Everything Varies

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EVERYTHING
VARIES!
UNIT 8
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Homepage

• Introduction
• Exercise 1 Recognizing Individuals as Unique
• Exercise 2 Matching Leaves
• Exercise 3 Using Math to Make Decisions about
  Variation in the Characteristics of Leaves
• Exercise 4 Matching Shells

Exercise List continued
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• Exercise 5 Making Sense of Variation The Game
• Exercise 6 Finding Species
• Exercise 7 Mechanisms
• Exercise 8 Mystery Identification,
• Suggested Readings Links

Return to Homepage
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Introduction

• There are over 2 million named species of plants
  and animals on Earth and many scientists feel
  that the actual number of types of organisms is
  well over 5 million.
• To start with, we need to know what variation is.
  It means to be a little bit different from others
  or from some typical pattern.
• Organisms vary because
• The Earth offers many different types of places
  to live physical structures such as mountains
  and water bodies create habitat variation.
• Climate adds to the environmental variability
  offered by physical structure in habitats.
  Climate (the weather patterns different parts of
  the world experience over time) is influenced by
  the shape of the Earth and its pattern of
  rotation around the sun

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The Student Will

• Learn the different methods scientists use to
  separate species by the characteristics that vary
  between them.
• Use both qualitative (observational) and
  quantitative (measurement) methods to make
  decisions about which organisms should be grouped
  together versus placed in different groups.

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Materials

• Container A
• 35 unique Leaves
• Container B
• 30 leaves each of two species
• Container C
• 31 mollusc shells with red dot on each
• Container D
• 30 mollusc shells with blue dot on each
• Container E
• Mystery Shell

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Objectives
Exercise 1. Recognizing individuals as unique.

• Exercise 1 helps students recognize the
  individual organisms unique traits by testing
  your skills in committing a leaf to memory that
  will vary a lot from many of the leaves available
  but only slightly from some.

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Directions

• 1st Run Before discussion
• Locate the Container marked A, which holds 35
  leaves from trees and shrubs that have been
  laminated. Each leaf has a unique number on its
  underside.
• The teacher will spread these out on the front
  table.
• Each student should select one leaf.
• Examine the top of this leaf for a few minutes
  with the object of committing it to memory.
• After studying the leaf, turn it over and copy
  the number that is on the back onto a sheet of
  paper. (Alternatively, the teacher might also
  keep track of the numbers for the students).
• Return each leaf to the front table.

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• The teacher will mix the leaves up on the front
  table, making sure that all are face up on it.
• Each student attempts to find his or her
  special leaf without looking at the number on
  its underside.

• Check the number of the leaf you picked from the
  table against the number written down for the
  leaf you originally examined.
• Count the number of students that were successful
  in finding their leaves. What was the class or
  group success rate (number correct/total number X
  100)?
• Example if 10 individuals out of 30 found their
  leaves, the success rate was 30.
• Discuss what characteristics the students used to
  remember their leaves and make an ordered list
  from the most frequently used characteristic to
  the least used trait.

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Fig. 1 Leaf traits that might vary among species
and even individuals.
To Exercise 1 cont
To Exercise 3a
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• For older students, prepare a bar graph, showing
  the relative importance of the characteristics
  the class used in identifying their leaves.
• Fig. 2 Example of relative importance of various
  characteristics in distinguishing among leaves.
  A. Absolute count. B. Relative count expressed as
  percent of individuals (Number of individuals
  using a particular trait/total number of
  individuals).

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• Play the game again with each student picking a
  new leaf.
• Check to see if the finding success rate has
  increased.

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Exercise 2 Leaf Match (very simple)

• Materials
• Leaves from container A
• Picture guide to tree leaves (Leaf Guide)
• How to play
• The tree guide sheet has pictures of all of the
  leaves that might be found in container A of this
  unit.
• Your goal is to find leaves in the batch that
  match each picture of a leaf that is shown to
  you.
• Below the picture is the common name (what local
  people call the tree or shrub) and scientific
  name (internationally registered name that
  reflects this species relationship to other trees
  as well as its unique characteristics.
• The scientific name consists of two parts the
  Genus (close relatives will all have this name)
  and species (only individual trees that might
  possibly interbreed share this name).

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Objective

• Exercise 2 helps students learn how to make
  comparisons among individual leaves to find those
  that are more similar to one another than are the
  majority of leaves in a sample.

 
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Directions

• Separate the class into groups of three or four
  students
• Partition the leaves in container A into piles so
  that each group has a unique pile of leaves to
  examine.
• The teacher will show on the screen three leaf
  species at a time.
• Each group of students should check their pile to
  match leaves in it to the images shown
• A representative from each group should bring
  potential matches up to the front for comparison
  with the image.
• Form piles of the different leaf species on the
  front table with all correctly matched leaves.
• label each pile by its common name and scientific
  name

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Directions continued

• Continue on to the next set of leaf images.
• Attempt at the end to place remaining leaves in
  the group piles into the correct species piles on
  the front table.
• Have a class discussion on what characteristic
  (s)
• makes each pile of leaves unique.
• makes some species piles more similar to one
  another than to others.

To Leaf images
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Image 1
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Image 2
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Image 3
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Image 4
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Image 5
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Image 6
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Exercise 3. Using math to make decisions about
variation in the characteristics of leaves

• For scientists, it is not enough to look at two
  individuals and decide that they differ in one or
  more characteristics (i.e., are qualitatively
  different).
• The differences must be quantified.
• The traits must be measured and expressed in
  numbers so that the differences between them can
  be statistically compared.
• Statistics is that branch of mathematics that
  organizes data such that central tendency
  (average or mean value) and the levels of
  variation around it can be found and compared
  between or among samples.

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3a. What is the expected value for a trait?

• Traits vary but within a local group of
  individuals there is a trait value that can be
  considered as typical (mean or central tendency)

Objective

• In this exercise we will quantitatively
  determine what is the typical size leaf of the
  sample of 30 leaves found in container B.

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• As a class, decide what measure of leaf size you
  would like to measure on one of the two sets of
  30 leaves in container B.
• You might want to base your estimate on the width
  of the leaf blade at its widest point, or its
  blade length from its tip to its base at the
  attachment of the petiole (Fig. 1). You could
  also get a rough estimate of the leafs blade
  area by multiplying the leaf blade width (W)
  times its length (L) as in (W X L).
• To Fig. 1
• Each student will receive a leaf and take the
  measures the class or group has agreed on.
• Lets say that you will round off each
  measurement to the nearest 1 cm (centimeter).
• Make a column for the trait on the board as shown
  in table 1.
• For Table Template, see Table 1

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Directions continued

• Find the smallest trait value and the number of
  students having measured this value.
• Record each value in column 1 of Table 1.
• Repeat for the next smallest trait value until
  all values have been recorded and the numbers are
  ordered from smallest to largest as shown in
  Column 1 of Table 1.
• Make two additional columns on the black board.
• Column 2 should list the possible leaf sizes
  (from 1 cm (the smallest size possible) to the
  largest width leaf in your batch.
• Column 3 lists the number of leaves you have
  representing each size. Put a 0 in your list for
  any size interval that has no leaves as shown for
  the 1 cm size class in Table 1.
• Save your data table for use in Exercise 3b

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Table 1.
Column 2 Possible leaf widths (cm) smallest to la
rgest
Column 1 Leaf widths ranked by size (cm)
Column 3 Number of leaves in size classes listed
in Column 2
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• It is time to make a graph.
• In the example shown in Fig. 1, the horizontal
  line at the bottom (X-axis) of the graph has 23
  1-cm intervals as this was the width of the
  largest leaf in the sample data set presented in
  Table 1.
• The Y-axis should have more intervals placed on
  it than the number of individuals of any one leaf
  size.
• In the example in Table 1, the largest number of
  individuals of a given size was 8 for a leaf
  width of 8 cm (Look in column 3.). The height of
  the Y-axis in the graph was thus set at 9.
• You are ready to plot your data. Put a dot for
  the leaf numbers present (Y- axis) for each width
  (X-axis) as shown in the sample graph.
• Finally, draw a line between each dot. This is
  your distribution of leaves with respect to an
  estimate of size (leaf width in the example).

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Fig. 1. Example plot of leaf width distribution
from 30 leaf sample.Y-axis
In the plot above, three mathematical estimates
mean, median and mode all indicate that typical
leaf width equals 8 cm in this example
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• The mean or average is calculated by adding up
  all of the leaf widths in column 1 of Table 1 and
  dividing this sum by the total number (n) of
  leaves measured (30 in this case) 238/30 7.9.
  Because we are measuring our leaves to the
  nearest cm, 7.9 is rounded off to 8.
• The median is determined by dividing the total
  number of leaves measured (n) by 2 and counting
  down the list of leaves ranked by size (from
  column 1) to the size designating that leaf. In
  the example 30 leaves/2 the 15th leaf and from
  column 1, we see that the 15th leaf 8 cm in
  width.
• The mode is equal to the most numerically
  prominent or common leaf. From columns 2 and 3 in
  Table 1, we see that the maximum number of leaves
  of a particular width in the sample was 8 (from
  column 3) and that these eight leaves were each 8
  cm in width (from column 2).

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• Note that these measures of central tendency or
  typical trait value do not always come out to be
  the same number.
• The estimate that is used most is the mean as it
  provides the most accurate number (i.e., 7.9
  compared to 8 for median and 8 for mode).
• The spread of leaf widths to the left and right
  of the peak at 8 cm on the graph provides a
  measure of variation in leaf width. The wider the
  number of intervals on the X-axis, the greater is
  the variation.
• Calculate the mean, median, and mode for leaf
  width in your sample of 30 leaves from Container
  B. Mark the location of the mean, median and mode
  on the graph you have made.

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• Just as the typical value can be calculated, so
  can the variation in trait values around this
  typical value.
• Variance is the most common measure of how
  variable a trait is.
• Variance is calculated from the mean.
• Calculate variance for your sample of leaves
  using the equation summarized on the next slide.

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• Subtract each leaf width value from the mean
  value you calculated under 8 and square the
  result of each subtraction ((mean leaf width
  value leafn)2). (Leaf values would come from
  column 1 of Table 1 but would not need to be
  sorted as in this column).
• Write your results down as you complete these
  difference calculations.
• Then add (sum (S)) all the new values together,
  and divide this sum by the quantity (n1), where
  n the total number of leaves in the sample.
• The following equation prescribes the calculation
  of variance
• Variance S((mean value leaf1)2 (mean
  value leaf2)2 mean value leaf3)2 mean
  value leaf4)2 . . . ) /n-1.
• The larger the value for variance relative to its
  mean, the greater is the variability in leaf size
  among leaves in the sample.

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3b. Comparing the sizes of leaves from two
samples.

• Container B contains two batches of 30 leaves
  collected from two different trees or shrubs.
• In this exercise we will quantitatively compare
  the typical leaf sizes of two species of trees.

Objective

• Students will learn
• how to determine whether two estimates are
  different from one another or not.
• the degree to which trait variability limits the
  ability to discriminate species as being
  different

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• Develop a distribution curve for each species as
  you did under Exercise 3a
• first making a table with the needed columns
• and then developing a graph with two axes.
• (You can use the data from the leaf sample you
  measured under 3a and only measure the other leaf
  sample here).
• This time you will plot two sets of points on
  your graph. Be sure to use different symbols for
  the two sets so that you do not confuse them when
  you are drawing your lines between points.
• Fig. 2 provides a model for you to follow in
  preparing this graph. Be sure to make the X-axis
  long enough to accommodate the largest size leaf
  present in the two samples and the Y-axis should
  accommodate the largest number of individuals of
  a given size.

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Fig. 2. Example of two species comparison of leaf
size distributions as estimated by leaf width.

• Examine Figure 2. Which species has the
  typically smaller leaves? Which species has the
  broader distributions of leaf sizes (greater
  variation)? Ask the same questions about your own
  two species comparison.

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• Calculate the mean, median, and mode for the two
  species you are comparing and show these results
  with arrows on your figure.
• Calculate the variances of leaf widths for
  species A and B. Do your numerical results fit
  what you would conclude from looking at your
  graph? They should, unless the two species are
  very similar in size.

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• We can actually determine whether the two means
  (central tendencies) noted for your leaf widths
  or other measures differ enough from one another
  to be able to say that the difference is
  meaningful or significant.
• Scientists conclude that two samples differ
  significantly from one another in trait value if
  at a probability of 95, the difference could not
  be accounted for by chance.
• When we make such comparisons, we are doing
  statistics or a statistical analysis. We apply a
  Students t-test to determine whether two sample
  means differ from one another.

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• The student's t-test for the comparison of two
  means uses the samples sizes (N), means,
  variances and standard deviations of the two
  samples.
• Because you know the sample sizes and have
  already calculated the mean and variance for
  species A and also for species B, you will not
  need to recalculate these here.
• Instead, find these values and list them as
  follows on a sheet of paper or the blackboard
• MeanA VarianceA NA SDA
• MeanB VarianceB NB SDB

The standard deviation (SD) is equal to the
square root of the sample variance.
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• Take the square roots of VarianceA and VarianceB
  and record your values in the table you are
  setting up under species SDA and SDB.
• Now you need to calculate the pooled estimate of
  standard deviation for the two species samples A
  and B. Lets call it SDAB.
• To calculate SDAB, you need to
• 1. Multiply the number of leaves minus one (NA
  -1) for species A times its variance. Write this
  value down (NA -1) VarianceA _____.
• 2. Do the same for species B (NB-1) VarianceB
  ________.
• 3. Add the two numbers together ((NA -1)
  VarianceA (NB -1) VarianceB _______
• 4. Take the square root of the sum just
  calculated under step 3. Write this number down
  as SDAB.
• 5. Sum the two sample sizes (NA NB) and
  subtract 2 from this sum as in (NA NB)-2 ____
• _.
• 6. Finally Divide the values calculated under
  step 4 by the sum calculated under step 5.
  value4/value 5 SDAB

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• The equation that describes these calculations
  looks like this
• SDAB (NA -1) VarianceA (NB -1) VarianceB
• (NA NB) - 2
• Congratulations! You have calculated the pooled
  standard deviation of the two means.
• We also have to calculate the test statistic t
  that will be compared to a standard from a table
  to determine whether the means differ
  significantly.
• ___ MeanA - Mean B
• SDAB 1/NA 1/ NB t

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• Subtract the mean for species B from that for
  species A _____
• Divide the sample size (number of leaves) of
  species A into 1 ________.
• Divide the sample size (number of leaves) of
  species B into 1 ________.
• Sum the results for steps 2 and 3 and take the
  square root of this sum.
• Multiply the result from 4 by the pooled species
  estimate of standard deviation. _____
• Divide the difference between the two means from
  1) by the result of the calculation in 5).

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• Now you need to check your value for t against a
  predicted distribution.
• We have looked this up on a table of statistical
  values for a sample size of 30 leaves for each of
  two populations.
• If your value for t is greater than 2.83, the
  sizes of your two species of leaves differ
  significantly at P lt 0.05.
• This means that 95 of the time, the differences
  you measured would not be due to errors in
  measuring the leaves or some other random effect.

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Exercise 4 Simple Shell Match (very simple)

• Materials
• Shells from containers C or D

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Objective

• Exercise 2 helps students learn how to
  discriminate between shells that are of like and
  unlike type

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Directions

• The teacher will spread the shells in Container C
  out on a desk at the front of the room.
• Each student will pick out a shell and take it to
  his or her seat
• The teacher will then display the different shell
  types on the screen at the front of the room
• When a student sees the image of the shell in his
  possession
• raise your hand
• bring the shell up to the front for verification
  and return to the table

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Directions continued

• If some students still have shells, remaining at
  the end of one round of showing pictures, have
  them bring them up to the front where they can
  attempt to find a matching shell that has been
  returned.
• At this point, ask the class the name of the
  unknown shell.
• Return to the images to locate that shell type
  and discuss the characteristics of this shell
  that make it unique.
• Put all of the shells back in Container C
• For images and names of the shells in Container
  C
• Repeat the exercise with the shells from
  Container D
• For images and names of the shells in Container D
  

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Container C Moon Shell
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Container C Turban Shell
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Container C Calico Scallop
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Container C Pear Whelk
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Container C Spindle Shells
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Container C Rose Cockles
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Container C Osyters
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Container C Nucleus Scallops
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Container C Orange Scallop
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Container D White Nautica
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Container D Ecphora (Fossil)
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Container D Nerite
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Container D King Crowns(highly variable)
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Container D Cebu Beauty
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Container D Lettered Olive
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Container D Candy Cane

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Container D Money Cowrie
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Container D Babylon
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Container D Horse Conch
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Container D Lace Murex
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Container D Apple Murex
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Container D Yellow Land Snail
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Shell Guide Box D continued
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Exercise 5. Making Sense of Variation The
Matching Game

• Mollusc shells come in a truly amazing variety of
  shapes, sculptures, patterns and colors.
• There are about 100,000 species in all, each with
  its own special combination of features.
• Each mollusc shell in Container C has
  characteristics that distinguish it from every
  other shell in the box. Yet some individuals
  share more characteristics in common than do
  others.
• We should be able to sort these shells on the
  basis of size, color pattern, shape, texture, and
  so on.
• This exercise and Exercise 6 demonstrate how
  biologists make decisions about the relationships
  among organisms based on the characteristics they
  share and dont share with other individuals.

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• Before starting this exercise, check to see that
  each shell in Container C has a red dot on it
  somewhere.
• Use Fig1. below to review some of the features
  of mollusc shells and Fig. 2 on the next slide

Fig. 1 Basic features of Gastropod/snail
Bivalve/clam shells Handout
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Fig. 2. Examples of different characteristics of
shells handout
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Directions

• Spread the shells from Container C out on a table
  at the front of the room.
• Each student should pick one of these shells.
• After everyone has a shell, each individual
  should seek out others in the class who have
  shells that resemble his/hers.
• Each group then
• Should make up a group name and write this at the
  top of two sheets of paper
• Should develop a list of characteristics that
  accurately describe the shell type all members of
  the group share and list these characters on both
  sheets of paper
• On the first sheet of paper, list the numbers
  found on each shell

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• Return your shells to the front desk and give
  your teacher the second sheet that has only your
  group name and the list of shell characteristics
  you have developed
• After all groups have turned in their shells and
  shell descriptor lists, the teacher will give
  each group a list of descriptive characteristics
  made by a different group.
• The teacher should now spread the shells out on a
  long table.
• Using the lists of shell descriptive
  characteristics, the new groups should try to
  find the correct shells.
• Once they think that they have found all of the
  correct shells
• each group will read the descriptive list of
  traits out loud to the rest of the class showing
  the matching shells they found as they do so.
• They should also read the numbers on the shells
  so that the group making the list can compare
  these numbers to their list of shells.

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• If a new group had trouble identifying the
  correct shells, the class should discuss what
  changes might be made in the list of descriptive
  characters that would make it easier to identify
  the correct shells.
• Please check to see that all red dot shells are
  placed back in Container C.
• At the end, the class can check the names and
  general types of their shells on the Shell Guide
  C

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Exercise 6. Finding Species

• The Animal Kingdom is at the apex or top of a
  hierarchy.
• This Kingdom consists of multicellular organisms
  that obtain their food by eating other organisms.
  
• The Kingdom is divided into approximately 30
  Phyla, which share similar body plans.
• The phylum is at the next highest level of the
  hierarchy.

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• The Mollusca is a phylum characterized as having
  a body cavity that houses organs, a mantle cavity
  that in part provides gas exchange, and some form
  of shell for protection.
• Each Phylum is divided into classes the Mollusca
  has five classes. The two classes of molluscs
  used in this exercise include the Gastropoda
  (snails) and the Bivalvia (clams and their
  relatives).
• Below the Class level in the hierarchy, there are
  four additional levels of organization Order,
  Family, Genus and finally Species.

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Objective

• In this exercise, you will learn about
  classification and how it is hierarchically based.

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Directions

• Your goal in this exercise is to partition the
  samples of molluscs into two groups down a
  hierarchical scale to the point at which only
  very similar shells are present in each group
  left.
• During the process, you will develop a tree
  similar to the one shown below

• If you are successful, each group of shells that
  you have derived by your successive splits of the
  original sample of 30 shells will represent a
  species.

Step 1. As a class examine Figures 1-2 on the
next 3 slides that show different features and
characteristics of shells and discuss how a
sample of shells was partitioned in a
hierarchical scheme.
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Fig. 1. Names and locations of traits on a
gastropod (snail) shell (upper diagram) and
bivalve (clam) shell (lower diagram).
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Fig. 2. Examples of different characteristics of
shells
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Fig. 3. Partitioning example

• In the example, the first split separates one
  species from all others a clam (Bivalvia) - on
  the right (B in the figure) versus 8 species of
  snails (Gastropoda) on the left (A in the
  figure).
• The character used was whether the shell was
  spiraled or not
• Inspection of the figure shows that there were
  five splits completed to get down to species
  level 1 split separated bivalves from gastropods
  and 4 additional splits in group A were required
  to get down to the species level in the snails.

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Directions continued

• Step 2. Spread the shells from Container C (red
  dot) out on a table in the front of the room.
• Step 3. Divide the class into groups of 3-4
  students.
• Step 4. Each group should have the opportunity to
  visit the shell sample and complete a number of
  splits on them to arrive at the individual
  species, using the handouts at the table
• Characteristics features of shells
• Example of hierarchical scheme
• Step 4. Each group should make a drawing of the
  hierarchy made on a sheet of paper.
• Be sure to label the trait differences you used
  in each split.
• Write down the numbers on each shell where it
  ends up in the diagram
• Include the names of all members of the group
• When all the groups have completed to exercise,
  they will each copy their hierarchy onto the
  board, including the shell numbers

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Time To Check Your Answers
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• Box C Each shell has a unique number, listed
  below by type

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Directions continued

• Have a class discussion that compares and
  contrasts the
• different hierarchies formed using the same
  sample of shells.
• Some Example Questions For Discussion
• What splits are similar, dissimilar?
• Did they all end up with the partitioning of
  species?
• Are the clams and snails separated early on in
  the hierarchy?

• Be sure that all red dot shells are put back in
  Container C.
• Repeat this exercise using the Shell sample
  found in
• Container D.

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Box D Each shell has a unique listed in
table by type
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Shell Guide Box D continued
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Exercise 7. Mechanisms

• Thus far in our exploration of variation, we have
  learned that individuals differ in various traits
  and that we can measure these differences.
• We have also learned that individuals vary in the
  degree to which they differ from other organisms
  and that individuals are grouped with other
  individuals that are most similar into species,
  to those who are a bit less similar into genera,
  to those who are even less similar into families
  and so on up a hierarchical scale that deals with
  the classification of organisms.
• This particular exercise introduces you to the
  mechanisms that may underlie trait variation. We
  will use leaves and flowers as examples here.

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Objective

• Exercise 7 helps students understand why
  organisms vary.

 
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Directions

• Examine the leaves in Fig. 1. These leaves were
  collected off of the same tree. In fact, they all
  came from the same branch but note that the
  individuals differ in size.
• Differences in size as well as other traits
  frequently are associated with development or
  growth.
• As the young leaf unfolds from its leaf bud, it
  now has room to grow into a larger size. All it
  needs is nourishment and time to increase in size
  to that typical for the mature (adult or fully
  grown) leaf of this species.
• Rank the leaves in Figure 1 from smallest to
  largest?
• STOP ! The answer is next

From smallest to largest C, A, B
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Fig. 1.Variation in leaf size as it is influenced
by age.
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• Do you know which tree species the leaves in
  Fig. 1 belong to?

STOP ! The answer is next
Tulip Poplar (Liriodendron tulipifera)

• Examine the two sugar maple leaves in Fig 2.
  Unlike those shown in Fig. 1., these two leaves
  are both mature. Yet they differ markedly in
  size.
• These two leaves were collected from different
  parts of the tree one from a branch near the top
  of the tree where it is exposed to the sun and
  the other from one of the lower branches, where
  there is less sun.
• These leaves are influenced by the environment in
  which they are growing.

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Fig. 2. Two mature (adult) leaves from different
locations on the same tree.
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• Leaves have genes that program plasticity or
  flexibility in growth. This plasticity permits
  individual leaves to adjust to the light
  environment in which they are located in so that
  each leaf can contribute as much food to the tree
  as possible.
• Which leaf in Fig. 2 was collected from the
  lower branch that was in the shade and why is
  this leaf the size it is compared to the other
  leaf?

STOP ! The answer is next
Leaf A came from a top branch that was exposed to
the sun, and B from a lower branch that was in
the shade. Because leaves trap sunlight to
produce food for the plant, leaves that are in
the shade need a greater surface area over which
to catch the suns rays than leaves that are in
full sun. If both leaves were the same size,
then A would be producing a much larger quantity
of sugar than B.
104

• Examine the sassafras leaves in Fig. 3.
• Note that there are four different leaf shapes
• 1. elliptical (balloon-shaped)
• 2. two lobes, one on either side of the main part
  of the leaf blade
• 3. a single lobe on the left side
• 4. a single lobe on the right side
• Morphs 3 and 4 look like left and right-handed
  mittens. Find each of the four leaves on the
  diagram.

Fig. 3.
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• Sassafras is a tree that produces all four leaf
  types and often on the same branch.
• We call a distinct variant that has a genetic
  basis a morph.
• Sassafras exhibits a shape polymorphism (many
  morphs) consisting of 4 morphs.
• Can you think of other examples of polymorphisms?

STOP ! The answer is next
Other examples of polymorphism include butterfly
and flower colors within the same species. Some
spider species have different body patterns such
as the Hawaiian Happy Face Spider that has
several different faces. Many animals including
fishes have different male morphs such as a
small individual that sneaks matings and a large
male that defends females from other males.
These are just a few examples.
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• We have learned that leaves vary in size and
  shape for a number of reasons.
• Examine the flowers in Fig. 4 Can you see
  differences between the two flowers shown from a
  red lily plant (Fig. 4a) or among the many
  flowers pictured of purple coneflowers in Fig.
  4b?
• Of all the parts of a plant, the flower is the
  least variable. We call traits that show little
  inter-individual variation conserved traits.
• Flowers house the reproductive organs of the
  plant and to reproduce, flowering plants need
  animals to carry pollen produced by the male
  anthers to the female organ (pistil) of another
  plant.

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Fig. 4. Examples of lack of variation in flower
structure a) red lily, b) purple coneflower
B.
A.
108

• It is important that the animal species that
  pollinates a particular plant species recognizes
  its flowers as unique from other species so that
  it can visit only flowers of the correct type.
• Every mistake that a bee makes in visiting the
  wrong flower is wasted pollen to the plant the
  bee has visited earlier.
• Pollen is very costly to make and is species
  specific. Seeds will not form if the wrong pollen
  is encountered. Thus the flowers of a particular
  plant species do not vary much from one another.

109
Exercise 8. Mystery Shell

• What is it?
• STOP!!! The Answer is next!!!
• The mystery shell is actually the operculum of a
  snail. The operculum is a shell door, which many
  but not all snails have. With this door, the
  snail is able to close its shell to survive
  periods of drought and to gain protection against
  predators. Snails that live along the seashore
  are often exposed to air when the tide goes out
  and there are many animals that feed on snails,
  especially birds.

110
 
111
Suggested Readings
Survival of the Sickest A Medical Maverick
Discovers Why We Need Disease by Sharon Moalem,
Jonathan Prince, Jonathan Prince The Three
PigsDavid Wiesner, David Wiesner
(Illustrator) Mutants On Genetic Variety and
the Human BodyArmand Marie Leroi Calculus of
Variations by I. M. Gelfand, S. V. Fomin, Richard
A. Silverman (Editor), Richard A. Silverman
(Translator)
112
Links
Exercise 6 http//science.education.nih.gov/custom
ers.nsf/HSGenetic.htm http//www.standards.dfes.g
ov.uk/schemes2/secondary_science/sci07d/
113
Table 1.
Column 2 Possible leaf widths (cm) smallest to la
rgest
Column 1 Leaf widths ranked by size (cm)
Column 3 Number of leaves in size classes listed
in Column 2