Jasmine Hawkins
Principles of Biology II
February 13, 2013
Natural Selection in the Isopod Population
Introduction:
Organisms that inhabit a specific location is the definition of a population. In one single
population there is almost never an organism that consist of individuals that phenotypes are
exactly the same (Johnstone-Yellin, 2013). In natural selection the four most important aspects
are variability, heritability, reproduction, and survivability. Phelan defines natural selection as
“A mechanism of evolution that occurs when there is heritable variation for a trait and
individuals with one versions of the trait have greater reproductive success than individuals with
a different version of that trait (Johnstone-Yellin, 2013).” In easier terms natural selection is the
result of survival and reproduction success in a population. Darwin was the first scientist to look
into the importance of variation in natural selection (Johnstone-Yellin, 2013). Darwin suggested
that over time in a population, organisms change in trait, this can occur due to natural selection.
Isopods are common terrestrial crustaceans, which have over three-thousand species.
These isopods are primarily nocturnal and eat decaying organic matter. In the spring the activity
of these creatures is increased, is when most of the mating occurs. Female isopods carry seven to
two-hundred eggs for three to seven weeks after mating, and the offspring then remains in the
females pouch for six more weeks. After the six weeks the offspring will leave and begin mature
on their own. (Johnstone-Yellin, 2013)
The research done by Darwin on natural selection impacted my hypotheses. This
experiment had three hypotheses 1) the shorter the length, the higher the speed the isopod will
have, 2) the more dorsal plate numbers, the faster the isopod, and 3) the higher the sprint speed
the higher survival rate. To go along with my hypotheses I predicted that the smaller the isopod,
the faster it will be, the more dorsal plates would equal to more legs, and the faster the isopod,
the less chance it has of being caught. What is being tested in this lab is the variation in several
traits of the isopods and whether some of these traits give advantage to the survival in the
isopods, and whether the survival advantages depend on the type of hunter. (Johnstone-Yellin,
2013).
Results:
Figure 1. Individual data of length frequency (mm) of isopods (Isopoda) divided into victims and
survivors. Isopods were taken and those that were caught in a short frame of time were named
the victims and those that were not caught were named the survivors. Taken by random, the
isopods were then measured. The results shown displays the survivability based on size.
Figure 2. Individual data of dorsal frequency of Isopods (Isopoda) divided into victims and
survivors. Isopods were taken and observed under a microscope, so that the dorsal plates could
be counted. The isopods were also taken at randomly and assigned a number, then tested on. The
results shown displays the relationship between survivor and victim isopods based on the number
of dorsal plates.
Figure 3. Individual data of speed frequency (cm/s) of isopods divided into victims and
survivors. Isopods were taken by random selection to race from the start line to the finish line,
which was twenty centimeters, once the isopod reached the finish line the time was then stopped
and recorded. The results display the speed of victims and survivors based on the number of
isopods.
Figure 4. Pooled Data of average number of length (mm) in isopods divided into predator type.
All the data was taken and then averaged into the numbers you see above. The survivability
differs on predator type. The results shown displays the survivability based on size and predator
type.
Figure 5. Pooled data of the average number of dorsal plates in isopods based on predator type.
The results display that the combs as a predator type is more likely to survive rather than spoons
or forks
Figure 6. This graph represents pooled data of the average number of speed (cm/s) in isopods
based on predator type and speed. The results display that speed is a key component in
survivability within this Isopod population, regardless of predator type.
Table 1. Based on individual data where the predator type is spoon and the traits which are:
length, dorsal plates, and sprint speed. The results show that the Isopods had little to no
significance different when it came to survivability.
Trait
Sig?
Length
Mean
Variance
Range
d.f
C.V
tStat
p
2.25′
2.875″
2.214286′
1.839286″
5′
6″
14
2.144787
-0.87802
0.39475
0.34475
Dorsal
Plates
10′
10″
0′
0″
0′
0″
0
0
65535
0
0.05
Speed
4.2′
5″
7.2′
′ = victims ″ = survivors
5.5″
3.3′
4″
8
2.306004
-0.50196
.629214 0.579214
Table 2. Based on pooled data where the predator type is a spoon and the traits. The results show
that there are definitely differences when you look at the traits between victims and survivors.
The numbers for survivability is lower when looking at this table.
Trait
Mean
Variance
Range
d.f
C.V
tStat
p
Sig?
Length
33.45′
4.5″
943.945′
33.842″
19′
17″
38
2.024
4.140
0.0
0.5
Dorsal
Plates
90′
88.5″
0′
0.5″
0′
1″
2
4.302653
3
0.1
0.5
Speed
42.294′
44.470″
253.171′
′ = victims ″ = survivors
297.534″
2.09′
2.20″
100 1.983
-0.662
0.510
0.5
Table 3. Pooled data based off of the fork predator type and the length, dorsal plates, and sprint
speed as the traits. The results show that there is no significance difference in dorsal plates and
length, but in sprint speed the victims have slight higher numbers.
Trait
pLength
Mean
Variance
Range
d.f
C.V
tStat
Sig?
33.125′
33.438″
767.317′
677.063″
14′
13″
30
2.043
-0.032
0.974
0.92398
Dorsal
Plates
80′
80″
0′
0″
0′
0″
0
655
0
0
0
Speed
42.875′ 37.594″
167.790′
′= victims ″ = survivors
182.829″
1.91′
2.06″
62
1.999
1.596
0.116
0.065689
Table 4. Based on pooled data where the predator type is a comb. The results show that there is
little to no significance in length, dorsal plates, and sprint speed.
Trait
Mean
p
Sig?
Length 34.867′
33.467″
Variance
Range
d.f
C.V
tStat
541.124′
550.267″
14′
12″
28
2.048
0.164
0.871
0.821
2.145
0.599
0.559
0.509
0
1
0.95
Dorsal
Plates
70.875′
70.125″
5.840′
6.696″
7′
6″
14
Speed
37.396′
37.397″
186.791′
211.566″
2.16
′
1.90″
124 1.980
′= victims ″ = survivors
Discussion:
The experiment supported the hypothesis on speed and length because the results
corresponded with the predicted results. Figure 3 shows that the higher the speed frequency is the
more of a chance the isopod has to survive, but in Figure 6, the results tell us that the
survivability is, solely based upon the type of predator. We can take from these two figures that
speed is a trait that is needed for the survival of the isopods. When it comes to the length of this
species, the shorter isopods seem to have survived more than those that are longer in length,
when looking at Figure 4. When looking at the length frequencies you will see that the
survivability ranges and it is not based on if the isopods are longer or shorter. In other studies
there has been a correspondence between body size and gender to survivability. Grunder and
Sutton (1989) found that adult Philoscia muscorum had higher survivorship than smaller bodied
juveniles (Knight & Ozga, July 2001). Other studies have shown that habitat might have an
impact on growth (Knight & Ozga, July 2001). Looking at the data that from the dorsal plates
majority of the isopods had the same number of dorsal plates, meaning in my prediction was
rejected. The number of dorsal plates does not relate to the amount of legs that the isopods have.
The length and dorsal plates was a part of the experiment that needs to have more trials done, so
it can be compared to this and previous data. Future directions could be testing other arthropods
to see if the traits of survivability are the same for them also.
Works Cited
Johnstone-Yellin, T. (2013, February 5). Natural Selection Lab. Bridgewater, Virginia, United
States of America.
Knight, S. L., & Ozga, M. (July 2001). Costs of Reproduciton in the Terrestrial Isopod Porcellio
laevis Latreille (Isopoda: Oniscidea): Brood-Bearing and Locomation. Journal of Kansas
Entomological Society Vol. 74, No. 3, pp.166-171.