by Dr. Gary Rosenberg
Three closely related terms --
classification, systematics and
taxonomy -- are important for understanding how
biological groups are recognized and named.
Classification is the
distribution of objects into groups. In biology, the objects
are organisms (individuals), populations, species, or groups
of higher rank.
Systematics is the science
of classifying biological diversity. Biologists use various
methods for estimating how closely species are related to each
other, including comparative anatomy, genetics, and paleontology
(the study of fossil organisms). The groups reflect biologists'
best estimate of the relationships of the organisms.
Taxonomy is the naming
of biological groups. These groups are called "taxa."
The singular of "taxa" is "taxon." In the
hierarchy of names above, each of the examples is a taxon.
To biologists, similarity per
se is not what allows recognition of groups, because not all
similarity indicates relatedness. For example, whales and sharks
both have fins, but many other features of their anatomy lead
us to believe that they evolved fins independently. Biologists
attempt to identify similarities due to descent from a common
Organisms can have primitive
similarity because of distant ancestry, or derived similarity
because of immediate ancestry. Derived similarities in particular
allow identification of biological groups. For example, both
humans and chimpanzees have four limbs and hair, and they nurse
their young, but these features do not tell us that humans and
chimps are closely related, because all mammals have these features.
They are primitive characteristics that reflect distant ancestry.
These same features are derived similarities in relation to
other groups. Thus having four limbs is a derived feature that
separates tetrapods (mammals, birds, amphibians, reptiles and
some fish) from all other organisms. Hair is a derived feature
that separates mammals from other tetrapods. This means that
the immediate common ancestor of all mammals had hair, and no
other organism did.
The pattern of derived similarities
is used to infer the relationships of the taxa, and is presented
in a branching diagram called a cladogram (clados is Greek for
branch), which is an evolutionary tree. It can be very difficult
to determine whether a given similarity is convergent (as in
fish and whale fins), primitive, or derived. This means that
it can be very difficult to find the correct tree showing the
relationships of a group of organisms. Biologists are continually
refining their ideas of the relationships of organisms and,
as a result, groups are often redefined or undergo name changes
as our knowledge increases.
There is always a temptation
to take evolutionary trees as representing progress. People
often refer to one organism as primitive and another as advanced,
but an organism is never entirely primitive -- it will have
some primitive features and some derived ones. The broadest
definition of evolution is simply "change over time."
Evolution does not necessarily mean progress. A species that
is restricted to caves might become blind -- this is change,
but if it is progress, it is progress in only a local sense.
The above material
has been adapted from Dr. Rosenberg's The Encyclopedia of Seashells,
published by Robert Halt, Ltd., London, 1992.