by Gordy Slack
About two million kinds of plants and animals have so far been
described by science. That represents only a small percentage
of the estimated ten million to one hundred million kinds of organisms
on Earth, but it is still a lot of names to keep track of. To
eliminate the ambiguity that would arise from the casual application
of names to plants and animals, Carl Linnaeus, an eighteenth-century
Swedish medical doctor and botanist, developed a rigorous system
for naming and organizing them. The Linnaen classification system
assigns every known kind of organism to a kingdom, phylum, class,
order, family, genus, and species.
The "atomic" unit of classification is the species, and each
kind of organism is to be universally referred to by a two-part
name (binomial) that includes its genus and its species names.
That binomial is usually derived from Greek or Latin, the languages
used for scientific writing worldwide in Linnaeus's day. Binomials
are traditionally written in italics to show that they are scientific
and not common names. The genus name is always capitalized, and
the species name is always in lower case. Here is the hierarchy
of classification for the okapi (Okapia johnsoni).
Artiodactyla (even-toed ungulates)
The okapi has numerous common names in many different languages,
but whenever a scientist says or writes the name Okapia johnstoni,
other biologists around the world know precisely what kind of
animal he or she is referring to.
Over the past two centuries, Linnaeus's system has been revised
in some substantial ways. For example, the Linnaean system originally
used the number and type of reproductive parts in plants for dividing
them into different categories, or taxa. That approach resulted
in some very awkward and "unnatural" groupings and was eventually
replaced by one made popular by the English naturalist John Ray.
Ray's system looks at morphological attributes from all parts
of an organism in all stages of its development to draw conclusions
about how to group it. But both Linnaes's system of hierarchical
classification and his binomial nomenclature are still in use
today, and Linnaeus is generally credited as the founder of modern
Discovery of the evolutionary nature of the history of life on
Earth brought a new dimension to the science of taxonomy. Not
only do modern scientists want to group organisms based on their
physical characteristics, or morphology, but they also want their
groupings to reflect the relationships between different organisms
over time on the branches of the evolving tree of life. Phylogenetics
is the science that studies the evolutionary relationships, or
the phylogeny, of organisms. It expresses these relationships
through diagrams, called cladograms, which show the relative genetic
similarity of species.
Today, when a new mammal species is discovered, biologists examine
its morphology, range, behavior, habitat requirements, and usually
its DNA before determining just where in the Tree of Life it should
be placed. Modern genetic techniques sometimes turn up big evolutionary
surprises, showing organisms that look quite similar, for instance,
to be only distantly related. But this kind of surprise predated
genetic techniques as well. The okapi is a good example of an
animal first thought to be related to one group (the zebras) and
later found, through detailed morphological examination, to be
instead a very close relative of another (the giraffes).
At the American Museum of Natural History and in other museums
and universities around the world, the Linnean project of naming
and classifying Earth's diversity of life continues. But with
only a small percentage of life forms described, biologists have
likened our attempts to understand life and the systems that sustain
it to doing chemistry while knowing less than a fifth of the elements
in the periodic table.
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