Download SYSTEMATICS: INTRODUCTION by Dr. Adriana Aquino

Seminars on Science: Diversity of Fishes
SYSTEMATICS: INTRODUCTION
by Dr. Adriana Aquino
This is a course about the diversity of fishes, an amazing group of vertebrates
with a 530-millon-year-old history. We're going to start this week by looking at the
way we scientists at the American Museum of Natural History in New York study
fishes. Our thinking and methods combine both scientific and detective work. By
the end of this course you will know not only a lot more about this group of
animals, but also about how someone like myself studies organisms. I will give
you an insight into the kinds of questions I ask myself and then try to answer.
Questions like: What's out there in the biological world? How did it get there?
How does it all fit together? You will be able to participate in my detective work.
The name of the branch of science that studies the diversity of life is systematics.
I am a fish systematist. (I can also be called an ichthyologist, from the Greek
word ichthys, meaning fish.) There are systematists who specialize in groups of
organisms such as birds, primates, insects, plants, or bacteria. The American
Museum of Natural History (AMNH) is one of the top research institutions in the
world concerned with the study of systematics and biodiversity. I am going to
take you through the thinking process of a systematic biologist like myself stepby-step. Over the next few weeks, you'll see how I approach a problem.
But, how do I go about studying the diversity of a group of organisms?
Systematists describe living and extinct species, some already discovered and
some formerly unknown to science. We develop ways of ordering the diversity of
species. In other words, we find ways to define natural groups. This involves
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arranging organisms into groups that share a history of transformation and
diversification, making sense of the origin and extinction of a species.
I am a systematist who studies systematics for its own sake. However, I want you
to consider a more pragmatic reason for working on systematics: Researchers
dedicated to any other kind of biological study (ecology, evolution, physiology,
biochemistry, ethology, genetics, cytogenetics, anatomy), researchers working
for industries involving the use of nature (food, textiles, drugs), people whose
main concern is the protection of natural environments, all need to know the
details of the species they work with.
In order to introduce you to how I approach my work — the method, terms and
concepts I use — let’s imagine the following sequence of three historical events
during a period of 50 million years:
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STAGE
SITUATION-EVENT
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Fifty million years ago (m.y.a.),
Species A lived in an isolated
region.
DIAGRAM IN SPACE
DIAGRAM IN TIME
A
A
(Fig. 1)
Fig.1
2
A
Thirty m.y.a., a mountain chain
formed. The population was
divided into two separate
populations. After millions of
years, these two populations
evolved into two distinct species
(B and C).
(Fig. 2)
C
B
B
Fig. 2
C
A
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D
Fifteen m.y.a., in the distribution
area where B was found, a new
mountain formation divides the
population into two subgroups.
As a result, B evolves into two
distinct species, D and E.
C
B
E
Fig. 3
D
E
C
(Fig. 3)
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Year 2000: Here we come, the systematists of the 21st century,
trying to understand the diversity of the species living in this
region. We sample and determine that there are three different
species (C, D and E). See Fig. 3 for distribution.
C
E
D
We want to classify these three species in a way that reflects the
history of their interrelationship. We won’t be able to indicate the
ancestor of the Species D and E, but we can tackle this
investigation like detectives, and propose an HYPOTHESIS:
Species D and E are more closely related to one another than either
one is to C.
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HOW do we do this? The CLUES for solving this mystery are the
CHARACTERS of the organisms.
C
E
D
We find that all three species share a character (X) that is unique to
them and not shared by any other species. Thus we can propose our
hypothesis. They all had a common ancestor with character X.
Graphically, it looks like this:
X
E
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After a thorough anatomical study of our fish, we find another
character (Y). We find Y exclusively in D and E. It is not shared by
C. Therefore we can propose our hypothesis in the form of a
branching diagram, called a CLADOGRAM (clade = branch).
D
Y
The method of classification we used to create this branching
diagram, based on new and shared characters, is called
CLADISTICS.
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Now, let's propose a CLASSIFICATION system to illustrate our
hypothesis, one that will show the evolutionary relationships
between organisms. We classify Species D and E as belonging to
Genus 1, and Species C to Genus 2. Species C is classified in a
single genus while Species D and E are classified in a two-species
genus, i.e. there is only one species in Genus 2 and two species in
Genus 1.
The most important advantage of this method of classification is
that we can interpret the PHYLOGENETIC, or evolutionary,
information from the scientific names of each species: We can read
their names and conclude that Species D and Species E are more
closely related to each other than either is to Species C.
C
X
CLASSIFICATION
GENUS 1
SPECIES D
SPECIES E
GENUS 2
SPECIES C
We didn’t discover the actual ancestor of Species D and Species E,
but, with some certainty, we have been able to support our
hypothesis that those two species SHARE AN ANCESTOR, that is,
they share a COMMON ANCESTRY.
Our classification, therefore, is based on our hypotheses of natural
groupings, that is, groups that share an evolutionary history.
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An important use of phylogenetic hypotheses is prediction. Let's look at an
example. We find the presence of a chemical substance in a rare species which
is a key ingredient in a medicine. We know that this species has a very close
relationship with another species that has a wider geographic distribution. Based
on our hypothesis of common ancestry, the chemical substance should be
present in both of these species. This has economic implications for the
medicine. Phylogenetic hypotheses are being used to predict the presence of
particular characters. Therefore, systematics allows scientists to demonstrate
relationships that might otherwise be merely based on a researcher's intuition.
It's also important to be aware of the importance of the geographic distribution of
the species you are studying. Consider the earlier example in Figure 3: Three
species occupied three different areas, separated from each other by mountains.
Even if we don't know the geological history of these mountains, assume that
there's a temporal correlation between their formation and the evolution of two
new species from one common ancestor. We now have a proposed phylogenetic
hypothesis in which we claim that Species D and E are more closely related to
each other than either is to Species C. We also propose that the three species
together have an ancestor common to all three.
ASSIGNMENT
Use the information above to develop a geographic hypothesis of how these
species descended from a common ancestor. Post your hypothesis in the Forum.
There is another concept related to Systematics: TAXONOMY. This is the part of
systematics that focuses on the theory, process, and practice of classification. In
our example, we mentioned the categories SPECIES and GENUS. These
categories are part of a system that is in almost universal use today. It was
invented about 200 years ago by a Swedish botanist named Carl Linnaeus. The
LINNAEAN SYSTEM uses a hierarchy of categories to describe the evolutionary
(or phylogenetic) relationships: species, genus, family, order, class, phylum.
In the case of a species, a two-word name is used. This is called BINOMIAL
NOMENCLATURE. The first name refers to the organism's genus while the
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second name tells us the species name. We humans belong to the species
Homo sapiens. Another species of this genus is the extinct Homo erectus.
The ultimate objective of BIOLOGY is to understand the world.
SYSTEMATICS is the study of the natural world in terms of its biodiversity.
When the STUDY OF THE DIVERSITY OF LIFE is viewed through the lens of
the theory of EVOLUTION, it becomes equivalent to the STUDY OF THE
HISTORY OF LIFE.
As we work our way through this study, we are going to look closely at the topics highlighted below:
•
SYSTEMATICS as an academic sub-discipline
•
CLASSIFICATION as an activity within systematics
•
CLADISTICS as a method of classification
•
ASKING QUESTIONS IN SYSTEMATICS: FORM – TIME – SPACE
•
SOURCE OF DATA
•
CHARACTERS
•
FORM – DIFFERENTIATION: OBSERVATION and DESCRIPTION
•
WHAT GROUPS SHOULD BE RECOGNIZED IN THE CLASSIFICATION? HOW SHOULD
THEY BE SUBDIVIDED? TIME -CLADISTICS
•
WHAT NAMES AND RANKS SHOULD THE GROUPS AND SUBGROUPS HAVE IN THE
CLASSIFICATION? NOMENCLATURE
•
SPACE - BIOGEOGRAPHY
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