Computer Simulations on Evolution – BiologyLabs On-line
The following was modified from http://www.biologylabsonline.com/protected/EvolutionLab/
Objectives: EvolutionLab will allow you to study important principles of
evolution by examining small populations of finches on two different islands,
"Darwin Island" and "Wallace Island." You will manipulate important an
ecological parameter that imposes natural selection and aspects of the
populations such as their genetic structure and then track how these
variables alter patterns and rates of evolution through natural selection.
This laboratory exercise mimics the kind of research that evolutionary
biologists frequently do because evolutionary changes can take many
generations to become apparent and because it is not usually possible to
experimentally alter ecological conditions on the scale necessary to observe
evolution in larger organisms. When you have completed this lab, you should:
1. Understand the link between population characteristics such as
mean, variance, and heritability of a trait and evolutionary changes in
phenotype
2. Recognize the relationship among ecological factors, variation in
phenotype, and variation in individual fitness
3. Understand how differences in ecological conditions between
populations can lead to divergent evolution
Background Information
Evolutionary biology is the study of the genetic, structural, and functional
modifications of populations that occur from generation to generation
through selection on individuals. Understanding how life evolves is a central
concept in biology. The current diversity of organisms and the correlation of
a species’ phenotype to its environment are the direct result of evolution
that has occurred over very long periods of time.
The English naturalist Charles Darwin is generally recognized as the person
who first proposed that evolution could occur through natural selection and
sexual selection. These theories, well supported by over 100 years of data,
are the basis for our current-day understanding of evolution.
Several years after Darwin's initial voyage to the Galápagos Islands, he
developed several hypotheses that he felt explained how diverse species of
tortoises and finches may have developed on the different islands. He
realized that although the populations of these organisms could grow rapidly
(as each individual has many offspring), tortoise and finch populations did
not increase in an uncontrolled manner; rather, the populations of these
animals were relatively constant. This indicated that, on average, each
breeding pair only produced two surviving offspring. If some individuals have
higher success, then other individuals must have lower success or the
population size would be increasing.
Darwin noted that individual organisms in a population varied in phenotype
(structure and function). He reasoned that if some individuals within a
population were better suited to the environment (found food more quickly,
escaped predators more easily, digested food more efficiently), they would
be more likely to leave offspring compared to individuals with different
phenotypes. If phenotype is inherited from generation to generation then
the population would, over several generations, come to more closely
resemble the type that had the better phenotype: it would be adapted to
the environment.
Darwin named this force of evolution natural selection, to contrast it with
the artificial selection that humans impose upon domesticated animals and
plants. It is important to realize that while natural selection happens to
individuals, the population can only be said to be evolving if there are
changes in mean phenotype over time.
In the 1970s, the evolutionary biologist Richard Lewontin formalized the
requirements for a population to evolve through natural selection, writing a
paper with the following list:
1. There must be variation in phenotype among individuals
2. This variation must be heritable (passed from parent to offspring)
3. This variation must result in differences in fitness among phenotypes
If any one of these three components is absent – if there is no variation in
phenotype (all humans are born with four limbs), or the variation that occurs
is not heritable (tatoos), or if the variation that does occur does not alter
fitness (blood type does not determine how many offspring a person has) –
then natural selection cannot cause evolutionary changes in phenotype.
Evolution is a central theme that unifies and connects virtually all disciplines
of biology. However, because evolution occurs over long spans of time and is
difficult to observe directly, the mechanisms that drive natural selection do
not lend themselves to experimentation during a few hours or even a
semester spent in the biology lab. In EvolutionLab, you will have the
opportunity to simulate experiments in evolution to help you understand how
traits of an organism can change in response to different biological and
environmental conditions. You will study how beak size and population
numbers for two hypothetical populations of finches on two different islands
can evolve in response to ecological factors that you will manipulate by
changing environmental conditions on these islands. EvolutionLab is not an
exact simulation of finch beak evolution; rather it is a model designed to
demonstrate important principles of evolution by natural selection.
The finches on Darwin and Wallace Islands feed on seeds produced by plants
on those islands. The simulation includes three categories of seeds: small,
soft seeds, large, hard seeds, and an intermediate category of seeds. The
plants producing the small seeds are more abundant in rainy years and those
producing the large seeds are more abundant in dry years. EvolutionLab is
based upon a model for the evolution of quantitative traits – characteristics
of phenotype that are controlled by a large number of genes (each gene with
a small affect). These traits are studied by looking at the statistical
distribution of traits in a population, and this simulation produces data
describing how the distribution of beak size within a population changes over
time. The fitness of a particular individual will be determined by the
environmental characteristics and also by random (stochastic) events that
affect which individuals actually survive and reproduce. In the experiments
described below, you will investigate how the effects of natural selection
are altered by the three components listed as required by Lewontin.
Introductiory run: First, change no settings and run the experiment. Were
the default settings set at a point where there will be evolutionary change?
What evolutionary change do you see?
Explore the different outputs: what does each graph present?
Experiment 1: The role of seed size in the evolution of finch beak depth.
One of the classic studies in evolution is the work by the Grants on the
Galapagos Island finches. Their data, covering over 30 years of study, show
the importance of precipitation in determining the size of seeds available,
and the size of seeds in determining which birds survive the dry season. We
can mimic this research, and then expand upon it, using the simulations in
EvolutionLab. We will start by documenting the evolutionary force imposed
by changes in precipitation and seed size.
Click on “Change Inputs”. Note that you have a great deal of control on the
biology and environment experienced by the birds on these two islands. To
start, we will leave everything at the default setting except precipitation
/seed size. Set the precipitation on one island to generate seeds that are
88% small and soft, and on the other island to generate seeds that are 88%
large and hard.
Record your inputs here:
Darwin
Wallace
beak size
variance
heritability
island size
initial population size
Precipitation
We will start with the hypothesis that “Differences in seed size between
the two islands will cause the evolution of differences in beak size between
the two populations.” List your predictions for the results of this experiment
below as answers to the following questions. Note that “no difference” is a
perfectly reasonable predicted result for an experiment. These are
predictions for your experiment and comparisons of your predictions with
your results constitute a test of the hypothesis.
a. Which individuals will survive on each island at year 10? Year 150?
b. After 300 years, which population will have larger bills?
c. After 300 years, which population will have more individuals?
Choose 300 years as the running time for this experiment, and run the
model. To test your predictions, look at the data outputs it generates.
a. Click on Histogram. The first plots show the beak size of surviving
individuals from the first year of the experiment, for both islands. Do
these data support or refute your first prediction?
Scan forward in time 150 years. Has the pattern of which animals survive
changed?
b. Look at the change in beak size over time. Is your second prediction
supported or refuted?
c. Click on population. Do these data support or refute your third
prediction?
Overall, what is the relationship between beak depth and seed
size/precipitation?
Overall, do the data cause you to accept or reject our initial hypothesis? If
they refute the hypothesis, write a revised hypothesis here:
Experiment 2: The importance of phenotypic variation. Click “New
Experiment” to restart the simulation.
Click “change inputs” and choose “variance”. The variance is the measure of
how variable the individuals in the population are for beak depth. Leaving all
other settings at their default values (equal between the two islands), set
the variance of one population to be less than 1.0, and the variance of the
other to be twice as high (in other words, one population is 2x more variable
than the other, and the one with lower variance has variance less than 1.0).
Record your inputs here:
Darwin
beak size
variance
Wallace
heritability
island size
initial population size
precipitation
Based upon what you learned in the first experiment, and upon the
introduction, what is the role of phenotypic variation in evolutionary change?
Write your testable hypothesis here (you need a statement of how the
difference in variation will alter evolution through natural selection).
Before running this experiment, answer these questions to generate
predictions based upon your hypothesis (again, answering “no difference” is
an acceptable prediction). Comparison of the results with these predictions
will allow you to refute or accept your hypothesis.
a. Which individuals survive on each island (large or small beaked)?
b. Which population will evolve more rapidly?
c. After 300 years, which population will have larger bills?
d. After 300 years, which population will have more individuals?
Choose 300 years as the running time. To test your hypothesis, look at the
data outputs.
a. Click on Histogram. The first plot shows the beak size of surviving
individuals from the first year of the experiment. Do these data match your
first prediction?
Scan forward in time 150 years. Has the pattern of which animals survive
changed?
b. Look at the graph presenting average beak size over time. Do these data
match your second prediction?
c. Do the data match your third prediction?
d. Click on population. Do these data match your fourth prediction?
Overall, do the data cause you to accept or reject your hypothesis? If they
refute your hypothesis, write a revised hypothesis here:
Experiment 3: The role of heritability. Heritability is a term referring to
how much of the phenotype of the offspring is determined by their genetics,
and is estimated by comparing offspring phenotype to parent phenotype. In
simple terms, the stronger the role of genetics, the higher the heritability
(this breaks down when there is no variation at all in phenotype among
individuals).
Click “New Experiment” to restart the simulation.
Click “change inputs” and choose “heritability”. Leaving all other settings at
their default values (equal between the two islands), set the heritability of
one population to be 1/4 the heritability of the other population. Record
your settings here:
Darwin
Wallace
beak size
variance
heritability
island size
initial population size
precipitation
Before running this experiment, formulate your hypothesis about how
differences in heritability will alter the evolution of beak size:
To test your hypothesis, answer these questions to produce predicted
results for your experiment (again, answering “no difference” is an
acceptable prediction).
a. Which individuals survive on each island (large or small beaked)?
b. Which population will evolve more rapidly?
c. After 300 years, which population will have larger bills?
d. After 300 years, which population will have more individuals?
Choose 300 years as the running time. To test your hypotheses, look at the
data outputs.
a. Click on Histogram. The first plot shows the beak size of surviving
individuals from the first year of the experiment. Do these data agree with
your first prediction?
Scan forward in time 150 years. Has the pattern of which animals survive
changed?
b. Look at the plot of beak size over time. Do the data agree with your
second prediction?
c. Do the data agree with your third prediction?
d. Click on population. Do these data agree with your fourth prediction?
Overall, do these data support or refute your hypothesis? If they refute
your hypothesis, write a revised hypothesis here:
Experiment 4: Genetic drift.
What happens if there are no differences among individuals in fitness?
Clearly, there is no natural selection if phenotypic differences do not cause
differences in fitness. Does evolution stop? To test this hypothesis, we will
run the following simulation 3 times.
For both islands, set precipitation at 66 cm. Set the island size for one
island at the maximum, and the minimum for the other. Then set population
size on the larger island at the maximum, and on the smaller island at the
minimum. Record your settings here:
Darwin
Wallace
beak size
variance
heritability
island size
initial population size
precipitation
Run the simulation and examine the plots of beak size versus time, and
population size; summarize the patterns you see in the table (below) for “run
1”. Hit “revise experiment” but do not alter the inputs, and run the
experiment a second time. Enter your results for “run 2” and then run the
experiment a third time.
Run 1
Run 2
Run 3
What you are observing is genetic drift: evolutionary changes (change in
mean beak size) without selection, due simply to the random selection of
which birds will survive and reproduce each generation. To be sure you
understand what is happening, answer the following questions:
Did the larger or smaller population change more?
Were the lines tracking changes in beak size over time smooth or irregular?
Did population size affect the “smoothness” of the line?
Did the larger or smaller population vary more across the three runs?
If you could push this out to 600 years, do you think the pattern would
change?
References
1. Darwin, C. On the Origin of Species by Means of Natural Selection, or
the Preservation of Favored Races in the Struggle for Life. New York:
New American Library, 1963.
2. Lewontin, R. C. (1970). "The Units of Selection". Annual Reviews of
Ecology and Systematics 1: 1-18.
3. Grant, P. R. "Natural Selection and Darwin's Finches." Scientific
American, January 1991
4. Weiner, J. The Beak of the Finch: A Story of Evolution in Our Time.
New York: A. Knopf, 1994.