Natural selection operates on populations that have variable characteristics.
If some of the variants are "better" than others, they are more likely
to allow individuals that possess them to survive and/or reproduce. These better
variants will then increase in frequency in subsequent generations.
We will explore variation in the weight of bean seeds in order to illustrate
the concept of variation.
1) Weigh 50 individual bean seeds to the nearest 0.01 g and record the data
for each seed in the table
below. Discard any beans that are broken or split in half. Before you begin, be sure to zero the scale by pressing the tare button
or bar. Click
here for additional instructions on using the scale.
After 50 beans have been weighed, it is suggested that you work on another
part of this exercise in case there is not enough time during this period to
finish. The remainder of this part can be finished at home if necessary.
Find the smallest weight in the table above and record it in the first cell
(under Weight) in the table below. Count how many beans from the table above
had this weight and enter it in the cell to the right.
2) Next, add 0.01 to the number in the first cell below and record this new
number in the cell underneath. Then, count how many beans had this weight. If
there were none, put a zero.
Repeat this procedure until all of the beans have been counted.
3) Make a bar graph of the data in the table above by plotting weight on the X
axis and number of beans on the Y axis. Use a computer graphing program for your graph.
Plot your data so that you have between 7 and 14 bars on the graph. If you
have more than 14 weight categories, combine categories. For example, if your
smallest seed weighs 0.40 and your largest seed weighs 0.60 (20 categories)
combine 0.40 and 0.41 into one category. Then combine 0.42 and 0.43 etc. If you
have 30 categories, it will be necessary to combine them in groups of 3.
4) Beans have approximately 1940 calories per gram. Calculate the number of
calories of energy that was required by the plant to produce the largest
bean that you weighed. (Note- If you do not have a calculator, use the one
on the computer. Click Start, Programs, Accessories, Calculator).
5) Calculate the number of calories of energy that was required by the
plant to produce the smallest bean that you weighed.
6) Suppose that an average plant has 100,000 calories available to produce beans. How many of
the smallest beans can it produce?
7) How many of the largest beans can it produce?
8) If all the bean seeds survive and germinate in a warm climate, which
plant will have the greater reproductive output, the one that produces
small beans or the one that produces large beans?
9) Suppose that during a cold year, larger beans survive and germinate
better than small beans because they have more stored energy available for
germination. Suppose also that the climate changes and becomes very cold
such that the smallest beans have a 25% survival rate and the largest ones
have an 95% survival rate. If an average plant has 100,000 calories
available, calculate the reproductive output in a cold
climate for the plant that produces the smallest beans. (First calculate
how many seeds that this plant will produce, then calculate how many of
these will survive.)
10) Calculate the reproductive output for the plant that produces the
largest beans.
11) Which plant will have the greatest reproductive output in a cold
climate? Which plant will have the greatest reproductive output in a warm
climate? (See your answer to the answer to #5 above.)
12) How will natural selection act on the size of bean seeds in areas where
the climate is slowly changing from warm to cold?
13) Suppose that in extremely cold conditions such as are found in arctic
areas, only the very largest beans are capable of survival. What would likely
happen to a population in in the arctic if there were no variation in the weight
of bean seeds; all seeds medium in size and weighed the about the same?
Model Population
In this exercise, we will explore what happens to the gene frequency of a
population from one generation to the next.
The model population will consist of diploid, sexually-reproducing insects.
Each individual in the model population will consist of two plastic beads
snapped together.
Although each individual has many thousands of genes, we will let each bead
represent a gene for the color of the insect. In this species there are two
colors: red and yellow. Red beads will be used to represent the gene (allele)
for red-colored individuals. We will also use the letter "A" to
represent this gene. Yellow beads will be used to represent the gene for
yellow-colored individuals. The letter "a" will also be used to
represent the yellow gene. Each individual must have two beads because animals
are diploid.
Construct an initial population that has the following characteristics:
Number of
Individuals |
Genotype |
| 2 |
AA |
| 8 |
Aa |
| 8 |
aa |
Gene frequency refers to the
proportion of alleles
that are of a particular type. For example, if 60% of the alleles in a population are
"a" and 40% are "A", then the gene frequency of "a" is 0.6
and the gene frequency of "A" is 0.4.
1) Calculate the value of p (the gene frequency of "A") in the population that you constructed? [Hint- The total number
of genes in this population is 36.]
2) Calculate the value of q (the gene frequency of "a").
Gametes
Normally males produce sperm and females produce eggs. In order to simplify
our model, we will ignore the sex of the individual. Assume that any
individual can mate with any other. Also for simplicity, we will have each individual produce four gametes (sperm
or eggs) when it reproduces.
3) How many "A" gametes (single red beads) will an "AA"
individual produce?
4) How many "a" gametes (single yellow beads)?
5) On average, how many "A" gametes will an "Aa"
individual produce?
6) How many "a" gametes?
7) How many "A" gametes will an "aa"
individual produce?
8) How many "a" gametes?
Gametes produced by each individual will be placed in a beaker. Each of the
two "AA" individuals produces 4 gametes for a total of 8 gametes.
Place 8 single red beads in the beaker to represent the gametes produced by these
individuals. Do not disassemble the initial population; use additional beads
to represent the gametes.
Each "Aa" individual produces 4 gametes each for a total of 32
gametes. Place 16 red and 16 yellow beads in the beaker to represent
reproduction of "Aa" individuals.
Each "aa" individual produces 4 gametes for a total of 32
gametes. Place 32 yellow beads in the beaker to represent reproduction by
"aa" individuals.
Mating
Mate selection in this population is approximately random, so you will
simulate mating by mixing
all of the beads in the beaker.
9) Randomly select two beads from the beaker to represent a sperm and an egg.
Snap the two beads together to represent a new individual produced by a sperm
fertilizing an egg. Put a tally mark in the table below next to the genotype of this
individual.
Continue removing the beads two at a time, snapping them together, and tallying them below until you have
removed and counted all of the individuals produced by these gametes.
| Genotype |
Tally |
| AA (two red beads) |
|
| Aa (one red and one yellow) |
|
| aa (two yellow beads) |
|
10) The gene frequency of "A" and "a" in the second
generation can be calculated from the table above. Calculate these frequencies
and record your answers in the table below.
| |
Initial Generation |
2nd Generation |
| Frequency of "A" |
0.33 |
|
| Frequency of "a" |
0.67 |
|
11) What happened to the gene frequency from the initial generation to the
next generation?
12) If each individual produced 1,000 gametes instead of 4, do you think
that the gene frequency would change?
13) If this experiment were carried out for another generation, what do you
predict would happen to the gene frequency? If you are unsure of the answer to
this question, continue the experiment for another generation.
Natural Selection
In this exercise, you will examine what happens to the gene frequency of an
allele when natural selection acts to change the frequency.
Assemble an initial population using the information in the
table below. You may reuse the initial population from the previous exercise.
Number of
Individuals |
Genotype |
| 2 |
AA |
| 8 |
Aa |
| 8 |
aa |
Suppose that the yellow (aa) insects produce fewer offspring than red (AA and
Aa) insects. This could be due to a number of reasons. Perhaps yellow insects
have lower survival because their predators can see them easier. Maybe they are
not as efficient at finding food or nesting sites. Whatever the reason, each
"aa" individual produces only 2 gametes (2 offspring). The other two
genotypes (AA and Aa) produce 4 gametes each.
When this population reproduces, each of the two AA individuals will produce
4 gametes for a total of 8 gametes. Place 8 red beads in a beaker to represent
these gametes.
When each Aa individual reproduces, 4 gametes will be produced, two red and
two yellow. A total of 16 red and 16 yellow beads should be placed in the beaker
to represent the gametes produced by these 8 individuals.
Each aa individual produces only 2 gametes. A total of 16 yellow beads should
be added to the beaker.
14) Draw the beads two at a time to represent diploid offspring and tally
their genotype in the table below.
| Genotype |
Tally |
| AA (two red beads) |
|
| Aa (one red and one yellow) |
|
| aa (two yellow beads) |
|
15) Calculate the gene frequencies of "A" and "a" from
the table above and record your answers in the table below..
| |
Initial Generation |
2nd Generation |
| Frequency of "A" |
0.33 |
|
| Frequency of "a" |
0.67 |
|
16) What was the effect of natural selection on the frequency of
"A"?
17) What was the effect of natural selection on the frequency of
"a"?
The Founder Effect and Migration
Place the individuals of your original population in a beaker (2 AA, 8 Aa,
and 8aa).
Randomly select two individuals to start a "new" population. These
two individuals represent emigrants to a new area.
18) Enter the number of each genotype in the new population.
| |
Initial Population |
New Population |
| Number of AA |
2 |
|
| Number of Aa |
8 |
|
| Number of aa |
8 |
|
19) Record the gene frequency of the new population in the table below.
| |
Initial Population |
New Population |
| Frequency of "A" |
0.33 |
|
| Frequency of "a" |
0.67 |
|
Starting a new population from a few individuals is called the
founder effect.
20) Did the founder effect result in a change in gene frequency when compared
to that of the original population?
The gene frequency of the initial population may change when the emigrants leave.
21) Count the number that remained in the initial population after
emigration. Enter this number in the table below.
| |
Initial Population |
Number Remaining |
| Number of AA |
2 |
|
| Number of Aa |
8 |
|
| Number of aa |
8 |
|
22) Calculate the gene frequency of the initial population after the emigrants
left and record it in the table below.
| |
Initial Population |
Initial Population
After Emigration |
| Frequency of "A" |
0.33 |
|
| Frequency of "a" |
0.67 |
|
23) Did emigration change the gene frequency?