CLASSIFICATION AND EVOLUTION
Robert
P. Gendron
Indiana
University of Pennsylvania
Humans
classify almost everything, including each other. This habit can be quite useful.
For example, when talking about a car someone might describe it as a
4-door sedan with a fuel injected V-8 engine.
A knowledgeable listener who has not seen the car will still have a good
idea of what it is like because of certain characteristics it shares with other
familiar cars. Humans have been
classifying plants and animals for a lot longer than they have been classifying
cars, but the principle is much the same.
In fact, one of the central problems in biology is the classification of
organisms on the basis of shared characteristics. As an example, biologists classify all organisms with a backbone
as "vertebrates." In this
case the backbone is a characteristic that defines the group. If, in addition to a backbone, an organism
has gills and fins it is a fish, a subcategory of the vertebrates. This fish can be further assigned to smaller
and smaller categories down to the level of the species. The classification of organisms in this way
aids the biologist by bringing order to what would otherwise be a bewildering
diversity of species. (There are
probably several million species ‑ of which about one million have been
named and classified.) The field
devoted to the classification of organisms is called taxonomy [Gk. taxis, arrange, put in order + nomos,
law].
The modern
taxonomic system was devised by Carolus
Linnaeus (1707‑1778). It is a
hierarchical system since organisms
are grouped into ever more inclusive categories from species up to
kingdom. Figure 1 illustrates how four
species are classified using this taxonomic system. (Note that it is standard practice to underline or italicize the genus and species names.)
|
KINGDOM |
Animalia |
Plantae |
||
|
PHYLUM |
Chordata |
Arthropoda |
Angiospermophyta |
|
|
CLASS |
Mammalia |
Insecta |
Monocotyledoneae |
|
|
ORDER |
Primate |
Carnivora |
Hymenoptera |
Liliales |
|
FAMILY |
Hominidae |
Canidae |
Apidae |
Liliaceae |
|
GENUS |
Homo |
Canis |
Apis |
Alium |
|
SPECIES |
sapiens (human) |
lupus (wolf) |
mellifera (honeybee) |
sativum (garlic) |
Figure 1
In
the 18th century most scientists believed that the Earth and all the organisms on
it had been created suddenly in their present form as recently as 4004 BC. According to this view, Linnaeus' system of
classification was simply a useful means of cataloging the diversity of life. Some scientists went further, suggesting
that taxonomy provided insight into the Creator's mind ("Natural
Theology").
This view of taxonomy changed
dramatically when Charles Darwin published On The Origin of Species in 1859.
In his book Darwin presented convincing evidence that life had evolved through
the process of natural selection. The evidence gathered by Darwin, and
thousands of other biologist since then, indicates that all organisms are
descended from a common ancestor. In
the almost unimaginable span of time since the first organisms arose (about 3.5
billion years) life has gradually diversified into the myriad forms we see
today.
As
a consequence of Darwin's work it is now recognized that taxonomic
classifications are actually reflections
of evolutionary history. For
example, Linnaeus put humans and wolves in the class Mammalia within the phylum
Chordata because they share certain characteristics (e.g. backbone, hair,
homeothermy, etc.). We now know that
this similarity is not a coincidence; both species inherited these traits from
the same common ancestor. In general, the greater the resemblance
between two species, the more recently they diverged from a common
ancestor. Thus when we say that the
human and wolf are more closely related to each other than either is to the
honeybee we mean that they share a common ancestor that is not shared with the
honeybee.
Another way of
showing the evolutionary relationship between organisms is in the form of a phylogenetic tree (Gk. phylon,
stock, tribe + genus, birth, origin):
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|
Figure 2
The vertical
axis in this figure represents time.
The point at which two lines separate indicates when a particular
lineage split. For example, we see that
mammals diverged from reptiles about 150 million years ago. The most
recent common ancestor shared by
mammals and reptiles is indicated by the point labeled A. The horizontal axis represents, in a general
way, the amount of divergence that has occurred between different groups; the
greater the distance, the more different their appearance. Note that because they share a fairly recent
ancestor, species within the same taxonomic group (e.g. the class Mammalia)
tend to be closer to each other at the top of the tree than they are to members
of other groups.
Several types of
evidence can elucidate the evolutionary relationship between organisms, whether
in the form of a taxonomic classification (Fig. 1) or a phylogenetic tree (Fig.
2). One approach, as already discussed,
is to compare living species. The
greater the differences between them, the longer ago they presumably
diverged. There are, however, pitfalls
with this approach. For example, some
species resemble each other because they independently evolved similar
structures in response to similar environments or ways of life, not because
they share a recent common ancestor.
This is called convergent evolution because distantly related
species seem to converge in appearance (become more similar). Examples of convergent evolution include the
wings of bats, birds and insects, or the streamlined shape of whales and
fish. At first glance it might appear
that whales are a type of fish. Upon
further examination it becomes apparent that this resemblance is superficial,
resulting from the fact that whales and fish have adapted to the same
environment. The presence of hair, the
ability to lactate and homeothermy clearly demonstrate that whales are
mammals. Thus, the taxonomist must take
into account a whole suite of characteristics, not just a single one.
The fossil
record can also be helpful for constructing phylogenetic trees. For example, bears were once thought to be a
distinct group within the order Carnivora.
Recently discovered fossils, however, show that they actually diverged
from the Canidae (wolves, etc.) fairly recently. The use of fossils is not without its problems, however. The most notable of these is that the fossil
record is incomplete. This is more of a
problem for some organisms than others.
For example, organisms with shells or bony skeletons are more likely to
be preserved than those without hard body parts.
The
Classification and Evolution of Artificial Organisms
In this lab you
will develop a taxonomic classification and phylogenetic tree for a group of
imaginary organisms called Caminalcules
after the taxonomist Joseph Camin who devised them. At the back of this chapter are pictures of the 14
"living" and 58 "fossil" species that you will use. Take a look at the pictures and note the
variety of appendages, shell shape, color pattern, etc. Each species is identified by a number
rather than a name. For fossil
Caminalcules there is also a number in parentheses indicating the geological age of each specimen in millions
of years. Most of the fossil
Caminalcules are extinct, but you will notice that a few are still living (e.g.
species #24 is found among the living forms but there is also a 2 million year
old fossil of #24 in our collection).
The purpose of
this lab is to illustrate the principles of classification and some of the
processes of evolution (e.g. convergent evolution). We do these exercises with artificial organisms so that you will
approach the task with no preconceived notion as to how they should be
classified. This means that you will
have to deal with problems such as convergent evolution just as a taxonomist
would. With real organisms you would
probably already have a pretty good idea of how they should be classified and
thus miss some of the benefit of the exercise.
Exercise 1: The Taxonomic
Classification of Living Caminalcules
Carefully examine the fourteen living species and
note the many similarities and differences between them. On a sheet of notebook paper create a
hierarchical classification of these species, using the format in Figure 3. Instead of using letters (A, B, ...), as in
this example, use the number of each Caminalcule species. Keep in mind that Figure 3 is just a
hypothetical example. Your
classification may look quite different than this one.
|
PHYLUM CAMINALCULA |
|||||||||||
|
CLASS 1 |
CLASS 2 |
||||||||||
|
ORDER 1 |
ORDER 2 |
ORDER 3 |
|||||||||
|
FAMILY 1 |
FAMILY 2 |
FAMILY 3 |
FAMILY 3 |
||||||||
|
GENUS 1 |
GENUS 2 |
GENUS 3 |
GENUS 4 |
GENUS 5 |
GENUS 6 |
||||||
|
A |
G |
H |
D |
B |
J |
L |
E |
K |
C |
F |
I |
Figure
3
The first step in this exercise is to decide which species
belong in the same genus. Species
within the same genus share characteristics not found in any other genera
(plural of genus). The Caminalcules
numbered 19 and 20 are a good example; they are clearly more similar to each
other than either is to any of the other living species so we would put them
together in their own genus. Use the
same procedure to combine the genera into families. Again, the different genera within a family should be more
similar to each other than they are to genera in other families. Families can then be combined into orders,
orders into classes and so on.
Depending on how you organize the species, you may only get up to the
level of order or class. You do not
necessarily have to get up to the level of Kingdom or Phylum.
Exercise 2. The Comparative Approach to Phylogenetic
Analysis
Construct
a phylogenetic tree based only on your examination of the 14 living
species. This tree should reflect your
taxonomic classification. For example,
let us say you have put species A and G into the same genus because you think
they evolved from a common ancestor (x). Their part of the tree would look like the
diagram on the right.
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When there are three or more species in a
genus you must decide which two of the species share a common ancestor not
shared by the other(s). This diagram
indicates that species E and K are more closely related to each other than
either is to C. We hypothesize that E
and K have a common ancestor (y) that is not shared by C. Similarly, two genera that more closely
resemble each other than they do other genera presumably share a common
ancestor. Thus, even in the absence of
a fossil record it is possible to develop a phylogenetic tree. We can even infer what a common ancestor
like y might have looked like.
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Exercise 3. The Phylogeny
of Caminalcules
Using
a large sheet of paper, construct a phylogenetic tree for the
Caminalcules. Use a meter stick to draw
20 equally spaced horizontal line on the paper. Each line will be used to indicate an interval of one million
years. Label each line so that the one
at the bottom of the paper represents an age of 19 million years and the top
line represents the present (0 years).
Cut out all the Caminalcules
(including the living species). Put
them in piles according to their age (the number in parentheses). Beginning with the oldest fossils, arrange
the Caminalcules according to their evolutionary relationship. Figure 4 shows how to get started.
Like
this Not
this
Figure 4
Hints,
Suggestions and Warnings
a. Draw lines faintly in pencil to indicate the
path of evolution. Only after your
instructor has checked your tree should you glue the figures in place and
darken the lines.
b. Branching should involve only two lines at
a time:
c. Some
living forms are also found in the fossil record.
d. There are gaps in the fossil record for
some lineages. Also, some species went
extinct without leaving any descendants (remember the dinosaurs, Fig. 1).
e. The Caminalcules were numbered at random;
the numbers provide no clues to evolutionary relationships.
f. There is only one correct phylogenetic
tree in this exercise. This is because
of the way that Joseph Camin derived his imaginary animals. He started with the most primitive form
(#73) and gradually modified it using a process that mimics evolution in real
organisms. After you complete your
phylogeny compare it with Camin's original.
Problems
1. You will notice that some lineages (e.g. the descendants of species 56) branched many times and are represented by many living species. Discuss the ecological conditions that you think might result in the rapid diversification of some lineages (A real world example would be the diversification of the mammals at the beginning of the Cenozoic, right after the dinosaurs went extinct.)
2. Some
lineages (e.g. the descendants of species 58) changed very little over
time. A good example of this would be
“living fossils” like the horseshoe crab or cockroach. Again, discuss the ecological conditions
that might result in this sort of long-term evolutionary stasis.
3. Some Caminalcules went extinct without
leaving descendents. In the real world, what factors might increase or decrease
the probability of a species going extinct?
4. Find two additional examples of convergent evolution among the Caminalcules. This means finding cases where two or more
species have a similar characteristic that evolved independently in each lineage. The wings of bats, birds and bees is an example of convergence
since the three groups did not inherit the characteristic from their common ancestor. Write your answers in complete sentences
(e.g. “Species x and y both have ____ but their most recent
common ancestor, z, did not”).
List two additional real-world examples
of convergent evolution (ones that we have not already talked about in class)
and discuss what might have caused the convergence.
5. Describe two examples of vestigial structures that you can find
among the Caminalcules. These are
structures that have been reduced to the point that they are virtually useless. Ear muscles and the tail bones are examples
of vestigial structures in our own species.
Explain
how vestigial structures provide clues about a species’ evolutionary past. Illustrate your argument with vestigial
structures found in humans or other real species.
