Name: ______________________
The Never-Ending Tournament
Lab Activity: Genetic Variety in Goldfish
In natural populations of plants and animals, variation
always exists among members of the same species. Just
consider the many differences between you and your
classmates! But in other species, this variation might not
seem so obvious. At first glance, all goldfish may look more
or less alike, and the differences between them may seem
small and insignificant. Yet appearances can be deceiving.
(Of course, to a goldfish all humans probably look the same,
too!)
The Far Side by Gary Larson
The variety of traits within a population is natural and
normal. It arises from a handful of reproductive mechanisms
that are forever reshuffling, recombining, and remaking our
genes:
o Mutations (DNA makes a mistake during replication,
or “self-copying”)
o Crossing over and independent assortment (during sperm and egg production, each
gamete receives a different mix of genes and chromosomes)
o Sexual reproduction (any of a father’s genetically unique sperm can fertilize any of a
mother’s genetically unique eggs)
o Random mating (any female can mate with any male within a population)
Most of these mechanisms merely create novel combinations of the genes and traits that
already exist. DNA is like a deck of cards, and sex is a way of shuffling the deck and dealing
out different hands to one’s offspring. Mutations, however, actually create new genes and new
traits. It’s like adding a few wild cards, never before seen, to the deck. That means that the
potential for genetic variety within a population is not only enormous, but really endless!
In this lab you’ll explore the variation that exists in a small population of goldfish, with
respect to three traits: body mass, fork length, and caudal span. You will use this data to
predict how differences in these traits might affect survival, reproduction, and the population’s
future.
Procedure
1. Proceed to the first aquarium and use a dip net to obtain one goldfish. Place it in a petri
dish and take it to your lab table. (Don’t worry – the fish won’t suffocate. Goldfish were
chosen for this lab because they are hardy and can withstand several minutes out of water.
Even so, make your measurements as swiftly as possible. And be nice to the fish!)
2. Accurately measure to the nearest millimeter (not centimeters!) the fish’s caudal span, the
height of the tail from tip to tip (see diagram). Try to fan the tail open in a natural way, not
spread unnaturally wide, yet not folded up like fan. For reference, have a look at the caudal
fins of goldfish still swimming in the tank. Record data in the table below.
3. Now measure the fish’s fork length, the distance in millimeters from the tip of the nose to
the crook where the tail’s upper and lower lobe meet (see diagram).
4. Finally, measure the fish’s body mass to the nearest tenth of a gram. Don’t forget to
subtract the weight of the dish!
5. Return goldfish to the second aquarium (so as not to measure the same fish twice).
6. Repeat steps 1-5 for nine more goldfish.
7. Now for each fish, calculate two indices of tail size relative to overall body size: (a) the ratio
of caudal span to fork length, which we’ll call the “Tail-to-Length Ratio” and express as a
percent, and (b) the ratio of caudal span to body mass, which we’ll call the “Tail-to-Weight
Ratio” and express in millimeters per gram. Use these formulas:
Tail-to-Length Ratio
Caudal Width (mm)
=
 100%
Fork Length (mm)
(expressed as a percent)
Tail-to-Weight Ratio
Caudal Width (mm)
=
Body Mass (g)
(expressed in mm/g)
DATA TABLE
Fish number
1
2
3
4
5
6
7
8
9
10
Caudal span (mm)
Fork length (mm)
Body Mass (g)
Tail-to-Length Ratio (%)
Length Class
(rounded to nearest 5%)
Tail-to-Mass Ratio (mm/g)
Weight Class
(rounded to nearest even #)
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8. Round your raw Tail-to-Length Ratios to the nearest 5% and record as “Length Class.”
Round your Tail-to-Mass Ratios, to the nearest even number and record as “Weight Class.”
9. Tally your Length Class and Weight Class data on the board for all 10 fish.
10. Once data for the entire population of goldfish is up on the board, construct two histograms
of the whole class data, one for Length Class, one for Weight Class. A histogram is just a
bar graph that plots the frequency (y-axis) of each trait (x-axis) in the population.
Class Data (all fish)
Weight Class
Frequency
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Frequency
Histogram of Length Class
Length Class (%)
Histogram of Weight Class
Frequency
Class Data (all fish)
Length Class
Frequency
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Weight Class (mm/g)
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Analysis & Interpretation
1. What is the Mean Length Class for the entire population of goldfish? What is the Mean
Weight class? (The “mean” is the average. You can use the histogram to figure this out:
(a) multiply each length by its frequency in the population, (b) add all these products
together, and (c) divide by the total number fish in the population.)
2. What is the Modal Length Class for the entire population? The Modal Weight Class? (The
“mode” is the most frequent or most common value.)
3. In any species of plant or animal, you can measure almost any trait and you will usually get
a bell-shaped distribution of frequencies. Both the mean and mode will typically occur
near the peak of the bell, and as you move left or right away from the mean and mode, the
traits will diminish in frequency. The lowest frequencies will usually occur at the “tails” of
the range of traits. Roughly speaking, does the goldfish data obey this pattern? Describe.
4. The tail size of each goldfish (relative to its body length and weight) is probably a
polygenic trait, influenced by multiple genes at multiple loci on multiple chromosomes. As
you can see, this trait varies, and so therefore do the underlying genes. Now, pet store
goldfish are raised in captivity where the size of a fish’s tail matters little to its survival. But
suppose we were to release this population of goldfish into a wild habitat, like a freshwater
pond. In the wild, which fish from your histograms would you predict to survive longest?
Why? Describe some situations where you predict tail size could affect a fish’s odds of
survival.
5. Given your prediction in #4 above, which goldfish will be able to produce more offspring?
Why?
6. Given your predictions in #4 and #5 above, what would probably happen to the Mean and
Modal tail ratios over many generations? Explain your reasoning. At the level of genes for
tail size, what would be happening?
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Frequency
6000
4000
2000
0
0
10
20
30
40
50
Tail-to-Body Ratio, Present Day
Frequency
6000
4000
2000
0
0
10
20
30
40
50
Tail-to-Body Ratio in 500 Generations
6000
Frequency
7. The distribution of traits in a
wild population very often
takes the shape of a “bell
curve,” like the one on the
right. This shows that most
members of the population
have traits that are fairly
“average,” yet a few have
traits toward the extremes.
Draw curves in the second
two
graphs
that
are
consistent
with
your
prediction in #6 above. Be
careful! Make certain you
know what the vertical y-axis
IS (the frequency of each
size class) and what it ISN’T
(it’s NOT tail size). For the
purpose of this exercise, let’s
assume that this is a stable
population of goldfish, with
each fish that dies being
replaced by exactly one
newborn. In that case, the
TOTAL number of goldfish in
the pond wouldn’t change
much from generation to
generation, even though the
trait distribution might.
4000
2000
0
0
10
20
30
40
50
Tail-to-Body Ratio in 1,000 Generations
8. Look around. If you were to measure the height of your classmates, would it plot out as a
bell curve? Now suppose a diabolical biology teacher locks your entire class in a gymnasium
without any bleachers, tables, etc. There’s nothing in there except basketball goals. Then
he or she releases into the gym a huge pack of starving, bloodthirsty miniature poodles.
Which classmates would be most likely to survive this ordeal and live long enough to pass
on their height genes? Explain.
Suppose instead that this insane, sadistic teacher locks you all in a cafeteria that is empty
except for some doghouses scattered about. Then, rather than poodles, he or she releases
a gang of ferocious, hungry, lumbering Barney dinosaurs. Now who would make it? Why?
The Moral: What REALLY determines whether a particular trait is “good” or “bad,” beneficial
or harmful?
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