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Classification and Evolution
Provided by Dr. W. Harmon and Dr. B. Tripp
Learning Objectives:
1. Observe the variation of species within a genus.
2. Classify species based on characters and character states.
3. Understand the differences between stasigenesis, cladogenesis, and anagenesis.
4. Recognize examples of convergent and parallel evolution.
5. Recognize examples of analogous and homologous characters.
Background:
The world is filled with a wide diversity of living organisms. How did this diversity of species come about?
This is a central question to the understanding of living systems and central to biology. This exercise
again builds your observational skills while illustrating good examples of several evolutionary concepts.
Materials available:
examples of dichotomous keys
6 different species of Mammillaria or Conifers per lab table
Limulus
Psilotum
series of cacti and euphorbs illustrating parallel and convergent evolution
examples of analogous structures
examples of homologous structures
Terms:
dichotomous key
characters
character states
stasigenesis
cladogenesis
anagenesis
convergent evolution
parallel evolution
analogous characters
homologous characters
1. Cherernoff Faces
Class Participation Activity
BIO 101 Lab Classification and Evolution
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2. Variations of species within a genus
Conifers
Gymnosperms (“naked seeds”) are trees and shrubs that
produce exposed seeds, usually in cones, rather than seeds
enclosed in an ovary, as in the angiosperms (flowering
plants). Conifers, ginkgos, and cycads are all gymnosperms. The Pacific Northwest’s native
gymnosperms are all conifers.
Conifers (also known as “softwoods”) have leaves that are needlelike (long and slender) or scale-like
(small and overlapping), typically evergreen, and well adapted for drought and freezing temperatures,
thanks to a thick waxy coating and other protective features.
The distinctive cone is a reproductive structure comprised of a central axis with spirally arranged scales
hearing pollen or seeds. A single tree usually has both pollen-bearing (male) and seed-bearing (female)
cones; males are usually carried on lower branches, or lower down on the same branches as females.
Male cones appear in spring, shed pollen, and soon fall from the tree. Female cones are larger, woodier,
and have scales that protect the seeds until the cones expand to release them. Cones described in this
guide are female.
Most conifer species in the Northwest belong to the pine family (Pinaceae). In our area, those commonly
known as pines (genus Pinus) bear long needles in bundles of two to five. Other pine family members
have much shorter needles. Larches (genus Larix) bear needles in brush-like clusters that are deciduous
(shed seasonally) and that turn yellow in autumn; cones are stalked and round to egg-shaped.
Hemlocks (genus Tsuga) bear needles on woody cushions and have small cones at the branch rips.
Spruces (genus Picea) have rough twigs, hanging cones, and sharp, four-sided needles borne on tiny,
raised, woody pegs. Douglas firs (genus Pseudotsuga) also have hanging cones and flat needles, but the
needles grow directly from the branches. True firs (genus Abies) have upright cones that shed their
scales and seeds while still on the tree, and needles arising from tiny depressions on the branches.
Other conifers in the Northwest include those of the cypress family (such as cedars and junipers) and the
yew family. Most members of the cypress family (Cupressaceae) have narrow, scale-like leaves covering
their branches; their small cones are round and bell-shaped or (in the junipers) fleshy and berry-like.
Yews (family Taxaceae) have needles in two opposite rows and bear seeds nor in cones but individually
and surrounded by a fleshy, cup-shaped, berry-like structure called an aril.
In our area, conifers are usually part of a continuous forest canopy in which the shapes of individual trees
cannot be discerned. Most conifers are pyramidal in shape when young, especially if they grow in an
open area; they mature to a ragged columnar shape with a conical (commonly broken) top and a limbless
lower half. Observe the collection of conifer species provided at your table. In order to identify the
differences which are used to separate species, it is very useful to write a key to the species. However, in
order to write a key it is essential to identify the characters which might be used to describe the
differences between species.
BIO 101 Lab Classification and Evolution
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Characters and Character states of Conifers
Character
Possible Character States
1.
2.
3.
4.
5.
Cactus (if available)
The genus Mammillaria is a large genus in the Cactaceae (cactus family). It possesses many
morphological variations which are readily observable. Observe the collection of Mammillaria species
provided at your table. This genus of cactus differs from other genera of cacti in that the spine clusters
are born on tubercles or nipples (thus giving rise to the genus name). In order to identify the differences
which are used to separate species, it is very useful to write a key to the species. However, in order to
write a key it is essential to identify the characters which might be used to describe the differences
between species.
List below the characters and character states (alternate forms of the character expression) of as many
characters as you can identify. For example, the character may be color of spine and the possible
character states are white, yellow, black, and brown. Work as a group with other students at the same
table.
List below the characters and character states (alternate forms of the character expression) of as many
characters as you can identify. For example, the character may be color of stem and the possible
character states are white, yellow, black, and brown. Work as a group with other students at the same
table.
Characters and Character states of Mammilaria
Character
Possible Character States
1.
2.
3.
4.
5.
BIO 101 Lab Classification and Evolution
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Dichotomous Key Example
Keys used in the identification of taxa separate these taxa by use of contrasting descriptive statements
known as couplets.
For example:
1. Plants with spines............................................................
go to couplet 2
1. Plants without spines.......................................................
go to couplet 3
2. Plants with white spines..................................................
2. Plants with yellow spines.................................................
go to couplet 4
go to couplet 5
3.........................................................................................
????? go to couplet?
3........................................................................................ ????? go to couplet?
4.................................................................
4.................................................................
.Spines plumose (feather-like) M. plumosa
Spines not feather-like go to couplet 6
5...........................................
5...........................................
?????
?????
go to couplet?
go to couplet?
6...........................................
6...........................................
?????
?????
go to couplet?
go to couplet?
etc
In the first couplet all the plants with spines are separated from those which lack spines.
Those with spines are further divided in couplet 2 on the basis of spine color and then
divided again (couplet 4) on the basis of spine morphology. Eventually this process
leads to only one kind of plant for each choice, resulting in the identification of the
specimen. [In the above example, the choices in bold illustrate the decision path to
identify Mammillaria plumosa. This is a dichotomous key because there are two and
only two choices at each step. In writing keys, taxonomists ordinarily would not rely on
a single specimen for species descriptions, but would observe many individuals from as
many populations as possible to derive the species descriptions.
Look at your list of characters. Which are the most distinctive and easily separated into
two contrasting groups of character states? When you decide which is the best
character for couplet 1, it helps to physically divide plants into two groups. One group
will be directed to the second couplet and the other group will be directed elsewhere.
Continue the process with each subgroup until you only have one plant in each group.
BIO 101 Lab Classification and Evolution
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Your Task
Write a dichotomous key to the species of Cactus provided on your table similar to the example above.
[Note the example above is incomplete.] Rather than using the actual species names you will use letters
A-F to identify the different species. Work in groups.
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BIO 101 Lab Classification and Evolution
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Your Task
Write a dichotomous key to the species of Conifers provided on your table similar to the example above.
[Note the example above is incomplete.] Rather than using the actual species names you will use letters
A-F to identify the different species. Work in groups.
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BIO 101 Lab Classification and Evolution
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3 . Stasigenic, cladogenic, and anagenic evolution
In the evolution of plants and animals, changes appear in populations which may be very small involving
only minor details in the organism or changes may be very large resulting in drastically new survival
strategies.
Stasigenesis occurs when an organism does not change morphologically over long periods of time.
Psilotum in, a plant which has not changed very much for at least 350 million years, is a good example of
stasigenesis. Limulus, the horseshoe crab is a good example of an animal that has not changed over 250
million years. Specimens of both organisms are on display on the side table. When you look at these
organisms imagine going back in time. Many other organisms would be different yet the ancestors of
these two organisms would be so similar you may mistake them for siblings instead of great-grandparents
from millions of generations past.
Most groups of organisms show at least some degree of change through time. Observe the variation
between the species of plants within the genus Mammillaria or the variation of rodent species(Skulls). The
variations you observe within these two genera are examples of cladogenic variation. Each genus
represents a clade, or a branch off the same family tree.
Larger evolutionary changes are represented by differences expressed genera, families, and larger
taxonomic groups. The larger differences represent anagenic changes. Anagenic changes result when
plants or animals evolve major changes to adapt to basically new or novel strategies for survival.
Observe the group of cacti on the side table. Observe the differences in the genera from the more
“primitive” genus to the more modern genus. The degree of intergeneric change between the genera
represents anagenic change. Note such changes as the size and production of the leaves, size and
number of the spines, and overall size and shape of the plants. What are some of the adaptive strategies
represented by the differences?
Define:
Stasigenesis
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Cladogenesis
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Anagenesis
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BIO 101 Lab Classification and Evolution
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4. Convergent and parallel evolution
As climates change over periods of millions of years, all the plants within a region are subjected to many
of the same changes in selective pressures. The strategies employed by plants to adapt to changing
climates depends largely on their genetic potential and the nature of the climatic changes.
For example, there are many different ways in which plants have adapted to the rigors of desert
environments. Occasionally plants from different ancestries assume the same adaptive strategies to fill a
new niche, resulting in the evolution of similar morphological appearances. This process is known as
convergent evolution. In fact, many times the similarities are so great that it becomes difficult to identify
taxonomic relationships. if these convergent taxa continue to evolve to changing climate conditions
through time, they may produce a series of comparable morphological forms. This process is termed
parallel evolution. It refers to the series of convergent steps.
Perhaps one of the best examples of convergent and parallel evolution can be found by comparing the
cactus family (Cactaceae) and the euphorb family (Euphorbiaceae). Observe the demonstration on the
side table of convergent forms within these two families. Note the parallel trends in the evolution of these
families. Although the plants look alike, many of the similarities are superficial. For example, the spines of
the cactus are always modified leaves, while the spines of the euphorbs may be stipular spines or
even thorns (modified stems).
Define:
Convergent evolution __________________________________________________
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Distinguish convergent from parallel evolution
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Define physiological response
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5. Adaptive Gradation, Analogous structures and Homologous Structures
Visit the stations around the room and define the following terms. Work to understand
these terms via the examples provided.
Define and give an example of each (not one provided in class)
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Homologous Structure __________________________________________________
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Analogous Structure __________________________________________________
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Adaptive Radation __________________________________________________
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