2005. The University of California Museum of Paleontology,
Berkeley. http://evolution.berkeley.edu/
11/2005 by L. Blumer.
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