Artificial Selection
in
Brassica rapa
Biology 100 – Advances in Biology in the Age of the
Human Genome Project
I242 – Light, the Universe, and Everything
Spring 2002
Table of Contents
Artificial Selection in Brassica rapa
Introduction............................................................................................................................... 1
Selective Breeding ................................................................................................................ 1
Artificial vs. Natural Selection.............................................................................................. 2
Research Origins of Fast Plants ............................................................................................ 4
Life Cycle of rapid cycling Brassica rapa............................................................................. 5
Variable Traits in Brassica rapa.......................................................................................... 13
Fast Plants and their Butterfly, Pieris rapae........................................................................ 13
Our Experiment....................................................................................................................... 16
Sowing Fast Plant Seed....................................................................................................... 16
Counting Hairs.................................................................................................................... 17
Selecting the parents of the next (second) generation......................................................... 18
Pollination of selected plants .............................................................................................. 19
Harvesting seeds ................................................................................................................. 20
Establishing the second generation..................................................................................... 20
Counting the hairs in the second generation....................................................................... 20
Data Analysis .......................................................................................................................... 21
Inheritance Patterns............................................................................................................. 21
Analyzing the data to describe the variation in the population........................................... 26
Thinking about variation..................................................................................................... 29
Writing the report.................................................................................................................... 30
General Guidelines.............................................................................................................. 30
The Conventional Format ................................................................................................... 31
Artificial Selection in Brassica rapa
Adapted and edited by Katherine Dorfman1
Introduction
Selective Breeding
A trip to a supermarket, farm, pet store or garden center will offer nearly endless examples of
the products of selective plant and animal breeding by humans. Over hundreds, and in some
cases thousands of years, humans have altered various species of plants and animals for our
own use by selecting individuals for breeding that possessed certain desirable characteristics,
and continuing this selective breeding process generation after generation. In many cases the
end results have been dramatic. Domestic dog varieties, from Chihuahuas to great Danes,
trace their separate lineages to a common wild ancestor, the wolf (Canis lupus). Domestic
fowl varieties are all derived from the wild jungle fowl (Gallus gallus), while most modern
breeds of domestic cattle originate from the now-extinct giant wild ox (Bos primigenius).
One example that particularly impressed Charles Darwin was that of domestic pigeon
varieties (such as tumblers, fantails, carriers, pouters, and many others), which are derived
from wild rock doves (Columba livia) over a period of some 5000 years.
There are many similar examples among plants, including those that humans have bred for
food as well as beauty. One plant group especially important to humans for food is Brassica
genus of plants in the mustard family. A wide variety of familiar and highly nutritious
vegetables originate from just a few species of wild Brassicas, in particular B. rapa, B.
oleracea, and B. junce. Some varieties have been bred specifically for root production,
others for leaves, flower buds, oil production, etc. The group is of great economic
importance, and because of this, the genetic relationships among the various forms have been
thoroughly studied and are now fairly well understood. It may surprise you to learn that these
familiar vegetables so different in appearance have the same species as a common ancestor.
Centuries of artificial selection have produced such widely divergent forms. For example, the
following varieties originate from these wild species (see Figure 1):
Brassica oleracea: cauliflower, broccoli, cabbage, Brussels sprouts, kohlrabi, collards,
savoy cabbage, kale
Brassica juncea: leaf mustard root mustard, head mustard, and many more varieties of
mustard
Brassica rapa: turnip, Chinese cabbage, pak choi, rapid-cycling
Figure 1. Varieties (cultivar groups) of Brassica rapa.
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Artificial Selection in Brassica rapa
The next time you’re in a grocery with a large produce section or a large farmers’ market, see
how many of these different varieties you can find; there are numerous other varieties not
listed above.
That plant and animal breeders have been able to dramatically change the appearance of
various lineages of organisms in a relatively short period of time is an obvious yet profound
fact. It certainly did not escape the attention of Charles Darwin, who devoted the first
chapter of The Origin of Species to the topic of artificial selection by humans (“Variation
Under Domestication”). He used many examples of selection by humans to help support the
case for his proposed mechanism resulting in evolution of natural populations—natural
selection.
In order to gain better understanding of selection and inheritance, researchers have
experimented with artificial selection involving a wide variety of traits in many different
species of plants and animals. The results, obtained in a relatively short period of time, are
often impressive.
For example, in one experiment (started in 1896), the average oil content of corn kernels was
increased from 5% to 19% in 75 generations of selective breeding. In another, average annual
egg production in a flock of white leghorn chickens was doubled in only 33 years (from 126
to 250 eggs per hen). Similar examples abound, for characters useful to humans (as above)
as well as more esoteric ones (such as the number of abdominal bristles on fruit flies, or the
fecundity of female flour beetles).
A general finding from these studies is that most variable traits in organisms respond to
artificial selection (i.e., it is usually possible—even easy—to increase or decrease the
frequency or average value of a trait in a lineage through careful selective breeding). In this
lab exercise, you will attempt to accomplish the same thing.
Artificial vs. Natural Selection
Natural selection is a deceptively simple concept, relatively easy to understand at a basic
level, but with profound implications that are intellectually challenging. The following
exercise in artificial selection will serve as an introduction to natural selection, and we hope
will help you to better understand this important concept. Fundamentally, artificial selection
and natural selection are quite similar, but there are a few important differences.
Natural selection
Very briefly, natural selection occurs when certain variants within a population of organisms
experience consistently greater reproductive success (i.e., leave more offspring) than do other
variants. Generally, within a population of organisms (of the same species), there is variation
among individuals for a great variety of obvious and not so obvious traits. Some of this
variability is a result of genetic differences among individuals, while some is probably a
result of different environmental influences. Here we are concerned only with variability that
has a genetic basis (i.e., is inheritable—passed on from parents to offspring). As a general
rule, organisms produce more (often far more) offspring than can survive to reproduce
themselves (in Darwin’s words, there is a “struggle for existence”). If individuals with
certain inheritable characteristics have an advantage in survival and reproduction over
individuals with other characteristics, then it follows that the next generation will differ fro
the previous one—perhaps slightly, perhaps considerably. Why? Because individuals with
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those certain favored inheritable characteristics are more likely to be parents by virtue of
possessing these characteristics; the next generation will contain a disproportionate number
of offspring of these “naturally selected” parents, and these offspring will tend to resemble
their parents. As a result, over many generations the genetic structure of the population
changes (in other words, the population evolves).
Artificial Selection
Artificial selection is essentially this same process. A population of organisms exhibits
considerable variability among individuals for a trait or traits that might be of interest to
humans for one reason or another (such as running speed in thoroughbred horses, petal size
and shape in tulips, number of kernels in an ear of corn, egg size in chickens, etc.). By virtue
of possession of certain traits, some individuals are selected by the plant or animal breeder or
scientist to be parents of the next generation, while the remainder of the population not
possessing those traits are excluded from breeding. To the degree that the desired trait is
inheritable, the offspring will tend to resemble their parents, and hence on average the next
generation will differ from the previous one—i.e., will be faster, or have larger petals, or
more kernels, or larger eggs, etc. Over time, substantial differences can be achieved between
the original population and the artificially selected subsequent ones.
As noted above, there are some important differences between artificial and natural selection.
In contrast to natural selection, artificial selection:
1) favors traits that for one reason or another are preferred by humans;
2) has a goal or direction toward which the selection process is directed;
3) generally is much faster than natural selection, because the next generation can be
absolutely restricted to offspring of parents that meet the desired criteria (rarely is
natural selection such an all-or-none phenomenon).
In artificial selection, humans are doing the selecting—purposely restricting breeding to
individuals with certain characteristics. In natural selection, the “environment” does the
selecting—individuals that survive and reproduce better in a given environment, for whatever
reason, are “naturally selected”. The environment includes such influences as competitors,
parasites, predators, food supply, climatic conditions, soil nutrients, and many, many others.
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Artificial Selection in Brassica rapa
Research Origins of Fast Plants2
Rapid-cycling Brassica rapa, Fast Plants, were developed by Dr. Paul H. Williams, Professor
of Plant Pathology at the University of Wisconsin-Madison. Fast Plants are a product of Dr.
Williams’ research to improve the disease resistance of plants in the family Cruciferae, a
large diverse group that includes mustards, radishes, cabbages, and other cole crops. In order
to speed up the genetic research in the crucifers, he began breeding Brassica rapa and six
related species from the family Cruciferae for shorter life cycles. The end result was a genetic
line of small, prolific, rapid-cycling Brassicas. These plants are now known as “Fast Plants”.
Dr. Williams continued to refine the rapid-cycling Brassicas to have characteristics most
suitable for laboratory and classroom use. He selected seed from stock that met the following
criteria:
•
•
•
•
•
•
•
Shortest time from planting to flowering
Ability to produce seeds at high planting density
Petite plant type
Rapid seed maturation
Absence of seed dormancy
Ability to grow under continuous fluorescent lighting
Ability to grow well in a potting mix
After about 20 years of planting, growing, and selecting, his breeding process had reduced a
six-month life cycle to five weeks. Further breeding produced relative uniformity in
flowering time, size, and growing conditions and yet the Fast Plants retained much variety.
Over 150 genetically controlled traits have been recorded and are useful in research.
These Brassicas have shortened traditional breeding programs and aided in cellular and
molecular plant research. Additionally, Fast Plants have become extremely useful as a
teaching tool in the classroom since all aspects of plant growth and development can be
easily demonstrated.
For more information on the development of Fast Plants and other rapid-cycling Brassicas
see: Paul H. Williams, Curtis B. Hill, 1986. Rapid-Cycling Populations of Brassica. Science,
New Series, vol 232 (4756), 1385-1389.
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Life Cycle of rapid cycling Brassica rapa
Under proper growing conditions, the life cycle is 35-45 days from planting a seed to
harvesting the next generation, as summarized in Figure 2, Figure 3, and Table 1.
Figure 2. Life Cycle of rapid cycling Brassica rapa
Figure 3 (overleaf): Growth of Fast Plants
6
Insert Figure 3 (overleaf): Growth of Fast Plants
Here.
Artificial Selection in Brassica rapa
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Table 1. Stages in the Life Cycle of Fast Plants: Concepts of Dependency3
Stage
State
Condition
Dependency
A. seed
quiescence
suspended growth of
(dormant embryo) embryo
B. germinating seed germination
awakening of growth
C. vegetative growth growth and
development
independent of the parent and many
components of the environment
dependent on environment and health of
the individual
dependent on environment
roots, stems, leaves grow
rapidly;
plant is sexually immature
dependent on healthy vegetative plant
D. immature plant
flower bud
Gametogenesis;
development
reproductive [male (pollen)
and female (egg)] cell
production
E. mature plant
flowering mating Pollination: attracting or
dependent on pollen carriers (bees and
capturing pollen
other insects)
F. mature plant
pollen growth
gamete maturation;
dependent on compatibility of pollen
germination and growth of with stigma and style
pollen tube
G.mature plant
double fertilization union of gametes;
dependent on compatibility and healthy
plant
union of sperm (n) and egg
(n) to produce diploid zygote
(2n);
union of sperm (n) and
fusion nucleus (2n) to
produce endosperm (3n)
interdependency among developing
H. mature parent
developing fruit, Embryogenesis;
plant plus embryo endosperm, and
growth and development of embryo, endosperm, developing pod
and supporting mature parental plant
embryo
endosperm and embryo;
growth of supporting
parental tissue of the fruit
(pod)
I. aging parent plant senescence of
withering of leaves of parent seed is becoming independent of the
plus maturing
parent;
plant;
parent
embryo
maturation of fruit; yellowing pods;
seed development drying embryo;
suspension of embryo
growth;
development of seed coat
J. dead parent plant Death;
drying of all plant parts;
seed (embryo) is independent of parent,
plus seed
Desiccation;
dry pods will disperse seeds but is dependent on the pod and the
environment for dispersal
seed quiescence
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Artificial Selection in Brassica rapa
Days 1-3: Germination and Emergence4
Germination is the awakening of a seed from a resting state. This resting state represents a
pause in growth of the embryo. The resumption of growth, or germination, involves the
harnessing of energy stored within the seed. Germination requires at least water, oxygen and
a suitable temperature.
The radicle (embryonic root) emerges. Seedlings emerge from the soil. Two cotyledons (seed
leaves) appear and the hypocotyl (embryonic stem) extends upward. Chlorophyll and
anthocyanin (green and purple pigments, respectively) can be seen. See Figure 4.
Figure 4. Germination and Emergence of Brassica rapa.
Days 4-12: Growth and Development
As a plant germinates and matures, it undergoes the processes of growth and development.
Growth arises from the addition of new cells and the increase in their size. Development is
the result of cells differentiating into a diversity of tissues that makes up organs such as roots,
shoots, leaves and flowers. Each of these organs has specialized functions coordinated to
enable the individual plant to complete its life cycle.
Days 4-9:
Cotyledons enlarge. True leaves appear and develop. Flower buds appear in the growing tip
of the plant. See Figure 5.
Both genetics and the environment play fundamental roles in regulating growth and
development, and determining the phenotype (appearance) of an organism.
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Figure 5. Development of true leaves
Days 10-12:
In Fast Plants, growth and development occur rapidly and continuously throughout the life
cycle of the individual. It is most dramatic in the 10 to 12 days between seedling emergence
(arrival at the soil level) and the opening of the first flowers.
The stem elongates between the nodes (points of leaf attachment), and flower buds rise above
the leaves, as shown in Figure 6. Leaves and flower buds continue to enlarge.
Figure 6. Fast Plant, just before flowering.
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Artificial Selection in Brassica rapa
Days 13-17: Flowering and Reproduction
Flowering is the initiation of sexual reproduction by the plant. The production of male and
female gametes (sperm and eggs) within the flower is the first step in preparing for
pollination and reproduction. Each part of the flower has a specific role to play in sexual
reproduction. The flower dictates the mating strategy of the species. One common strategy
involves employing animals, often insects, to carry pollen (containing male gametes) to the
pistil (female reproductive organ). The structure of a Fast Plant flower is shown in Figure 6.
Figure 6. Flower Structure 5
Pollination is the process of mating in plants whereby pollen grains developed in the anthers
are transferred to the stigma. The pollen grains then germinate, forming pollen tubes that
carry the sperm from the pollen to the eggs that lie within the pistil.
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Flowers can be cross-pollinated (from one plant to another) for 3-4 days. Pollen is viable for
4-5 days and stigmas remain receptive to pollen for 2-3 days after a flower opens. After final
pollination, the remaining unopened flower buds and side shoots should be pruned off to
direct the plant's energy into developing the seed. See Figure 7.
Figure 7. Reproductively mature plant
Days 18-40: Fertilization to Seeds
Fertilization is the final event in sexual reproduction. In higher plants, two sperm from the
pollen grain are involved in fertilization. One fertilizes the egg to produce the zygote and
begin the new generation. The other sperm combines with the fusion nucleus to produce the
special tissue (endosperm) that nourishes the developing embryo. In some plants endosperm
nourishes the germinating seedling. Fertilization also stimulates the growth of the maternal
tissue (seed pod or fruit) supporting the developing seed. See Figure 8.
Within 24 hours of pollination, the sperm has fertilized the egg. Immediately following
fertilization, the pistil and other maternal structures of the flower will grow and change
functions. Within the ovules, the embryos also grow and differentiate through a series of
developmental stages known collectively as embryogenesis.
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Artificial Selection in Brassica rapa
Figure 8. Pod development
Days 18-22: Petals drop from the pollinated flowers. Pods elongate and swell. Development
of the seed and young plant has begun and will continue until approximately Day 36.
Days 23-38: Seeds mature and ripen. Lower leaves yellow and dry. Twenty days after final
pollination (about Day 38) plants should be removed from water.
Days 38-45: Plants dry down and pods turn yellow.
On Day 45, pods can be removed from dried plants. Seed can be harvested. The cycle is
complete.
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Variable Traits in Brassica rapa
One can observe several fairly obvious variable traits in a large population of mature Fast
Plants, such as: arrangement of flowers, total number of flowers, total number of leaves,
arrangement of leaves, length of the lower leaf, surface area of the lower leaf, plant height,
total number of seeds per plant, total mass of plant, stem length between first and second
leaves, etc. In contrast, there are other traits that don’t appear to vary at all, such as: number
of petals per flower (4); number of sepals (green petal-like structures just below the petals)
per flower (4); petal color (bright yellow); and number of cotyledons or “seed-leaves” (2).
When you first have plants to observe, look carefully at them and note the variability you can
see. Remember, all your plants are almost exactly the same age, so differences you see (such
as height, leaf size, etc.) are not due to differences in age. Figure 9 shows a typical Fast Plant
at about two weeks old, in which the cotyledons are distinguished from the true leaves.
Figure 9. Fast Plant, about 14 days old.
One variable trait that you might not have noted in your list is “hairiness” of leaves, petioles
(leaf stalks) and stems, but if you look more closely (especially with a hand lens and desk
lamp) you should see these “hairs” on some plants. Technically, plant “hairs” are called
trichomes, and they have been shown to have a specific function. What do you suppose this
function might be?
Fast Plants and their Butterfly, Pieris rapae 6
Pieris rapae, the ubiquitous cabbage white butterfly, is found across North America in many
natural places and in gardens with cabbage, broccoli, and other crucifers. It hatches as a little
caterpillar from a tiny egg laid on the surface of a leaf by the mother. It goes through 5 such
larval stages in about 18 days, finally pupating and metamorphosing into an adult, which
emerges from the pupal case about 8 days later. The adult can live up to three weeks, making
the entire life cycle less than 2 months long.
Read: Jon Agren, Doublas W. Schemske, 1993. The cost of defense against herbivores: An
experimental study of trichome production in Brassica rapa. American Naturalist, Vol 141
(2):338-350, and Jon Agren, Doublas W. Schemske, 1994. Evolution of trichome number in
a naturalized population of Brassica rapa, American Naturalist, Vol 143 (1):1-13 for more
information on the relationship between Pieris and Brassica.
Figure 10 (overleaf). Life cycle of Brassica rapa and Pieris rapae.
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Artificial Selection in Brassica rapa
Paste part 1 of Figure 10 (overleaf). Life cycle of Brassica rapa and Pieris rapae. Here.
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Paste part 2 of Figure 10 (overleaf). Life cycle of Brassica rapa and Pieris rapae. Here.
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Artificial Selection in Brassica rapa
Our Experiment
Sowing Fast Plant Seed
•
Label your pots, following the example of the first one.
•
Put a MiraCloth wick in each pot. Make sure the pointed end comes out through the
hole in the bottom of the pot, and the top extends about 1 mm above the top of the pot
•
Fill each pot halfway with moist soil mix.
•
Put a fertilizer pellet on the soil in each pot.
•
Add more soil, up to the bottom of the pot lip.
•
Position the wicks so they extend out of the soil by about 1 mm.
•
Use the butt end of a dissecting needle to make a little depression in the soil for the
seed.
•
Touch a toothpick to a glue stick, then pick up one or a few seeds with the sticky end.
•
Carefully place one seed in the little depression, flicking it off the toothpick with the
dissecting needle.
•
Sprinkle vermiculite over the seed to cover it.
•
Put water into each pot with a plastic transfer pipet, until it drips out of the bottom.
•
Decide on a watering schedule, to ensure that the soil is kept moist.
•
Take the pots upstairs to the environmental chamber.
•
Conscientiously follow the watering schedule over the next week, taking care not to
tip over any pots.
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Counting Hairs
As you have probably guessed, the variable trait that you are going to attempt to alter in this
lineage is “hairiness.” In this “wild type” population, some plants should be noticeably hairy,
many are slightly hairy, while others are apparently hairless. But such an observation is too
general, and needs further quantification.
To accomplish this, you could count all hairs on all parts of each plant, but this would be a
time-consuming task, and an unnecessary one. It turns out that hairiness of one part of a
plant (such as the leaf surface) is strongly correlated with hairiness on other parts (such as the
leaf margin). In other words, a plant’s hairiness in general can be quantified by assessing
hairiness of a specific structure.
The structure we will use for this “hairiness index” is the petiole, or leafstalk, where
trichomes are large, conspicuous and rather easily counted, and the structure is relatively
small with a defined starting and ending point.
For consistency, we will use the petiole of the first (lowermost) true leaf, and we will define
the limits of the petiole as follows: from its junction with the main stem (usually marked by a
small bulge or ridge, often differently colored)to its junction with the lowermost leaf vein
(see Figure 11).
Figure 11. The limits of the petiole
An important note: the two lowermost squarish, two-lobed and rather thick leaves are
actually cotyledons (“seed leaves”), not true leaves. The first true leaf is just above these two
cotyledons. What you need to do now is to count the total number of trichomes on the
petiole of the first true leaf of each plant in your sample. Your data will then be combined
with data from the rest of the section.
Use the hand lens and desk lamp; the trichomes are most conspicuous if strongly illuminated
against a dark background, such as the black table top. If present, the trichomes will
generally be concentrated on the lower side of the petiole, but some could occur on the sides
or top as well. Be sure to rotate the plant so that you can see all aspects of its petiole. Count
a second time for verification, then record your data in your data sheet.
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Artificial Selection in Brassica rapa
Reliability of hair counts
The number of hairs we count in this experiment may vary from one observer to another
(inter-observer reliability), or between counts performed by the same person (intra-observer
reliability). So we can never be entirely certain about the number of hairs on a given plant.
Take some time to think about why the count includes this inherent variability. These kinds
of uncertainties are often called “error,” but that is not meant to imply mistakes on the part of
the experimenter (or the equipment manufacturer).
Making multiple measurements is a way of acknowledging the uncertainty in each one, and
reducing the risk of letting a single odd measurement carry too much weight. We then
represent our result as an average of the many repeated measurements we took. But this
glosses over the variation among measurements. We don’t have time to count each plant a
dozen times in this lab period, so we will compromise as follows:
•
To improve the accuracy of our counting, the hairs on each plant will be counted by two
students, and the counters will collaborate (or enlist the aid of a third counter) to come to
a fair estimate of the actual number of hairs.
•
Make a data table for yourself, in which you record the plant number and number of
hairs. After you have adjudicated the number of hairs with your counting partners, enter
your data in the official data sheet.
•
Enter your data in the spread sheet on the computer.
Compiling the data
When all groups have finished assessing trichome number of individual plants, your lab
instructor will coordinate the effort of combining the separate data into a single data table.
After tabulating the data from all sections, your instructor will make this summary available
to you.
Selecting the parents of the next (second) generation
Only a small fraction (about 10%) of this population of plants will be selected to be parents
of the next generation. These won’t be randomly chosen, but instead will be the hairiest 10%
of plants in the population, and the 10% least hairy.
Sort the data table by number of hairs. Find the top 10% in hairiness. Print out or write
down their ID numbers.
If more than 10% of the population has no hairs, then use a randomizing technique to choose
the 10% to be parents. Your instructor may have created a column in the spread sheet with
random numbers in it; pick the 10% of the population that is bald and has the highest
numbers in this column. (You may also pick numbers out of a hat or toss dice or coins to
pick the winners in this selection process.)
Indicate on the data sheet which plants have been chosen as breeders for which group.
Separate the chosen plants into hairy parents and bald parents, and dump the contents of the
remaining pots into the compost pile. From now on the two sets of parents must be kept
strictly separate.
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Pollination of selected plants
The plants you selected last week to be parents of the second generation should be in heavy
flower. Today, you will assist these plants in the process of sexual reproduction. Brassica
rapa plants need assistance because in nature they are totally dependent on certain insects for
transferring sperm-bearing pollen from the male part of the flowers of one plant to the female
part of the flowers of another plant. In these plants, the most conspicuous floral structures
are the yellow petals. These petals enclose the male sexual structures (stamens, terminating
in the pollen-producing anthers) and female structures (pistil, with the pollen-receiving
stigma, style and egg-producing ovary), as shown in Figure 6 and Figure 7. Although each
flower has both male and female parts, sperm from one plant are incapable of successfully
fertilizing eggs of the same plant. This self-incompatibility ensures out-crossing (mating
between different individuals).
The insect pollinators don’t act as pollen couriers just to be nice; the plants lure and reward
them with nutrients (nectar and edible pollen), and the insects inadvertently pick up the
sticky pollen on various body parts and then carry it with them to the next flower (Figure 12).
Honeybees are common pollinators of Brassicas in the wild. We will use artificial “bees”
(little pollinator wands) to help us pollinate these plants.
Figure 12. Honeybee (the usual pollinator of Brassica)
ü Designate sides of the room for hairy and bald plants. Do not mix plants or
pollinating wands! No student should pollinate more than one type!
Each student group should obtain one or two of the selected mature plants. The crosspollination activity will be accomplished over the course of the week, so that even early- and
late-bloomers can mate with each other. The object is to transfer pollen from each plant to
every other plant. This can be done in the following way. Using a single pollinating wand,
one group will lightly rub and twirl the bristly end for several seconds onto the anthers and
stigma of each open flower on their plant(s). Then pass the wand, now loaded with yellow
pollen, to the next group, who will repeat the process. After the bee stick has made one
complete round (all open flowers on all plants have been “visited” by the wand), make a
second round in the same manner. When finished, insert the wand into the soil (pointed end
down, “bee” end up) of one of the pollinated plants—don’t return the used wand to its
original container, where other students might confuse it with fresh, unused ones. Finally,
return all plants to the watering tray.
Fertilization will result in the development of the ovules (each containing an embryo) into
mature seeds, which will be contained in the fruit or seed pod (the elongated ovary). The
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Artificial Selection in Brassica rapa
length of time from fertilization to mature seed is about 3-4 weeks. We don’t need to do
anything more with these plants until it is time to harvest the seeds except to remove them
from water in 3 weeks, which hastens the seed-maturation process. Next week, you should
easily be able to see the elongating fruits. In two weeks, they will have become even longer
(2-5 cm) and the individual seeds inside (perhaps 5-20 per pod) will have become visible. In
three weeks, the maturing process is nearly complete. You will plant these seeds (generation
2) in about 4 weeks.
Harvesting seeds
The Fast Plants that you selected from the initial large population and pollinated should have
mature seeds ready to be harvested. Each student group should take one or two of the plants
to your table. Remove several pods, then break open the pods into a small plastic dish,
releasing the seeds. You should be able to retrieve 5-20 or so seeds from each pod. The
plant from which you removed the pod is the mother of each of those seeds; the father could
have been one or more of any of the other plants in the selected group. Similarly, your plant
could have been the father of seeds from any of the other plants in the selected group. Our
mass cross-pollination technique ensured that all individuals in the selected group had an
equal opportunity to be parents.
Establishing the second generation
The goal is to plant and grow generation 2 seeds in order to assess the average hairiness of
this new generation (the offspring of the selected generation 1 parents). Each student will be
responsible for planting a small number of seeds. (This time we will plant 2 seeds per pot,
since germination rates tend to be lower in classroom-harvested seeds than seeds we get from
the supplier.) Your lab instructor will give you further directions.
After planting, your section’s flat will be placed under the lights in the lab room, and
maintained by the staff. By next week’s lab, most of the seeds will have germinated. In two
weeks they will be ready for the final analysis of the experiment.
Counting the hairs in the second generation
Follow the counting procedure from the first generation, keeping separate records for the
offspring of the hairy parents and the offspring of the bald parents.
Mount Holyoke College, Spring 2002
21
Data Analysis
Inheritance Patterns
Is hairiness genetic?
Allele dominance and frequency
An allele is dominant if it takes only one to express a trait. Dominant alleles are generally
those that do something: specify a working version of an enzyme, an antigen, a hormone, a
transcription factor, a receptor, etc. Recessive alleles, on the other hand, are generally those
that represent a loss-of-function. That is, they do not specify a working version of the protein
in question. It seems more likely that alleles that result in production of trichomes are
dominant.
Allele frequency depends not on dominance, but on current prevalence and usefulness.
Alleles that reduce the reproductive capability of their bearers tend to become less common,
while those that enhance reproductive capability tend to become more common.
In a large population with random mating and no slection, allele frequencies do not change.
Allele frequencies change when there is selection for one allele over the other.
When you read The cost of defense against herbivores, by Agren and Schemske, you should
be thinking about how herbivores act as agents of selection on their food plants. If
herbivorous insects prefer to eat smooth stemmed plants, then the smooth stemmed plants
will, on average, produce fewer offspring. Therefore, if hairiness is a heritable trait, the
frequency of alleles for hairiness should increase in the population, and the frequency of
alleles for smooth stems should decline *.
If there are no predatory insects to maim or kill the smooth-stemmed plants, then the smoothstemmed plants may actually have an advantage – they can spend their energy on making
babies instead of making trichomes. In this case, the allele for smooth stems should increase
in frequency in the population.
For neither scenario (selection against smooth plants by herbivorous insects or selection for
smooth plants in the absence of herbivores) does it matter which allele is dominant.
Whichever allele is most useful under the current conditions will become more common in
the population.
We have acted as very severe agents of selection in our population of plants. We have
chosen two subsets of the original heterogeneous population to be parents, and have killed all
the rest! We reduced the reproductive capability of the non-chosen plants all the way to
zero! In one subset, only smooth-stemmed plants were allowed to breed. In the other subset,
only those in the hairiest 10% or so of the population were allowed to breed.
*
Of course, the plants also act as agents of selection on the insects. If all the plants are hairy, the individual
insects that cannot abide trichomes do not breed as much (because they don’t get enough to eat) as those that
find a way to eat hairy plants. Allele frequencies will change in the insects, producing a population more
willing to eat hairy plants. This will likely result in further selection by the insects against the relatively less
hairy plants, and an arms race of sorts will ensue.
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Artificial Selection in Brassica rapa
If hairiness is an inherited trait, then we should be able to create two populations by this
technique, one in which the hairy allele(s) is(are) common, and one in which the smooth
allele(s) is(are) common. No matter which allele is dominant, we can change its frequency in
the population. This should further reinforce the idea that either the dominant or the
recessive allele can be more common in the population, depending on the circumstances.
Realized heritability
Selection can act on any phenotypic variation, but can only cause evolutionary change if the
variation is genetic.
Population biologists often use an index called realized heritability, h2, to quantify the degree
to which a trait in a population can be pushed by selection. To calculate this index, we find
the response to selection by subtracting the average of the second generation from that of the
entire first generation. We also find the selection differential by subtracting the average of
the selected parents from that of the entire first generation. Realized heritability is the
response divided by the differential.
realized heritability =
realized heritability =
response to selection
selection differential
avg1st gen − avg 2 nd gen
avg1st gen − avg selected parents
A low h2 (for example, less than 0.01) occurs when the offspring of the selected parents
differ little from the original population, in spite of a big difference between the population
as a whole and the selected parents.
A high h2 (for example, greater than 0.6) occurs when the offspring of the selected parents
differ from the original population almost as much as the selected parents do.
Is hairiness controlled by a single gene?
Complete dominance
One of the first questions we might ask of our results is whether there is a single “on/off”
type gene for hairiness. To answer such a question, we need to predict what the results of our
experiment might be if that were the case. So here is a hypothetical inheritance pattern for
hairiness in Fast Plants:
Suppose there is a single gene which determines whether or not trichomes are produced.
Suppose that there are exactly two alleles of this gene, “H”, which directs the production of
hairs, and “h”, which does not.
Since every plant gets one of these alleles from its mother and one from its father, each has a
pair of alleles, and there are three possible pairs of alleles, or genotypes: HH, Hh, and hh.
Since, in our hypothetical situation, “H” allows hairs to be made, any plant with at least one
“H” will have trichomes, that is, plants with either the genotype HH or Hh will produce hairs
and plants with the genotype hh will not. The appearance of the plant with respect to the
genes in question is called its phenotype.
Mount Holyoke College, Spring 2002
23
According to this scheme, we cannot tell the genotype of a hairy plant by looking at it, but all
the hairless plants have the genotype hh. So when two hairless plants are mated, they each
put one “h” into their respective gametes, and their offspring would all be hh, as well.
On the other hand, some of the hairy plants might have the genotype HH and some, Hh, so
there are three possible matings: HH x HH, HH x Hh, Hh x Hh. It is possible in this third
mating for an offspring to be produced with the genotype hh, making it bald. It is left as an
exercise for the reader to figure out the probabilities of the various genotypes in the offspring
of these matings.
We can examine our data to find out whether all hairless parents had hairless offspring, and
some hairy parents had hairless offspring and thus answer the question of the presence of a
single on/off gene.
We can also examine our data for the converse: is there a single on/off gene whose dominant
allele prevents trichome production? Again, the reader is left to construct the relevant
diagrams and use the data to answer this question.
Incomplete dominance
Suppose the hairiness gene we’ve been discussing isn’t a simple on/off switch, but a
modulator of some sort. Suppose there is a “T” gene (for trichome), which influences the
number of trichomes, rather than the mere presence or absence of them. Suppose that there
are still two alleles, “T”, and “t”, but that “T” is incompletely dominant to “t”. This means
that the heterozygote, Tt, would have a phenotype (a number of hairs) intermediate between
that of the two homozygotes, TT and tt. Plants with the genotype TT would have many hairs,
those with the genotype Tt would have some hairs, and those with the genotype tt would
have very few hairs. Each “T” can be thought of as contributing one “phenotypic unit”,
which in this case would be some number of hairs. This is illustrated in Table 2.
Table 2. Genotypic and phenotypic categories for a singlegene, incomplete dominance model of trichome number
inheritance.
# of “phenotypic units”
2
1
0
Intermediately
phenotype Hairiest
hairy
smoothest
Genotype
TT
Tt
tt
Matings between similar homozygous parents (TT x TT, or tt x tt) should produce only
offspring just like the parents. Matings in which at least one of the parents is heterozygous
(Tt) would produce offspring with greater variation. Calculating the results of such matings
is left as an exercise for the reader.
24
Artificial Selection in Brassica rapa
Cross
T
T
t
Results:
t
Genotype:
Phenotype:
Proportion (fraction) of
offspring of a Tt x Tt cross:
We can examine our data for evidence that a single gene with incomplete dominance controls
trichome number in our plants. Are there offspring of two very hairy parents that have as
few hairs as the least hairy parents? Are there offspring of two bald parents that have as
many hairs as one of the hairy parents?
Is hairiness controlled by multiple genes?
Hairiness in Fast Plants most likely differs from these simple traits in that there are not just
two or three phenotypes, but a whole range (from zero to “many”). Explaining the
inheritance of such traits involves a more complicated model.
Traits such as body size or skin pigmentation in humans, or milk production in cattle, for
which there exists a continuum of phenotypes between two extreme values, are known to
geneticists as quantitative (or continuous) traits. Traits such as petiole trichome number in
Fast Plants, number of bristles on fruit fly abdomens, or number of eggs laid by house fly
females, are in a related category called meristic traits. These often exhibit wide variability
among individuals, but the variability is incremental, not continuous (e.g., an individual
might have 5 or 6 hairs, but not 6.3 hairs).
Despite the difference between these two kinds of traits, geneticists feel their mechanism of
inheritance is similar. Such traits are very important—most traits in most organisms are
quantitative or meristic, and relatively few are simple two-phenotype traits such as exhibited
by Mendel’s peas (red/white flowers) or the Macintosh rabbits (straight/floppy ears).
Quantitative and meristic traits are also often called polygenic traits, because the model that
best explains their inheritance involves multiple gene loci, with two or more alleles at each
locus.
Two loci. Let’s imagine a situation in which there two genes contribute to the hairiness of
the petiole in Fast Plants. As in the previous example, each locus has two alleles (“A” and
“a” for one locus; “B” and “b” for the other), and each “uppercase” allele contributes one
“phenotypic unit”. Thus, an individual with genotype AABB would exhibit a phenotype of 4
units; individuals with genotype AABb or AaBB would each have a phenotype of 3 units;
etc., as shown in Table 3.
Mount Holyoke College, Spring 2002
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Table 3. Genotypic and phenotypic categories for a 2-gene model of trichome number
inheritance.
Number of “phenotypic
units”
4
3
Very many
hairs
Many
hairs
2
1
0
Some
A few
Almost no
hairs
hairs
hairs
AAbb
Aabb
AABb
aabb
AaBb
Possible genotypes
AABB
aaBb
AaBB
aaBB
As before, when both parents have only the dominant alleles (AABB x AABB) or only the
recessive alleles (aabb x aabb), the offspring will have the same genotypes as their parents.
phenotypes
Suppose, however, that our hairiest parents were in category 3, that is, with genotypes AABb
and AaBB. There are three possible matings in this population: AABb x AABb, AABb x
AaBB, and AaBB x AaBB. What are the possible genotypes and phenotypes in the offspring
of these matings? What are the highest and lowest phenotypic categories that might result
from matings between parents with category 3 phenotype?
We will have to consider whether our data are consistent with a 2-gene system such as this.
Three loci. If there were 3 genes, “A”, “B”, and “C”, there would be 7 phenotypic
categories. The reader should fill in Table 4 like that for two loci, showing how many
different genotypes would there would be for each category.
Table 4. Genotypic and phenotypic categories for a 3-gene model of trichome number
inheritance.
# of
“phenotypic
units”
6
phenotype Hairiest
5
4
3
2
1
0
smoothest
Possible
genotype(s)
The more genes involved in adding to the number of trichomes, the more phenotypic
categories there are, and the more closely the theoretical distribution can match the real
distribution, which after all, had 24 different numbers of trichomes on their petioles. Another
factor is how many trichomes on average are added by each dominant allele. Biological
things being the way they are, we should expect a rather loose correlation between the
number of dominant alleles and the number of trichomes, as opposed to a precise one-to-one
correspondence.
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Artificial Selection in Brassica rapa
What happens in the second generation depends on the nature of the inheritance of trichome
number in fast plants. What if, for example, the hairiest individuals in our first generation
did not happen to have all possible trichome-generating alleles? (I.e., What if our hairiest
individuals were in fact heterozygous for one or more trichome genes?) How would that
affect the possible range of trichome number in their offspring?
What if there is more than one genotype that results in a smooth petiole, so that some of the
smooth individuals were heterozygous? What might we see in their offspring?
Analyzing the data to describe the variation in the population
In addition to the variation in counting, of course, there is variation between plants. Our goal
in this semester long experiment will be to see how much of this variation can be accounted
for by genetics. We will compare the hairiness of the original population with the next
generation, after selection. We need measures not only of hairiness, but also of variation in
hairiness.
The mean (average)
Calculating the average number of hairs per plant is one way to indicate hairiness. Suppose
the 6 plants planted by one student had the following number of hairs (arranged in
descending order): 15, 13, 10, 8, 5, 0. The average number of hairs is 8.5, calculated as
follows:
average =
15 + 13 + 10 + 8 + 5 + 0 51
=
= 8.5
6
6
The Excel file has been set up to calculate this automatically as the data are entered, since
adding up hundreds of numbers is tedious. You can use the function wizard (fx) to calculate
the average: pick a cell to contain the average, click on fx, choose AVERAGE, highlight the
column containing the number of hairs, and click OK.
Frequency distribution
One way to look at the hairiness in the population is to construct a histogram, or bar graph, to
illustrate the variation. To do this, we need to lump some of our data together in categories.
A convenient starting point would be to count how many plants have between 0 and 4 hairs,
how many between 5 and 9, how many between 10 and 14, etc. This effectively creates
hairiness categories.
The student at the computer highlights just those cells whose entries range from 0 through 4,
and counts them. If she keeps her finger down on the mouse, Excel will tell her, in the upper
left corner of the spread sheet, how many rows she has highlighted. If she lets her finger up,
she has to count them by hand.
A student at the board constructs a table showing the frequency distribution. Table 5 is such
a table for our 6 plant data given in the section on averages, above.
Mount Holyoke College, Spring 2002
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Table 5. Frequency distribution of number of
hairs on the first petiole of 6 week-old plants
number of hairs number of plants
0-4
1
5-9
2
10-14
2
15-19
1
Each student should construct a histogram illustrating the frequency distribution of hairs on
all the plants. The histogram illustrating the data in Table 5 is given in Figure 13. Figure 14
illustrates the distribution of trichome number in the first generation of Fast Plants grown in
spring 2001 for an experiment similar to ours. The height of each bar represents the number
of plants whose number of hairs fell into the category given below the bar. Talk with your
instructor about learning how to construct such a histogram with Excel.
600
unselected
selected smooth
selected hairy
500
#of first generation plants
number of plants
3
2
1
400
300
200
100
0
0
0 to 4
5 to 9
10 to 14
number of trichomes per petiole
0 to 4
15 to 19
Figure 13. Distribution of number of
hairs per petiole
5 to 9
10 to 14
15 to19
number of trichomes per petiole
20 to 24
Figure 14. Frequency distribution of
trichome number in first generation Fast
Plants, Spring 2001.
The standard deviation
One common method of expressing the variation within a population is to calculate one of
several conventional indices of uncertainty. An especially common such index is the
standard deviation. It is a statistical measure of the precision in a series of repetitive
measurements. It is calculated this way:
sd
=
2
∑ ( x − xi )
Where
sd is standard deviation,
n is the number of data,
xi is each individual measurement,
x̄ is the mean of all measurements, and
Σ means the sum of
n−1
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Artificial Selection in Brassica rapa
The standard deviation is something like the average deviation from the mean. We subtract
each measurement from the mean of the measurements. Since some of these are positive and
some are negative, and will cancel each other out when we add them together, we square
each deviation to make it positive. Then we add all these squares together, and divide by the
number of measurements (actually, we fudge it here, using n-1 instead of n, because this
gives a better result—sorry) to get a sort of average. Finally, we correct for having squared
the deviations by taking the square root of the whole thing.
The standard deviation has become popular because of an interesting feature. For a large
sample, two thirds of the values can be found within one standard deviation of the mean,
95% within two standard deviations, and 99% within three.
In the example above, the standard deviation would be 5.5, calculated as follows:
sd
=
(15 − 8.5) 2 + (13 − 8.5) 2 + (10 − 8.5) 2 + (8 − 8.5) 2 + (5 − 8.5) 2 + (0 − 8.5) 2
6− 1
=
=
6.52 + 4.5 2 + 1.5 2 + ( − 0.52 ) + ( − 3.52 ) + ( − 8.5) 2
5
42.25 + 20.25 + 2.25 + 0.25 + 12.25 + 72.25
5
=
149.5
5
=
29.9
=
5.47
A greater spread among my values would give a higher standard deviation; a smaller spread,
a lower one. Table 6 illustrates this: the data set from the example above is listed between
two other data sets, each with the same average and different standard deviations.
Table 6. Three data sets with the same average and different standard deviations.
raw data
12, 11, 9, 8, 7, 4
average standard deviation
8.5
2.88
15, 13, 10, 8, 5, 0
8.5
5.47
19, 17, 11, 3, 1, 0
8.5
8.34
It is a good thing we have computers to do the calculations for us, but for the answer to have
meaning, we need to know how it is done.
The standard deviation is often used in histograms, to illustrate the spread of values in the
data, as well as to help compare one group of data with another.
Figure 15 llustrates the use of means (the bars) and standard deviations (the lines) to compare
the number of trichomes in the various groups of Fast Plants from a previous semester’s
experiment. The heights of the bars tell us that the two sets of offspring were different from
the first generation of plants, and the standard deviation marks tell us that there was more
variation among the offspring than among the selected parents.
Mount Holyoke College, Spring 2002
29
25
number of trichomes
(mean + SD)
20
15
10
5
0
1st generation selected hairy
selected
smooth
2nd hairy
2nd smooth
Figure 15. Comparison of number of trichomes on the petiole of the first true
leaf of: all plants in the first generation, the selected hairy parents, the selected
smooth parents, and the 2 second generations.
Thinking about variation
Think about what would be expected to change about the variation in hair number from the
first to the second generation, after selection. Consider not only the mean number of hairs,
but also the standard deviation. Will overall variability stay the same? Will the average
change?
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Artificial Selection in Brassica rapa
Writing the report
General Guidelines
A research report, in communicating to your peers, should accomplish several things. In
addition to telling the story of what you did and what you found, it should also justify your
work. By this I mean that it should provide enough background information to provide
appropriate context, connect your findings to a larger body of knowledge, and make the
question seem intrinsically interesting or practically valuable. It should argue persuasively
for your conclusions. And it should do all this in the conventional style of scientific
communication.
As with other expository writing, a scientific report has an introduction, a middle, and a
conclusion. First you set the stage and try to get the reader’s interest, then you explain what
you did and what you found, then you sum up the significance of your results, returning to
the context established at the beginning. Write in your own voice; don’t aim for an
artificially stilted, formal style, but avoid slang. Use the active voice when possible (e.g., We
randomized the order of presentation, as opposed to The stimuli were presented randomly).
Be as explicit as possible. For example, when you say something like “increased hairiness in
the second generation”, be sure it is clear to the reader whether you mean more hairs per
plant, or more plants with hairs. Also, distinguish between “gene”, “allele”, and “trait”.
Scientific names
The genus name for an organism is a proper noun, so it is capitalized. The species name is
an adjective, and is not capitalized. Both are Latin, a foreign language, so the entire name is
italicized, like this:
Homo sapiens
Brassica rapa
Pieris rapae.
Because it is a proper name, it usually doesn’t act as an adjective. Say “Brassica rapa has
been studied”, or “plants of Brassica rapa have been studied”, rather than “the Brassica rapa
plants have been studied.” [This last sounds rather like “the Jane Smith person”.]
The second time you refer to a species, you may abbreviate its name by using the first initial
of the genus: B. rapa. [Obviously this only works when the name is unambiguous. In a
paper about human gut organisms, for example, you’d have to distinguish between
Eschericia coli (a bacterium) and Entamoeba coli (an ameba).]
Mount Holyoke College, Spring 2002
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The Conventional Format
Introduction
In this section, you present your background material and establish the context for your work.
Review previous work on this subject. Try to give enough background material so that your
project can be seen as part of a larger endeavor. What common knowledge or scientific fact
or particular observation led you to ask the question your experiment was designed to
answer? What does the reader need to know to follow your reasoning? E.g.,
In “Basic Color Terms”, Berlin and Kay (1999) give the terms white, black,
red, green, yellow, blue, brown, purple, pink, orange, and grey as the basic
color terms of English. …
In fall 1999, the Unity of Science class asked students to mark on the visible
light spectrum the location of the “best” red, orange, yellow, green, and blue.
There was very poor agreement in the placement of the “best” orange.
After setting the stage, tell the reader what you were trying to find out (not what you were
trying to prove), and how you came to that question from the background information. E.g.,
From the color naming data collected in class, we know that subjects place
light with a wavelength 580 nm in the red, the orange, or the yellow, so we
decided to ask our subjects to mark the red/orange and orange/yellow
boundaries, as well as the “best” red, orange, and red. We hoped to compare
the boundary results with the “best color” results.
Don’t give away the punch line, but do tell whether you had a particular expectation, and
why. E.g.,
We expected subjects to put the “best” color about halfway between its
boundaries with the neighboring colors, even when asked to do so out of
order. Further, we thought that there would be as much disagreement as to
the boundaries as there had been to the “best” colors.
Methods
What did you do and how did you do it? This section should be written as a description of
what you did, not a recipe or set of instructions. (A recipe sounds like this: Tell the subject
to sit in front of the color naming spectrometer, cover her head with black cloth, and look in.
Set the wavelength to 580 nm, and ask her to name the color. A report sounds like this: The
subject sat in front of the color naming spectrometer, covered her head with black cloth, and
looked at the spectrum inside. We set the wavelength to 580 nm, and asked her to name the
color.)
Be sure to give the reader information that cannot be obtained otherwise, e.g., which
trichomes you counted. You can leave out some organizational or administrative details
(such as the organization of your data sheet or the numbering of pots), unless you think this
bears directly on the data or your interpretation.
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Artificial Selection in Brassica rapa
Results
What happened in your experiment? Write about what you observed. Refer to each of your
figures and tables by their numbers in writing (because if you don’t, they are not really part
of your paper). Summarize the main features of your results, and point the reader, in writing,
to the appropriate table or figure for the details. Don’t interpret in this section. For example,
Although there was some variation, all of the subjects placed the “best
orange” somewhere between the orange/red and yellow/orange boundaries.
Table 1 summarizes our results.
or
As shown in Figure 1, subjects who placed the orange/red boundary farthest
to toward the red end of the spectrum also placed the “best orange” farthest
toward the red.
Figures
A drawing, graph, diagram, or photograph (in short, any kind of picture) is a Figure. Put its
label, number, and title (which should be as explicit as you can make it) beneath it, as shown
in this lab manual. (See, for example, Figure 1 on page 1, or Figure 15 on page 29.) Refer to
it as Figure __ at the appropriate place in your written account.
Color is used primarily to distinguish between two or more sets of data. For example, to put
two sets of data (e.g., hairiness of the entire first generation and that of the two sets of
selected parents in Figure 14) on the same graph in order to show the differences between
them, you must give the reader an easy way to distinguish between them. (Shades of gray or
hatch marks may stand in for color in a black-and-white printed sheet.)
You can copy and paste an Excel “chart” into a Word file, as I have done in this manual.
Then you can place your illustration wherever you think it best fits into your paper.
Excel doesn’t like to put titles below the graph, so I usually type them with Word. This
allows me to use Word’s automatic numbering capability. (Insert/caption lets you put in
figure and table numbers; Insert/cross-reference lets you refer to them later.) If you use this
automatic numbering, you can change the order of your figures and tables as you edit your
paper and they will renumber themselves, as well as all references to them. (You may have
to save, close, and reopen the file to see the renumbering occur, however.)
Tables
A list of numbers or words is a table, and gets numbered separately from the figures. Table
labels and titles go above the table, as illustrated in throughout this manual. (See for
example, Table 5on page 27, or Table 6, on page 28). Refer to tables by number as you do
figures.
Mount Holyoke College, Spring 2002
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Numbers
Reading a long list of numbers isn’t very interesting, so you need to come up with a way to
point out the salient features of a graph or table without telling every single value. Think
about what you find striking or instructive about the data, and point it out.
Avoid unnecessary decimal places. For example, it seems more efficient (and more
emphatic) to say that none of the plants selected for smooth petioles had any trichomes than
to say they had an average of 0.00 trichomes. (Since you count hairs, you should be using
whole numbers in any case.)
If you weigh it, or measure its volume, you have an amount; if you count them, you have a
number, e.g., an amount of dirt; a number of trichomes.
Discussion
This is where you interpret and explain your results. What do you think your results mean?
Are they as expected? Do they make sense? Are they interpretable? If not, why not?
(Don’t fall back on “experimental error.” This explains nothing.) Be explicit, e.g.,
The variability from one subject to the next suggests that naming of colors is
imperfect, at best. We think this variability is “real,” that is, it results from
differences between subjects rather than from inconsistencies in the
measuring device, since no subject placed the “best orange” outside the
boundaries she set at a separate time between orange and red or yellow.
or
The spectrum, as viewed in our device, was slightly skewed, and although we
had told subjects to use the top of the indicator line, perhaps getting better
alignment between indicator and spectrum would reduce some of the
variability from person to person.
References
It is important to give credit appropriately to the authors whose work you refer to (or borrow
from, or quote, or criticize, or otherwise make use of). This includes material you find on the
Internet as well as published articles in the scientific literature. Web citations should include
not only the URL, but also a title and as much as you can figure out as to the author or
sponsoring organization. The American Psychological Association publishes the APA Style
Guide at http://www.apastyle.org/elecref.html; this is a useful resource.
There are several citation formats in use in the scientific literature. One of the clearest and
most widespread is to put embedded references in the text and complete citations in
alphabetical order at the end of the paper. The embedded references look like this:
Williams and Hill (1986) describe the development of Fast Plants.
or
Fast Plants were bred for both research and teaching purposes (Williams and
Hill, 1986).
If there are more than 2 authors, you can abbreviate the citation like this: Jones, et al.
34
Artificial Selection in Brassica rapa
Page numbers are usually omitted when you cite the article in the body of your paper,
because most scientific articles are short. The complete citation at the end of the paper looks
like this:
Author, initial, year. Article title. Journal name, Volume (#): pages.
Williams, P.H., Hill, C.B., 1986. Rapid-Cycling Populations of Brassica. Science,
New Series, 232 (4756):1385-1389.
It’s not necessary to summarize your references one at a time. It might be more concise (and
more useful to the reader) if you combined the results by topic, something like this:
Although the number of trichomes was not correlated with the total number of
flowers or seeds, plants with more trichomes flowered later than those with
fewer (Agren & Schemske, 1994, 1995).
1
All references are from the Wisconsin Fast Plants web site: http://www.fastplants.org., retrieved 1/23/02.
The lab exercise is adapted from: Evolution By Artificial Selection: A 9-Week Classroom Investigation
using Rapid-Cycling Brassica rapa, Bruce A. Fall, Steve Fifield, and Mark Decker, University of Minnesota,
Minneapolis, MN 55455: http://www.fastplants.org/TeachingMaterials/Labs/FALL.pdf
2
Adapted from Research Origins of Fast Plants, http://www.fastplants.org/Introduction/Science.htm
3
Life stages material (Table 1, and Figure 10) adapted from Fast Plants Life Cycle: Seed to Seed in 35 Days,
http://www.fastplants.org/PDFs/PDF's%20for%20WFPID's/Growth%20and%20Development/Fast%20Plant
s%20Life%20Cycle.pdf
4
Day-by-day developmental material adapted from Fast Plants Life Cycle
http://www.fastplants.org/Introduction/LifeCycle.htm
5
Photograph from View a Fast Plant, http://www.fastplants.org/Introduction/ViewFP.htm, diagram from Fall
et al. (note 1).
6
Pieris material adapted from The Fast Plant and its Butterfly: Investigating the Life Cycles of Wisconsin
Fast Plants and the Brassica Butterfly, Wisconsin Fast Plants Notes, 2001, Volume 14 (1)
http://www.fastplants.org/PDFs/2001_Newsletter.pdf