Explaining the Evolution of Threespine Sticklebacks in Nature
Using Realistic Data (Simulation & Case Study)
Learning Goal
 Develop student understanding of natural selection.
Learning Objectives
 Understand that fitness is determined by environmental conditions.
 Understand that different selective pressures lead to phenotypic differences
between populations.
Prerequisite Understanding
 Understand that populations are limited by a finite amount of resources.
 Understand that individuals differ in their traits.
 Understand that variation in populations arises through mutation and
recombination.
 Understand that only inherited traits will affect variation in the next
generation.
 Understand that fitness is the reproductive output of an individual relative to
other individuals in a population.
ACTIVITY
Overview
In recent evolutionary history, the marine stickleback has colonized numerous
freshwater lakes in the northern hemisphere. Colonizations, and subsequent
recolonizations, by the small, common fish have produced coexisting, divergent
populations of the threespine stickleback Gasterosteus aculeatus.
The following activity includes simulations and case studies based on marine
stickleback colonizations. The simulations are designed to engage students in
thinking about how selection affects variation in a population and how fitness of
phenotypes is determined by the environment. After completing the simulations,
students apply the understanding gained through the modeling exercise to data on
the phenotypic differences between stickleback populations.
Part 1: Students model change in fitness and variation over one generation
Part 2: Students model change in fitness and variation over six generations
Part 3: Students use inferential reasoning to explain phenotypic variation
between stickleback populations in different lakes
Part 4: Students apply understanding of fitness and variation to develop an
explanation supported by data for the evolution of threespine
stickleback species pairs.
PART 1: Single generation simulation
Individuals from a source population of marine sticklebacks with four types colonize
three different lakes. Each population starts with the same number of individuals
and types of individuals, yet each environment presents different selective pressures.
Using the “stickleback cards”, simulate the evolution of the populations by following
these steps:
1. Split the class into 3 groups. Each group gets a colonizing population of
stickleback cards, a large piece of paper divided down the center, the rules of
the simulation, extra stickleback cards, and a worksheet. Each group will be
in charge of simulating the evolution of one of the four populations (Lake A,
Lake B, Lake C).
2. Each colonizing population is made up of 20 fish: 14 small/well armored, 2
large/poorly armored, 2 small/poorly armored, 2 large/well armored. (NOTE:
each of these characteristics are inherited.)
3. Simulating the first generation:
a. Each fish must find a mate in order to reproduce. Sticklebacks mate
assortatively, so each type only mates with others of its type
(large/armored only with large/armored).
b. The selective pressures of the lake determine the number of offspring
of each pair of each type.
c. Define the selection pressure on each lake population (Table 1)
Lake
Selective Pressure
Lake A
Lake B
Predators eat poorly armored AND small fish
Predators eat small fish AND diet does not have the right
nutrients for building armor
Predators eat large fish AND diet does not have the right
nutrients for building armor
Lake C
Table 1. Lakes and selective pressures.
d. Identify the fitness of each type (high, average, or low) and determine
the number of offspring (Table 2). Types can be classed into high,
average, or low fitness based on the selective pressures of the lake.
Fitness
High
Average
Low
Number of offspring per pair
4 offspring
2 offspring
1 offspring
Table 2. Number of offspring per pair of sticklebacks
e. Create the next generation using the stickleback cards. Tape both the
colonizing population and the next generation to the posterboard.
f. What has happened to the population after one generation of
selection?
Part 2: Multigenerational simulation
1. Using the rules of the simulation and a fitness worksheet, groups project the
frequency of each type in their lake over 6 generations. G0 is the colonizing
population and G1 is the first generation just simulated. Fitness may at times
include high, average and low value individuals, at other times may include
only high and low value individuals, and at other times may include only high
value individuals. Students must be able to explain the assignment of fitness
values based on their understanding of fitness.
2. After 6 generations how has the variation in the population changed? How
has the frequency of each type changed? How has fitness of each type
changed? How many generations does it take for the population to have only
one type?
3. As a whole group, discuss the variation in each population, how did variation
change over time given different selective pressures? How did fitness of each
type differ in each population?
4. Repeat the activity, this time starting with different frequencies in each
colonizing population. How did variation change over time given different
selective pressures? How did of each type fitness differ in each population?
5. How would you predict the outcome of a colonization?
6. If you found two lakes whose sticklebacks differed in their traits, what would
you predict would be the cause of this?
PART 3: Phenotypic Variation between Lake Populations
1. Using the data in Table 3 and the lessons learned in Parts 1 and 2 come up
with an explanation for why number of armor plate and maximum body
length of sticklebacks differ between lakes.
Lake
Average number
Maximum body
Predatory Fish
of armor plates
length (mm)
present
Kumdis
6
54
Yes
Watt
0.5
57
No
Loon
2.8
63
No
Mayer
6.6
95.4
Yes
Hickey
4.5
69.9
No
Woodpile
5.2
63.8
No
Wiggins
1.5
63.4
No
Yakoun
6.2
50.2
Yes
Table 3. Characteristics of Lake Populations of Sticklebacks in the Queen Charlotte
Islands. Modified from Moodie and Reimchen 1976.
2. How would you test your ideas?
Part 4: Phenotypic Variation within a Lake
In a few lakes in British Columbia, pairs of stickleback species exist in the same lake.
These populations have been studied carefully by scientists. Student teams will
develop a hypothesis using actual data and Darwinian evolutionary theory to explain
how specific ecological conditions within a lake favor the divergence of the two
populations of sticklebacks in Paxton Lake. They will present their reasoning in a
poster-session format. Students should be prepared to display their poster
illustrating their reasoning and to explain/defend their reasoning. The poster should
include:







Descriptions of the traits in present populations.
Evidence of variation in present population.
Evidence of the heritability of traits.
Descriptions of selective pressures (environmental conditions affecting traits).
Evidence of selective pressures.
Explanation of result of selective pressures based on Darwinian natural
selection.
Conclusion.
A. Background Information
Ecology of Paxton Lake
Paxton Lake is located on Texada Island in coastal British Columbia and was formed
after the last ice age about 12000 years ago (McPhail 1992). It varies in depth from
5-15 meters, with shallow littoral areas dominated by terrestrial and aquatic plant
cover and pelagic areas of deep open water (Larson 1976). These littoral and
pelagic habitats are distributed in patches around the lake. The most abundant
organisms in the pelagic habitat are zooplankton and the most abundant organisms
in the littoral habitat are invertebrates that live in the aquatic plants.
Natural History of Sticklebacks
Two different forms of sticklebacks are found in the lake (Figure 1). Benthics are
deep-bodied, large sticklebacks that have mouth structures for eating invertebrates.
Limnetics are slim-bodied, small sticklebacks that have gill rakers for eating plankton
(McPhail 1992). Genetic evidence suggests that these are two populations that
interbreed rarely and are separately descended from two colonizations of marine
sticklebacks into the lake.
Figure 1. Paxton Lake Sticklebacks. (A) Limnetic stickleback (B) Benthic stickleback.
Modified from McPhail (1992)
B. Morphological Characteristics of Limnetics and Benthics
Many of the morphological differences between Limnetics and Benthics found in the
wild are evident even when fish are raised in the lab.
Characteristic
Limnetics
Benthics
Maximum Body Length
65 mm
100 mm
Average Body Length
45.3 mm
45.9 mm
Average Body Depth
8.86 mm
10.77 mm
Average number of armor
5.55
0.38
plates
Average number of gill
23.75
18.75
rakers
Table 4. Morphology of Benthics and Limnetics males caught in the wild. Modified
from McPhail 1992.
Characteristic
Limnetics
Benthics
Average Body Depth
8.86 mm
10.78 mm
Average number of armor
5.51
0.41
plates
Average number of gill
23.62
18.32
rakers
Table 5. Morphology of Benthics and Limnetics males raised in the lab. Modified
from McPhail 1992.
C. Feeding Habits of Limnetics and Benthics
Limnetic fish have small mouths with gill rakers, well suited for straining plankton.
Benthic fish have large mouths, well suited for eating macroinvertebrates. These
characteristics lead to different foraging behaviors of each fish.
Type of Food
Limnetics
Benthics
Plankton
0.75
0.04
Macroinvertebrates
0.13
0.83
Other
0.12
0.14
Table 6. Diet composition of Limnetics and Benthics. Measured as percentage of
diet in fish caught in September. Modified from Schulter and McPhail 1992.
D. Predation on Limnetics and Benthics
Limnetics are armored (protected from predation) and faster swimmers than
benthics. Benthics are able to hide in vegetation, are camouflaged against lakebottom substrate. These characteristics lead to different vulnerabilities to predation
in open water (pelagic) versus shallow water (littoral) environments.
Predation in the two habitats
1
0.9
0.8
Percentage surviving
0.7
0.6
Limnetic
0.5
Benthic
0.4
0.3
0.2
0.1
0
Pelagic
Littoral
Habitat
Figure 2. Predation in the two habitats. Measured as percentage of fish surviving in
predation experiments with the predators of each habitat. Modified from
Vamosi 2002.
E. Distribution of sticklebacks in Paxton Lake
Limnetics spend more time in shallow, open water. Benthics spend more time in
deeper water near vegetation or cover.
Depth (m)
0-1
1-2
2-3
Table 1. Depth preferences
number of fish per 10 m2 in
Limnetics
Benthics
4.0
0
19.1
1.4
7.0
2.2
of Limnetics and Benthics. Measured as average
September 1970. Modified from Larson 1976.
Substrate
Limnetics
Benthics
Above vegetation
30.8
0
In vegetation
0
0
Edge of vegetation
0
1.2
Open mud
0
0
Mud with cover
0
2.3
Branches
0
0
Table 2. Substrate preferences of Limnetics and Benthics. Measured as average
number of fish per 10 m2 in September 1970. Modified from Larson 1976.
References Cited
Larson, G.L. (1976). Social-Behavior and Feeding Ability of 2 Phenotypes of
Gasterosteus-Aculeatus in Relation to Their Spatial and Trophic Segregation
in a Temperate Lake. Canadian Journal of Zoology-Revue Canadienne De
Zoologie, 54(2), 107-121.
McPhail, J.D. (1992). Ecology and Evolution of Sympatric Sticklebacks
(Gasterosteus): Evidence for a Species-Pair in Paxton Lake, Texada Island,
British Columbia. Canadian Journal of Zoology- Revue Canadienne De
Zoologie, 70(2), 361-369.
Moodie G.E.E. & Reimchen, T.E. (1976). Phenetic variation and Habitat
Differences in Gasterosteus Populations of the Queen Charlotte Islands.
Systematic Zoology, 25(1), 49-61.
Schluter, D. & McPhail, J.D. (1992). Ecological Character Displacement and
Speciation in Sticklebacks. American Naturalist, 140(1), 85-108.
Vamosi, S.M. (2002). Predation Sharpens the Adaptive Peaks: Survival Trade-Offs in
Sympatric Sticklebacks. Annales Zoologici Fennici, 39(3), 237-248.
Activity Comments and Expectations
PART 1: Single generation simulation
Students should be to identify the initial fitness of each type of stickleback in each
lake. In Lake A (predators eat poorly armored AND small fish) large armored fish
have high fitness, large poorly armored fish have average fitness, small armored fish
have average fitness, and small poorly armored fish have low fitness. In Lake B
(predators eat small fish AND diet does not have the right nutrients for building
armor), large poorly armored fish have high fitness, large armored fish have average
fitness, small poorly armored fish have average fitness, and small armored fish have
low fitness. In Lake C (predators eat large fish AND diet does not have the right
nutrients for building armor), small poorly armored fish have high fitness, small
armored fish have average fitness, large poorly armored fish have average fitness,
and large armored fish have low fitness.
Students should be able to pair up the sticklebacks and assign them the appropriate
number of offspring based on the fitness of their type.
After one generation of selection, the frequencies of high fitness types have
increased and the frequencies of low fitness types have decreased. The population
has also changed in the number of individuals.
Part 2: Multigenerational simulation
Using the worksheet, students should be able to calculate number of each type
based on the fitness of the type and the frequency of each type in the previous
generation. They should understand that when there is a single individual of one
type, it cannot find a mate and does not reproduce. When low fitness types are lost
from the population, average fitness types are now considered to have low fitness.
After 6 generations, all the populations should have only the high fitness type.
In all population, the variation in the population has been reduced by selection. The
frequencies of each type have changed according to the fitness of the type relative to
others in the population.
Repeating the activity with different amounts of variation in the colonizing
populations (providing each still has each type) should have no effect on the
outcome of 6 generations of selection. Thus, selection pressures and fitness are
predicted to be the most important determinate of the phenotypes of stickleback lake
populations that are descended from marine ancestors.
Part 3: Phenotypic Variation between Lake Populations
Students should be able to relate the absence of predators as the environmental
condition that leads to larger body size and fewer armor plates in stickleback
populations. It is not the only factor, as some populations (for example, Mayer
Lake) have large armored fish. This exercise is meant to show the realistic nature of
the stickleback simulation.
The hypotheses that students develop could be tested by collecting data on
differences in the environment and correlating certain factors with certain aspects of
stickleback morphologies.
Part 4: Phenotypic Variation within a Lake
Students should identify two separate environments exist within Paxton Lake and
that Limnetics have adapted to live in the pelagic habitat (feeding on plankton and
surviving predator attacks by fast swimming) and Benthic fish have adapted to live in
the littoral habitat (feeding on macroinvertebrates and surviving predator attacks by
hiding in the vegetation). The morphological adaptations to the environments that
Limnetics and Benthics have are heritable since they are still present in fish raised in
the lab. Students should be able to link the logic of the previous parts of the activity
to explain how populations of marine sticklebacks, experiencing different
environments and selective pressures, could diverge into morphologically distinct
populations, even in the same lake.