BIOL 119, Fall 2012
Natural Selection
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NATURAL SELECTION
Overview
This simulation of the process of natural
selection will illustrate how a population of seeds
can become adapted to an environment over a
period of generations. An initial population with
a large amount of variation will be exposed to a
predator (you), and changes in the proportions of
different prey phenotypes will be measured in
three different environments.
Introduction
The purpose of this lab is to demonstrate the
process of natural selection by simulating the
effect of a selective predator feeding upon a
variably colored prey population in several
different environments. The change in frequency
of different colored prey types in the population,
as a result of the predator’s feeding efforts, will
represent adaptation to the environment by the
prey population.
Ecology is the study of the interactions of
organisms with one another and with their
physical environment. These interactions are also
the driving mechanism of evolution via natural
selection. In the strictest sense, biological
evolution is defined as any change in gene
frequency. An organism’s genes in large part
determine the characteristics of that organism.
Thus, the genotype of your cells determine many
of your physical traits such as hair color or eye
color. These traits together make up your
phenotype, or the sum total of your physical
characteristics. In many cases an individual’s
physical characteristics determine its ability to
survive and reproduce in a given environment.
Natural selection is simply the process by which
individuals with certain phenotypes survive
better and reproduce more than individuals with
other phenotypes within a particular
environment.
Natural selection is only one of the forces that
can change gene frequencies (what are some
others?). However, most evolutionary biologists
agree that natural selection is the major driving
force of evolution. In this lab we will illustrate
how a common ecological event - predation - can
serve as an agent of natural selection. Predators
usually can find or capture some prey individuals
more easily than others. If the differences in prey
susceptibility are traits which are determined by
their genes, and therefore these traits are
heritable (offspring inherit these traits when they
get their genes from their parents), then the prey
population may evolve in the direction of
reduced susceptibility to capture.
The evolution of color patterns in the British
peppered moth, Biston betularia, is a classic
example of directional selection in response to
predation (and changing environmental
conditions). These moths, which rest on tree
trunks during the day, occur in a light colored
form with a sprinkling of dark spots and a dark,
or melanic, form.
In the woodlands around Manchester, England,
industrialization in the early part of this century
killed the lichens which normally grew on trees
and blackened the tree trunks with soot. As this
happened, the previously rare dark phenotype
came to dominate the woodlands and the light
form was rarely seen. In other areas, further from
the factories and smokestacks of Manchester,
lichens abounded on the tree trunks and the light
phenotype was common while the dark
phenotype was rarely seen. In each habitat the
common phenotype was better camouflaged.
Kettlewell (1955, 1961, 1965) knew that these
moths were preyed upon by birds during the day,
and he hypothesized that birds preyed selectively
on whichever phenotype was less cryptic,
causing the observed correspondence between
moth color and habitat color. By releasing both
phenotypes in each habitat and observing bird
predation from a hiding place he was able to
confirm this hypothesis. As further evidence,
industrial pollution around Manchester has
abated in recent years and the areas which had
soot-darkened, lichen-free tree trunks are
returning to their pre-industrial condition. In
BIOL 119, Fall 2012
response to this environmental change, the light
phenotype is once again replacing the dark
phenotypes in such areas (Berry 1990, Clarke et
al. 1990).
This type of selection, in which the distribution
of phenotypes is shifted in one direction (i.e.
light to dark, or dark to light) is called
‘directional selection’. Natural selection also can
act to maintain the existing distribution of
phenotypes in a population (stabilizing
selection), or selection can favor extreme
phenotypes at the expense of intermediate
phenotypes (disruptive selection). The goal of
this simulation is to learn how natural selection
functions, and to determine which of these three
types of selection is operating in each of the three
artificial environments.
Methods
Creating the Population
You will use ten different seed types that vary in
color and shape but are all approximately the
same size to represent the various phenotypes in
our population. This is meant to illustrate a
population of a single species in which there are
ten different color phenotypes. The initial
population will consist of 10 individuals of each
of the ten seed types (100 seeds in all). Each
group of students will conduct the experiment in
each of three habitat types: homogeneous white,
homogeneous packing peanut, and patchy.
Natural Selection
Scatter the seed population within the chosen
environment. We will imagine that the
population was formerly adapted to gaudy
surroundings; perhaps a tropical area with a great
array of flowering plants. A climatic catastrophe,
which the seeds have survived, has resulted in
these three new habitat types. One of you will act
as the predator (who also survived the
catastrophe). The predator ‘enters’ the populated
area and gathers seeds as rapidly as possible
within the guidelines for predator behavior given
below. The predator must always take the first
seed it sees. Seeds are to be taken one at a time
and must be dropped into an Erlenmeyer flask.
This will ensure that visual contact with the
foraging area is broken momentarily following
each seed capture. Foraging will continue until
the appropriate number of seeds has been taken.
The exact number of seeds taken in each seed
category must then be tabulated on the
Natural Selection
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appropriate line of the data sheet in order to
know which seeds remain in the population.
Determine the number of seeds of each type that
escaped predation by counting the number of
seeds of each type that were caught. After
counting them, place the captured seeds back in
the stock of unused seeds (please keep the seed
varieties separate). Record the number of seeds
of each phenotype that were captured and that
survived (see Sample Data Sheet).
Reproduction of Survivors
The reproductive process in our simulation will
be asexual to avoid the necessity of simulating
the genetic complexities of sexual reproduction
(and to keep your minds focused on the goals of
the lab!). Each survivor will produce offspring
that are genetically and phenotypically identical
to itself. Count out the appropriate number of
seeds of each variety, scatter these offspring onto
the habitat and start a new bout of predation.
Make sure you start with 100 seeds each time!
Different Habitats
Perform the experiment in each of the three
habitat types. A different student may be the
predator in different habitats, but all predators
should be able to distinguish the colors of all
seed types and have good visual acuity.
Rates of Predation
Pick one habitat and perform the experiment with
2 different predation rates. Be sure to balance the
rate of predation and the rate of reproduction so
that the total number of seeds is 100 at the start
of each predation session. For example, the
predator might take 50 seeds, in which case each
of the remaining seeds should produce a single
identical offspring. If the predator takes 80 seeds,
each remaining seed should produce 4 identical
offspring.
Summarizing Your Results
For each experiment, graph the number of seeds
of each type as a function of generation.
Alternative Experiment
Although natural selection is commonly referred
to as ‘survival of the fittest’, the true measure of
fitness is successful reproduction. An individual
that survives to a ripe old age, but who does not
leave any offspring, has a much lower fitness
than an individual who dies young after having
many offspring. This simulation deals only with
differential survival and does not address
BIOL 119, Fall 2012
differential reproduction. How might you design
an experiment to explore the interaction between
differential survival and differential
reproduction? How much of an increase in
reproduction would be necessary to balance a
higher mortality rate? You don't need to actually
do this experiment, but think through how you
would!
Hand In
1. A data sheet and a set of graphs for each
habitat type. The graphs should show the number
of seeds of each phenotype that were present at
the start generations 1 - 4.
2. Answer the following questions:
a. Which seed phenotypes were selected for or
against in each habitat type?
b. What type of selection (directional,
stabilizing, disruptive, other) is indicated by
each of the graphs? Did the type of selection
appear to remain constant for each habitat
type?
c. How did the pattern of selection differ
between the homogeneous and the patchy
habitats?
d. Did the effect of natural selection differ as a
function of the rate of predation?
e. What kinds of predators do you think would
be more or less likely to influence the
evolution of seed color?
3. Your alternative experimental design.
Literature Cited
Berry, R.J. 1990. Industrial melanism and
peppered moths (Biston betularia (L.)).
Biological Journal of the Linnean Society
39:301-322.
Clarke, C.A., F.M.M. Clarke, and H.C. Dawkins.
1990. Biston betularia (the peppered moth)
in West Kirby, Wirral, 1959-1989: updating
the decline in f. carbonaria. Biological
Journal of the Linnean Society 39:323-326.
Kettlewell, H.B.D. 1955. Selection experiments
on industrial melanism in the Lepidoptera.
Heredity 10:287-301.
Kettlewell, H.B.D. 1961. The phenomenon of
industrial melanism in Lepidoptera. Annual
Review of Entomology 6:245-262.
Natural Selection
Kettlewell, H.B.D. 1965. Insect survival and
selection for pattern. Science 148:12901296.
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BIOL 119, Fall 2012
Natural Selection
Sample Data Sheet
Habitat Type: _________________________
Seed Types:
Rate of Predation: ______________
BB:
Black Bean
PB:
Pinto Bean
BP:
Blackeyed Pea
SF:
Sunflower Seed
GP:
Green Pea
SP:
Split Pea
GN:
Great Northern Navy
SN:
Small Navy
KB:
Kidney Bean
YC:
Yellow Corn
Seed Type
BB
BP
GP
GN
KB
PB
SF
SP
SN
YC
Generation 1
10
10
10
10
10
10
10
10
10
10
Total
100
# captured
# surviving
# of offspring
Generation 2
100
# captured
# surviving
# offspring
Generation 3
100
# captured
# surviving
# offspring
Generation 4
# captured
# surviving
# offspring
100
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