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Variation in Natural Populations 1
Defining Species
Variation in Natural Populations
The Problem of Species
The most famous work by Charles Darwin was his book, On the Origin of Species. The
idea of species is central to all of biology. Just as energy is central to physics, and the
elements are central to chemistry, so the reality of species of animals and plants has
dominated biology for hundreds of years.
But what is a species? Textbooks give a variety of definitions. This should warn that the
term is not at all clear! the most common definition might be: a group of animals or
plants that share a set of phenotypic characteristics (anatomy, physiology, behavior[for
animals]) and share a similar genetic make-up that makes them distinct from all other
groups of animals or plants. This group is also reproductively isolated from other
groups such that individuals of the group cannot breed successfully with an individual of
other groups. Thus, a species of bird, say the yellow warbler (Dendroica petechia), is a
species because no other bird looks exactly like the yellow warbler and can breed with
it. A species is therefore a unit in the great diversity of life we see around us. It is a
natural unit because species really exist, not just in the mind of humans, but as real
entities in nature. Ever since Darwin, naturalists have added another part ot the
definition: a species has a special evolutionary history -- the species exists today
because all the individuals in the species share a common ancestry over evolutionary
time.
Let’s examine this definition in more detail. First, how different do two populations have
to be before we can call them different species? Usually, no single trait can define a
species. Very often, to define a species we must describe a number of differences and
some of these differences might not be simple either-or situations. The exercise you will
do in Centennial Woods will illustrate this further.
The issue of reproductive isolation is also difficult. The African Lion and the Tiger will, if
put together in a cage, either fight non-stop, or will eventually settle down, and if bored
enough, will breed. Baby Lion-Tiger hybrids are sometimes born alive (viable), but don’t
do very well and often die young. They are typically sterile. The domestic dog and the
coyote can interbreed and produce fertile offspring. These “coydogs” do not breed in the
wild because their reproductive cycle is so different from either dogs or coyotes. These
two examples show us that there are many cases of cross-specific breeding that can
lead to offspring, but this does not mean the two forms are in fact the same species.
What is important to remember about the whole issue of reproductive isolation? We
want to know that if in nature individuals of the two species will court, mate, and
produce living, healthy, fertile offspring. Behavior is an important component of
reproductive isolation. Lions and tigers today do not have overlapping geographical
distributions, but did in the past. Lions are social animals and breed within their groups,
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Variation in Natural Populations 2
tigers are solitary. Their behaviors are so different that it would be impossible for them
to breed in nature.
Some writers in the past have claimed that if we use artificial insemination techniques to
produce hybrids between two species this shows they are not really different species at
all -- that is, they are actually a single species. This is a gross error in understanding the
meaning of species.
Here is another problem with the reproductive isolation requirement in our definition of
species. What do we do about species that reproduce without sex? Many single-celled
organisms are asexual, and there are even 35 species of vertebrate animals (fish, frogs,
salamanders, lizards, and perhaps some snakes) that reproduce asexually (in the
asexual vertebrates there are only females in the species!). In these cases we must use
the rest of our definition of species, especially the similarity in genetic make-up
Evolution and the Concept of Species
If evolution is taking place now as well as in the past, then new species must be
evolving all the time. Therefore, in nature we should find that the boundary between
species may sometimes be unclear. Suppose there is a widespread species we call A,
and in a small valley in the Green Mountains a population of A is changing over time
and eventually will form a new species that we could someday call B. But, today this
population of almost B is different from A, may have some problems in reproducing
with old-fashioned A, but is not completely reproductively isolated. It is clearly not a real
new species. What do we call this? Although such populations might cause some
confusion to biologists who would like to give it a new name, we expect to find such
situations if evolution is indeed going on today. This is exactly what we sometimes find
in nature and is one of the best kinds of evidence we have that evolution is a real event
in nature. It was the existence of these almost B’s that finally convinced Darwin that the
special creation hypothesis was not correct and that each species around today has
evoved from some ancestral species.
The Goldenrod
As you enter Centennial Woods, you will soon notice a large field of goldenrod along the
path. This is the plant that we have come to study. Other patches can be found
elsewhere in the Woods, and are worth finding and sampling. Your TA will show you
which patch to sample this year.
There are many species of goldenrod (genus = Solidago). Here in Vermont, we know
when autumn is coming when we see the bold golden flowers of goldenrod appear.
Many people regard this plant as a “weed” and don’t want it in their gardens. But, in
Britian it is a popular plant and really looks nice as a border along paths. Also, some
people blame the goldenrod for autumn or late summer hayfever. The blame is
incorrect. Goldenrod makes very large, heavy pollen that cannot get up into the air to be
breathed in by hayfever sufferers. That is why the flowers are so colorful: to attract
insects that carry the heavy pollen grains from plant to plant. The plants that cause
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Variation in Natural Populations 3
allergies are those with inconspicuous flowers that spread their pollen by the wind.
Mostly grasses are the villains, but also ragweed with its dull flowers which let loose
with huge numbers of tiny pollen grains that end up in many a sensitive nose. At the
turn of the century there was a strong effort to make goldenrod the national flower. If
you don’t know what this plant looks like, a sample will be on display in the lab.
There are about 125 species of Solidago world wide, and about 90 are found in North
America and about half of these are in the NE USA. Often they are very difficult to tell
apart without microscopic examination of chromosomes. One writer stated that perhaps
the punishment in hell for a botanist would be to be forced to write a complete
monograph on the genus!
There are two goldenrod species common in Centennial Woods: the Canada Goldenrod
(Solidago canadensis) and the Rough-stemmed Goldenrod (Solidago rugosa). In both
species the flowers are carried in large clusters called inflorescences. Many species of
flowering plants produce flowers in such clusters. For example, the sunflower is actually
thousands of flowers grouped in a disk. In the Canada Goldenrod the clusters form
spreading plumes. The plant may grow to five feet or more. The Meskwaki Indians
believed that washing a baby in a solution from this plant would make the child cheerful
and blessed with a good sense of humor. The leaves of this plant do have a very nice
odor, so perhaps anyone bathed in such a solution would feel happy! The Roughstemmed Goldenrod is a “furry” plant with wrinkled leaves. It also can grow to large size,
sometimes to seven feet.
How to Identify Species
Biologists have the real, practical problem of identifying species in the field. Suppose we
wanted to study the reproduction of the Canada Goldenrod. We would want to be sure
that each individual plant we study is indeed the proper species. To help in identifying
species, taxonomists (those who classify species) often construct a key to the species
of a particular group. Attached here (see page 9) is a key to the New England
species of Solidago.
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Variation in Natural Populations 4
Pick out any individual Goldenrod plant. Let’s use the key to identify it. Go to the pair of
statements at number 1 on the key. Does your plant have all of its flowers at the top, or
terminal part, or are the flowers distributed all along the main stem? Almost certainly in
Centennial woods, the plant you are looking at has its flowers in clusters at the top. So
the first statement is true…go over to the right and see that now you go to statement 2
rather than statement 6. At statement 2 you see another pair of choices. Is the flower
cluster long and thin (4 times longer than broad) or fairly broad. Again, almost certainly
your plant will have a fairly broad cluster, so you next go to statement 3 (otherwise you
would have an individual of Solidago squarrosa). At statement 3 you have to decide if
you have a plant with narrow leaves and a slightly fuzzy stem, or broad leaves with a
very fuzzy stem. If the stem is very, very fuzzy you have Solidago rugosa, if the stem is
smooth you go to number 4. At number 4 you see that if you have a plant with a slightly
fuzzy stem you have Solidago canadensis.
Note on the key the statement about the veins in the leaves at number 4. You must
choose between “venation various” or “ leaves 3-veined”. Note that the first statement
includes the possibility of the leaves having three veins.
A key, then, uses natural characteristics that differentiate species to help us in
identifying our plants. The differences used in the key are certainly NOT the ONLY
differences among these species , but they are the easiest to use in the field.
Now, if you assume all the goldenrods in this patch are one of the two species (Canada
vs. Rough-stemmed), what are the differences between them that makes identification
easy?
Walk around in the field until you look over at least five individual plants of each species
(careful when you walk, you don’t want to make a mess of the field because lots of other
people are coming out to look at the plants!). Note the variation within each species in
height of the plant, stem diameter, number of leaves and so forth. We will examine this
variation in the next part of this lab period. Are there any plants that don’t seem to fit into
the two common species ? Is so, can you identify them using the key?
As you look the plants over, look for swellings on the stems that look like the plant
swallowed a pea. These are called galls and are caused by attack of a parasitic insect,
often a wasp. The insect lays its eggs in the plant’s stem and the larvae cause the plant
to produce the growth as a kind of home for them as they feed and mature. The galls
seem to harm the goldenrod because they reduce the number of seeds produced and
sometimes stunt the growth of the plant. We will do a project later in the semester on
how selection works on gall size and placement.
Variation in Natural Populations
An important prerequisite for evolutionary change is that there be variability in natural
populations. For natural selection to change the genetic nature of populations, there
must be variation in the phenotypes present. That is, morphology, anatomy, physiology,
and behavior must show variation among individuals. Biologists have long been
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Variation in Natural Populations 5
interested in variation in populations. Indeed, studies on the evolution of any population
must begin with a quantification and understanding of the variation in phenotypes and
genotypes present. In this laboratory exercise you will study phenotypic variation in two
species of plants, the Canada and rough-stemmed goldenrods; later in the semester
you will study variation in humans. You will use the data you gather and some
elementary statistics to test some hypotheses about the nature of variation in the
populations under study.
Methods
While in Centennial Woods, you will study Canada goldenrod (Solidago canadensis)
and the rough-stemmed goldenrod (Solidago rugosa). These species are dissimilar
morphologically and can be easily distinguished by using the key; you should already be
familiar with how to tell these two species apart.
Work in groups of three. Each group should have a meter tape; one person acts as the
data recorder while the other two take the measurements.
Step 1. Locate a plant at random and identify it to species using the key.
Step 2. Take the following measurements, and record them on the data sheet provided:
Height of the plant (from ground level to topmost branch).
Diameter of the main stem at 25 cm height.
Do this for 30 plants of each species.
Be sure to keep all data organized by individual plant.
Return to the lab.
Analysis
Our goal is to describe the variation in morphology within species, then compare the two
species. We might argue that the variation within species should be less that the
variation between species. Or, we might state that two closely related species of plants
cannot coexist in the same area without important ecological differences that are
reflected in their morphology. A third hypothesis might propose that the galls are
important in altering the morphology of the plants.
You have data for 30 individuals of each species, so your n will be 30 for each
descriptive statistic. Do the following for each of your measurements for each of the
species.
[A] Plot a histogram for each variable. A histogram is made by grouping the data in
small intervals, and then plotting the values observed for a variable on the X axis, and
the number of times each value was observed on the Y axis. Your instructor will
illustrate the method, and draw the histogram for the pooled data. You will draw the
histogram for the data that you collected. What is the effect of increased sample size on
the shape of the histogram? How close do your results and the class results resemble a
normal distribution?
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Variation in Natural Populations 6
[B] Find the mean, or one kind of average value for each variable. The mean is denoted
by
x and is calculated by summing all the observations and dividing by the total number of
observations. Using mathematical notation we write:
n
x=
∑x
i =1
i
n
where xi refers to the value of the variable and n is the sample size. Note that because
of the original design of our study we set n at 30.
[C] Calculate the variance for each of the variables. The variance is a measure of the
distance of each measurement from the mean and therefore tells us something about
the degree of variation in the data set. The standard deviation (s) is the square root
of the variance (s2). Variance is calculated as:
( xi − x ) 2
s =
(n − 1)
2
[D] Next you can compare the means using the t-test. The value of t is compared as
(subscripts refer to “species 1” and “species 2”…you decide which will be which)”:
t=
x1 − x 2
 (n1 − 1) s12 + (n 2 − 1) s 22  n1 + n2 



n
n
2
n
n
+
−
1
2

 1 2 
For this test there are n1 + n2 – 2 degrees of freedom. Find the probability of your result
from the table of t values provided in this manual and determine if the plants differ
significantly for each measure.
You can also use the t-test to compare gall-infected and gall-uninfected plants.
[E] Next we want to correlate the plant height and stem diameter. You might expect
them to be related: taller plants would have more to support and thus thicker stems. But
how tight is this relationship? A measure of the “tightness” of the relationship between
two variables is the correlation coefficient. There are both parametric and nonparametric
correlations.
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Variation in Natural Populations 7
Use the correlation work sheet to calculate the parametric product-moment correlation
coefficient, or r. You must have your data in pairs, call stem height “x” and stem
diameter “y” fir this analysis.
n
r=
∑ (x y )
i
i =1
n
i
n
∑ ( x )∑ ( y
i =1
2
i
i =1
2
i
)
The value of r can range from +1 to –1, with values close to +1 or –1 indicating a tight
relationship between the two variables. Positive correlations mean that an increase in
one variable is associated with an increase in the other. Negative correlations mean that
as one variable increases, the other decreases (see Figure on next page). To test if a
value of r is significantly different from zero, we use n-2 degrees of freedom ( in this
case 28) and look up the value of r in a statistical table. For df = 28 the value of P < .05
would be r > .45.
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Variation in Natural Populations 8
r = 1.0
r = -1.0
y variable
y variable
x variable
x variable
r = 0.0
r = 0.75
y variable
y variable
x variable
x variable
This figure illustrates positive and negative correlations between variables and the value
of r that would be measured in each case. If the points fall along a perfect line, then r =
1.0 for a positive slope (upper left panel), and r = -1.0 for a negative slope (upper right
panel). If the points are associated, but not perfectly aligned, the value of r will be
something less than 1.0 (lower left panel). Finally, if there is no association between the
two variables, r will be close to zero (lower right panel).
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Variation in Natural Populations 9
Key to the New England species of Solidago.
1. Flowers in a terminal, often branching, cluster
2
Flowers distributed along main stem in leaf axils
2. Terminal cluster at least 4x longer than broad
6
S. aquarrosa
Terminal cluster about as long as broad
3
3. Leaves narrow (less than 1.5 cm) or lance-shaped; stem smooth or hairy 4
Largest leaves broader than 2 cm, stem with long fuzzy hairs
S. rugosa
4. Main stem smooth, venation various
Main stem covered with fine down, leaves 3-veined
5
S. canadensis
5. Main stem usually covered with whitish bloom; leaves sharply toothed
S. gigantean
Main stem without white bloom; leaves entire
S.graminifolia
6. Leaves egg-shaped
S. flexicaulis
Leaves lance-shaped
S. caesia
9
Variation in Natural Populations 10
Solidago canadensis
Plant #
Height of Plant
Diameter of Stem
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
10
Variation in Natural Populations 11
Solidago rugosa
Plant #
Height of Plant
Diameter of Stem
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
11
Variation in Natural Populations 12
Sample Worksheet for calculating the
mean and variance of a sample.
Species: ______________________________
Variable: _____________________________
Observation
(xi)
Distance from the
mean
(xi –x)
Distance
Squared
(xi – x)2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
∑xi = ______
∑(xi – x)2 = ________
s2 = ________
s = ________
Mean = ______
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Variation in Natural Populations 13
Statistics Summary Sheet
Plant Height
Mean
s2
Stem Diameter
s
Mean
Canada
goldenrod
roughstemmed
goldenrod
t value
significance
level
correlation
coefficient
13
s2
s
Variation in Natural Populations 14
Worksheet for correlations
Species: _________________________
Let x = plant height and y = stem diameter
xi
xi2
yi
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
∑
14
yi2
xiyi