Biomedical Engineering Reference
In-Depth Information
swam in the ocean like fish, and this was all the proof they needed. For
about two thousand years following the death of Aristotle, biologists per-
sisted in their belief that dolphins were a type of fish. For the past several
hundred years, biologists have acknowledged that Aristotle was correct
after all; dolphins are mammals.
Aristotle, and legions of taxonomists that followed him, understood that
taxonomy is all about finding the key properties that characterize entire clas-
ses and subclasses of organisms. Selecting the defining properties from a
large number of morphologic, developmental and physiologic features in
many different species requires attention to detail, and occasional moments
of intellectual brilliance. To build a classification, the taxonomist must per-
form the following: (1) define classes (i.e., find the properties that define a
class and extend to the subclasses of the class); (2) assign species to classes;
(3) position classes within the hierarchy; and (4) test and validate all the
above. These tasks require enormous patience and humility.
A classification is a hierarchy of objects that conforms to the following
principles:
1.
The classes (groups with members) of the hierarchy have a set of properties or
rules that extend to every member of the class and to all of the subclasses of
the class, to the exclusion of all other [unrelated] classes. A subclass is itself a
type of class wherein the members have the defining class properties of the
parent class plus some additional property(ies) specific for the subclass.
2.
In a hierarchical classification, each subclass may have no more than one
parent class. The root (top) class has no parent class. The biological clas-
sification of living organisms is a hierarchical classification.
3.
At the bottom of the hierarchy is the species. In the classification of
living organisms, the species is the collection of all the organisms of the
same type (e.g., every squirrel belongs to a species of “squirrel”).
4.
Classes and species are intransitive. As examples, a horse never becomes
a sheep, and Class Bikonta never transforms into Class Unikonta.
5.
The members of classes may be highly similar to each other, but their
similarities result from their membership in the same class (i.e., conform-
ing to class properties), and not the other way around (i.e., similarity
alone cannot define class inclusion).
It is important to distinguish a classification system from an identification
system. An identification system matches an individual organism with its
assigned object name (or species name, in the case of the classification of
living organisms). Identification is based on finding several features that,
taken together, can help determine the name of an organism. For example,
if you have a list of identifiers: large, hairy, strong, African, jungle-dwelling,
knuckle-walking; you might correctly identify the organism as a gorilla.
These identifiers are different from the phylogenetic features that were used
