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Much as a
seven-year-old addresses his letters with a gosh-darn-cute hierarchy of
locations ("123 Main Street, Columbus, Ohio, U.S.A, Earth, the Solar System,
the Milky Way galaxy"),
taxonomists define an organism's "home" on the plane of living things
with a set of predicates that increase in specificity from "domain"
and "kingdom" to "genus" and "species."
Determining
which categories an organism falls into is no easy matter. In general, physical
characteristics determine grouping. For example, one important distinction
concerns how an organism gets its nutrients. Members of the plant kingdom
obtain nutrients through photosynthesis (and share certain other traits as
well).
Speciation--the
evolutionary process by
which one species of organism becomes two--is notoriously difficult to put one's
finger on. It doesn't help that there exists no universally accepted definition
of "species." Reproductive methods vary so wildly from one region on
the plane of living things to another that a single rule couldn't possibly
cover the diversity between (or even within) kingdoms. However, taxonomists
have devised certain regionally useful standards for differentiation. In
addition to having physical similarities, members of a sexually-reproducing
species must be able to successfully produce healthy, virile offspring.
(Breeding horses and donkeys may produce mules, but these hybrids are usually
sterile.)
In general,
two diverging groups in a species
are said to have undergone speciation
when they are reproductively isolated from one another--when their
distinctiveness is preserved across generations of members. For primates like us, this implies
an inability to mate with a member of the divergent species.
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