Biomedical Engineering Reference
In-Depth Information
GENERAL PRINCIPLES OF CLASSIFICATION
Oddly enough, despite the importance of classification in our lives, few
humans have a rational understanding of the process of classification; it's all
done for us on a subconscious level. Consequently, when we need to build
and explain a formal classification, it can be difficult to know where to
begin. As an example, how might we go about creating a classification of
toys? Would we arrange the toys by color (red toys, blue toys, etc.), or by
size (big toys, medium-sized toys), or composition (metal toys, plastic toys,
cotton toys). How could we be certain that when other people create a classi-
fication for toys, their classification will be equivalent to ours?
For modern biologists, the key to the classification of living organisms is
evolutionary descent (i.e., phylogeny). The hierarchy of classes corresponds
to the succession of organisms that evolved from the earliest living organism
to the current set of extant species. Historically, pre-Darwinian biologists
who knew nothing about evolution, somehow produced a classification that
looked much like the classification we use today. Before the discovery of
the Burgess shale (discovered in 1909 by Charles Walcott), taxonomists
could not conduct systematic reviews of organisms in rock strata; hence,
they could not determine the epoch in which classes of organisms first came
into existence, nor could they determine which fossil species preceded other
species. Until late in the twentieth century, taxonomists could not sequence
nucleic acids; hence, they could not follow the divergence of shared genes
in different organisms. Yet they managed to produce a fairly accurate tax-
onomy. A nineteenth-century taxonomist would have no trouble adjusting to
the classification used in this topic.
How did the early taxonomists arrive so close to our modern taxonomy,
without the benefit of the principles of evolution, geobiology, modern
paleontological discoveries, or molecular biology? For example, how was it
possible for Aristotle to know, about two thousand years ago, that a dolphin
is a mammal, not a fish? Aristotle studied the anatomy and the develop-
mental biology of many different types of animals. One large group of ani-
mals was distinguished by a gestational period in which a developing
embryo is nourished by a placenta, and the offspring are delivered into the
world as formed, but small versions of the adult animals (i.e., not as eggs
or larvae), and the newborn animals feed from milk excreted from nipples,
overlying specialized glandular organs (mammae). Aristotle knew that
these were features that specifically characterized one group of animals and
distinguished this group from all the other groups of animals. He also knew
that dolphins had all these features; fish did not. He correctly reasoned that
dolphins were a type of mammal, not a type of fish. Aristotle was ridiculed
by his contemporaries for whom it was obvious that dolphins were a type
of fish. Unlike Aristotle, they based their classification on similarities, not
on relationships. They saw that dolphins looked like fish and dolphins
